1 ■.: , ( Series in Population Biology McGRAW-HILL PUBLICATIONS IN THE BIOLOGICAL SCIENCES Marine Biological Laboratory Library Woods Hole, Massachusetts the process of evolution Recent McGraw-Hill Publications in the Biological Sciences Series in Population Biology Ehrlich and Holm The Process of Evolution Series in Organism Biology Mazia and Tyler The General Physiology of Cell Specialization This symbol of the cephalopod Nautilus appears on all McGraw-Hill Publications in the Biological Sciences. It was chosen to represent the just proportion of living structures and to suggest the harmonious workings and balanced arrangement of the parts and elements of living things. The color of the binding represents the Biological Series in which this book is published. the process of evolution PAUL R. EHRLICH RICHARD W. HOLM Department of Biological Sciences Stanford University Illustrated by ANNE H. EHRLICH McGRAW-HILL BOOK COMPANY, INC. New York San Francisco Toronto London \ The Process of Evolution Copyright® 1963 by the McGraw-Hill Book Company, Inc. All Rights Reserved. Printed in the United States of America. This book, or parts thereof, may not be reproduced in any form without permission of the publishers. Library of Congress Catalog Card Number 63-15891 19130 56789101112 HD MM 75^+3210698 To Ed'^ar Anderson Joseph II. Cumin Herbert L. Mdsmi Charles D. Michcnrr Robert R. Sokal and Robert E. Woodson preface Modern evolutionary theory is the great unifying concept of biology. It represents the major theoretical triumph of the biological sci- ences-an all-embracing theory which attempts to explain the mani- fold complexities of biological phenomena. The biochemist attempt- ing to understand the genetic code, the neurophysiologist probing the complex mechanisms of the mind, the embryologist seeking to understand how one tissue affects the development of another^ in- deed, all biologists, are working on problems whose theoretical sig- nificance can be measured only by their contribution to our under- standing of evolutionary phenomena. The biochemist may be able eventually to cure cancer, the neurophysiologist to vmderstand mental disorders, and the embryologist to discover how the genetic code is translated into an organism. But, without a theory that inter- relates all these phenomena, their work would have onlv applied significance. The central position of evolution in biology has long been recog- nized. Nevertheless, most laymen and many biologists are largely ignorant of modern evolutionary theory. This book is an attempt to supply a reasonably concise volume dealing with organic evolution. It has been written for the reader concerned more with the process of evolution than with its products per se. There are no pictures of dinosaurs, no taxonomic descriptions of organic diversity, and no discourses on the history of evolutionary thought. We have assumed that our readers have reached at least that le\el of biological sophistication attained in a rigorous university course in biology. We hope that the book will serve as a challenging text for an undergraduate course in e\'olutionary theory, as a basic text to be supplemented with outside reading for a graduate course, and as general reading for biologists in other fields who may wish a brief review of what is known of the process of evolution. Most of the material presented has been used in either the undergraduate course in evolutionary processes or the course in advanced topics in evolution at Stanford University. An attempt has been made to present evolutionary theory as a unified whole. Because we are assuming some familiarity, at least on a casual level, with phenomena such as selection and mitosis, wo have felt free to make passing reference to them before they are treated in detail. Life, meiosis, genetic systems, culture, and the like have not been taken for granted. Rather we have attempted to show how these phenomena are themselves the result of evolu- tionary processes. Necessarily this involves speculation, which \m- VII viii I Preface feel to be rewarding and stimulating. It is, however, important that it be recognized as speculation. In some areas, other evolutionists certainly will find our treatment heterodox. In particular, we have deemphasized taxonomic ideas such as species and subspecies, which we feel have channeled the thinking of biologists about evo- lutionary problems. The term adaptation has been given the rela- tively inconspicuous role that we feel it deserves. Our reasons are discussed in the final chapter. We have tried to make our descriptions and discussions as rigorous as possible, except where it becomes absurdly pedantic to avoid taxonomic concepts or the casual use of words such as selection and adaptation. Lapses into what may be termed teleology we regard as teleonomy. We hope the reader will agree that a some- what more unified and logical treatment of evolutionary phenomena is possible if a rigid taxonomic framework is not followed. Scientific names used in this book connote kinds of organisms and carry no implications of genetic attributes or phylogeny. At the end of each chapter is a list of references chosen, in part, because of their currency and extensive bibliographies. Each ref- erence is briefly annotated. Reference without a direct citation often is made to scientists closely associated with a particular concept or experiment; direct citations can be found in the bibliographies of general papers listed. A rather extensive glossary also has been included. Our intellectual indebtedness to a very large number of evolu- tionists will be obvious. We must specifically acknowledge the writ- ings of Edgar Anderson, C. D. Darlington, Theodosius Dobzhansky, Herbert L. Mason, Ernst Mayr, George Gaylord Simpson, G. Led- yard Stebbins, and Sewall Wright which have had a profound influence in interesting us in evolutionary problems and in shaping our thoughts about them. We should Hke to thank the following persons who have helped us in many ways in the task of preparing this book: Joseph H. Camin, Verne Grant, P. H. Greenwood, N. K. Johnson, Alan E. Leviton, George S. Myers, C. L. Remington, R. G. Schmieder, and Robert C. Stebbins. One or more chapters of the manuscript were read by Kenneth B. Armitage, William K. Baker, D. L. Bilderback, Marsden S. Blois, Winslow R. Briggs, Howell V. Daly, Ruth R. Ehrlich, M. M. Green, Robert W. Hull, Joan Johnston, Donald Kennedy, Charles D. Michener, Ashley Montagu, Robert M. Page, John F. Pelton, David D. Perkins, Timothy Prout, Peter H. Raven, David C. Regnery, G. G. Simpson, Robert R. Sokal, Michael E. Soule, John H. Thomas, Preface ix Robert P. Wagner, Norman K. WesselLs, and Charles Yanofsky. Theodosius Dobzhansky read and criticized the entire book. Tliis generous donation of time and effort on the part of all these indi- viduals is deeply appreciated. Many of the subjects considered here have been discussed in detail with colleagues and students in our Population Biology Seminar. The authors accept full responsibilitv for all errors of fact and interpretation, as they have not always been able to adopt the suggestions of the reviewers. Paul R. Ehrlich Richard \V. llnhn contents Preface vi PART 1 ORGANISMS: ORIGIN AND FUNCTION Chapter 1 ] The Origin of Life The Early Stages 6 Origin of Self-replicating Systems Energy Sources 9 Origin of Structure 12 Origin of the Genetic Code 13 Reading the Code 17 Summary 19 References 21 Chapter 2 ! Units of Replication 22 Structure of Cells 23 Cell Division: Mitosis 27 Cell Division: Meiosis 31 Summary 35 References 35 Chapter 3 Genetics 36 Variation and Mendelian Genetics 37 The Units of Heredity 38 Mendel's Laws 39 Recombination 40 The Expression of Genes 42 Mutation 44 Evolution of Dominance 45 Chromosomal Mechanisms 46 Sex Chromosomes 47 Alterations of the Chromosomes 48 Deletions and Duplications 48 Inversions and Translocations 49 '^ xii I Contents Continuous Variation 52 Summary 54 References 55 Chapter 4 | Development 56 Growth and Homeostasis 57 Life Cycles 61 Differentiation and Morphogenesis 62 Modification of the Developmental System 66 Summary 67 References 68 PART 2 ! POPULATIONS: PROPERTIES Chapter 5 | Populations 72 Individuals and Colonies 73 Spatial Distribution 75 Ecological Distribution 76 Structure 79 Numbers of Individuals 80 Environment 82 Communities 86 Summary 87 References 88 Chapter 6 | The Theory of Population Genetics 90 Mendelian Populations 91 Panmixis 91 Gene Pool and Gene Frequency 92 Hardy-Weinberg Law 93 Population Size 95 Effective Breeding Size 96 Genetic Drift 97 Decay of Variability 98 Loss of Mutations 100 Mutation 101 Contents xiii Selection 102 Fitness or Adaptive Value 104 Types of Selection 104 Homozygous Recessives Completely Unsuccessful 105 Homozygous Recessives Relatively Unsuccessful 108 Homozygotes Inferior to Heterozvgotes 109 Balanced Polymorphism and the Retention of Variability 110 Genetic Load 112 Heterozygotes Inferior to Homozygotes 114 Migration and Population Structure 114 Joint Pressures 116 Adaptation and Gene Combinations 120 Summary 122 References 123 Chapter 7 Changes in Populations 124 Examples from Nature 125 Differentia] Mortality in Sparrows 125 Industrial Melanism 125 Microevolution in British Lepidoptera 131 Polymorphic Land Snails 133 Island Water Snakes 137 Chromosomal Polymorphism in Drosophih 139 Examples from Man 145 Pasture Plants 146 Mimicry of Flax 147 Disrupti\e Selection in Mimetic Butterflies 147 Resistance to Antibiotics and Insecticides 148 Laboratory Populations 150 Genetic Homeostasis 153 Genetic Assimilation 154 Adjustment to the Environment 157 Summary 158 References 158 Chapter 8 I Genetic Systems I 160 Genetic Systems in Microorganisms 161 Transformation 161 Recombination in Viruses 162 Transduction 163 xiv I Contents Sexual Recombination in Bacteria 163 Microbial Genetics and Evolution 164 Genetic Systems of Other Organisms 165 Sexuality and Diploidy 166 Diploid Life Cycles and Alternation of Generations 167 Recombination and Genetic Systems 169 Reduction of Recombination 169 Mating Systems and Recombination 170 Inbreeding Systems 171 Outbreeding Systems 172 Summary 172 Chapter 9 | Genetic Systems II 174 Meiotic Drive 175 Ghanges in Ghromosome Structure 176 Inversions 176 Reciprocal Translocations 177 Ghanges in Ghromosome Size and Shape 182 Ghanges in Ghromosome Number 183 Polyploidy 183 Aneupolyploidy 184 Eupolyploidy 189 Apomixis 196 Summary 207 References 207 PART 3 I POPULATIONS: DIFFERENTIATION Chapter 10 | The Differentiation of Populations 210 Examples of Differentiation 212 Gontinuous Geographic Variation 212 Golor, Pattern, and Size Variation in Animals 212 Ecotypic Variation in Plants 213 Glinal Variation in Plants 214 Glinal Variation in Animals 215 The First Stages of Genetic Isolation 218 Glosely Related Isolates 220 Contents xv Species Swarms in Fishes 220 Sibling Species of Alpine Butterflies 220 The Galapagos Finches 224 Host Preference in Parasitic Organisms 229 Discussion of Observed Patterns 230 Geographic Variation in Selection Pressures 280 Exchange of Genetic Information 234 Cessation of Gene Exchange 236 Isolation 237 Fusion of Populations 238 Meeting with No Gene Exchange 238 Limited Gene Exchange 240 Selection against Hybrids 242 Patterns of Differentiation 243 The Galapagos Finches and African Cichlids 243 Sibling Butterfly Species 244 Differentiation of Parasites 245 Allopatric Speciation 247 Sympatric Speciation 247 Summary 248 References 249 Chapter 11 ( Major Patterns of Variation 250 Extinction and Biogeographic Provincialism 251 Extinction 251 Biogeographic Provincialism 256 Reticulate Variation 258 The Fossil Record 258 Modes of Evolution 259 Rates of Evolution 261 Major Evolutionary Patterns 264 Adaptive Radiation 264 Differing Rates of Evolution and Adaptive Zones 266 Competition 270 Convergence 270 Higher Categories 272 Evolutionary Trends 274 Increase in Size 274 Increase in Complexity 275 Summary 276 References 277 xvi I Contents PART 4 [ HUMAN EVOLUTION: PHYSICAL AND CULTURAL Chapter 12 | The Evolution of Man 280 Man's Evolutionary History 281 Culture 285 Summary 292 References 293 Chapter 13 | The Theory of Evolution 294 Anthropocentrism 295 Cultural Bias 296 Scientific Bias 298 Evolutionary Biology 308 Epilogue 312 References 314 Glossary 316 Index 332 the process of evolution 1 organisms: origin and function The Process of Evolution is divided info four major sections. These deal with ( 1 ) the origin and functioning of organisms, (2) the properties of populations of organisms, (3) the ways in which differentiation of populations occurs and results in major patterns of variation, and (4) the evolution of 7nan and his culture ( which includes evolutionary theory). This initial section deals to a large extent with subjects that often are taken for granted in discussions of evolution. The basic properties of life are themselves products of an evolutionary process. In these first four chapters, certain of the properties of living systems critical to the .study of evolution are outlined. Emphasis is given to the ways in which a continuity of information is maintained in the cyclic stream of life and to wai/s in which this inforynation is elaborated. Where possible, intelligent speculation about ways and means of ancient transformations and origins of ubiquitous mechanisms is included. Such speculation, no matter how inaccurate it may turn out to be, .serves to remind us that such things as photosynthesis, DNA, meiosis, dominance, and cellular differentiation did not always exist in their present forms. No attempt has been made to give an encyclopedic account of these major areas of biological thought; rather we have tried to set the stage for the consideration of the process of evolution in organisms as we know them today. the origin of life One is so accustomed to the axiom that all life originates from pre- existing life that he seldom considers the question of how life began in the first place. The ancients solved the problem with the idea of the spontaneous generation of such complex organisms as flies and mice from nonliving matter. But these, as well as more sophisticated ideas, were laid to rest by the experiments of Redi and Pasteur. As a result, however, the basic question. How did life originate?, was brought into focus. Without some type of spontaneous generation, how can the origin of the myriad entities which are called "alive" be explained? Often this problem has been confused by the tendency to equate life with the properties of highly complex organisms. The contrast between a bird and a rock or between a bacterium and an iron filing is self-evident. Indeed, it is so striking that the difference between the hving and the nonliving could be misconstrued as one of kind rather than one of degree. The great majority of biologists believe that there is no significant discontinuity between the living and the nonliving, even though they may not agree on a definition of "life" or even upon the "properties" of life. Many obstacles may be avoided merely by viewing life as a special property of matter at a certain stage of complexity and not attempting a rigorous definition. At least it can be said that living systems handle energy in a regulative manner so as to establish an energy potential between the organism and its environment; cer- tainly one of the most fundamental properties of life is the continu- ous and directed movement of electrons among the complex mole- cules of which living organisms are made. It is important to note that these energy transformations are precisely controlled. The regulated release of an amount of energy, which uncontrolled would cause a mild explosion, results in what is thought of as life. In addi- tion, living systems have the property of reproducing themselves. Thus when the problem of the origin of life is considered, answers must be sought to the questions of how the systems that extract and utilize energy from the environment could arise and how they could replicate. There seems to be a sort of twilight zone between the extremes of living and nonliving, an area in which these terms may not be applicable. In this zone of viruses, nucleic acids, and spe- cialized sorts of colloids some of the answers to our questions may be found. It does not seem likely that the spontaneous origination of life can be observed at the present time. If new life did appear spontane- ously, it would probably quickly be eliminated by modern hetero- The Process of Evolution trophic organisms even if the environment were favorable. The situation was quite difiFerent under the conditions that were probable on the earth billions of years ago. THE EARLY STAGES The sine qua non of the production of life as we know it is the de- velopment of certain organic compounds— compounds built around carbon and consisting, in the main, of this element joined in diverse configurations with nitrogen, oxygen, hydrogen, phosphorus, and sulfur. The early stages in the chemical evolution of the earth's surface must have been characterized by the presence of much simpler inorganic molecules. The questions that immediately arise are how these were combined to produce the more complex com- pounds found in living systems, and what the source of energy for such transformations may have been. There is considerable evidence to support the thesis of Oparin that the atmosphere of the early earth was reducing in character, being made up principally of methane, water vapor, ammonia, and hydrogen. The behavior of these substances under a variety of con- ditions has been studied. For example. Miller placed mixtures of these gases in an apparatus (Fig. 1.1) in which they could be ex- posed to electrical discharges. Circulation was produced by boiling water on one side of the apparatus and condensing it on the other. Chromatographic analysis at the end of the experiments revealed the presence of amino, hydroxy, and aliphatic acids— three basic types of organic molecules, including the unit of protein structure. The amino acids included glycine and alanine (the most common amino acids in proteins), aspartic acid, and glutamic acid. It is interesting to note that «-alanine predominated over ^-alanine in these experiments; modern proteins contain only a-amino acids, (a-amino acids have the NH2 and COOH groups both attached to the same carbon atom.) Miller argues that the same types of com- pounds would have been produced under the influence of ultra- violet light and electrical discharges if the primitive earth had had a reducing atmosphere. He further contends that organic compounds would not be produced if oxidizing conditions were present and points out that, if amino acids (and other organic compounds) are necessary for life, the presence of life on earth is evidence for a primitive reducing atmosphere. The theory that the early atmosphere was reducing in character now seems widely accepted. Free oxygen, which first appeared some 800 million to 2 billion or more years ago and which gives the The Origin of Life present atmosphere its oxidizing character, came from two sources: Part had a photochemical origin (from water undergoing photolysis in the upper atmosphere, with the hydrogen escaping into space)- the rest was produced photosynthetically by living organisms (the main source today). Fig. 1.1 I Spark-discharge apparatus. ( After Miller, 1957, Ann. N.Y. Acad. Sci. 69. ) In addition to the abiogenic formation of organic compounds as outlined above, simple organic compounds are formed by the inter- action of water vapor with carbides in magma brought to the sur- face by volcanic activity (3Fe,„C„ -|- 4mHoO = Fe.{04 + C;i„H,sm). Calvin and others have shown in experiments with ionizing radia- tions (of the sort that would be produced by radioactive materials or by cosmic rays) that, in the presence of molecular hydrogen, partial reduction of carbon dioxide can occur. Further irradiation of aqueous solutions of the substances produced (formic acid, formal- dehyde ) leads to the formation of compounds such as oxalic acid or acetic acid. Eventually molecules of two-carbon compounds (acetic acid) may combine to produce a four-carbon compound (succinic acid). In these experiments amino acids also are produced. In other experiments, Fox has shown that heating dry amino acid mixtures results in the formation of synthetic polypeptides (pro- 8 I The Process of Evolution teinoids). Proteinoids in water tend to form spherules of varying size and shape, depending upon their interaction with substances mixed with them. These spherules, in some respects, resemble co- acervates and other cell models. Among other things, the proteinoid spherules retain their integrity for rather long periods and are not destroyed by high-speed centrifugation. ORIGIN OF SELF-REPLICATING SYSTEMS Thus it can be seen that there were diverse ways in which organic compounds may have been produced on the primitive earth. It is not unreasonable to assume, therefore, that the primitive ocean was comparable to a thin soup of organic materials. There is little agree- ment as to how the first self-replicating systems developed in this "soup. " Obviously, what was first required was the selective con- struction of molecules. Calvin has pointed out that the phenomenon of autocatalysis has the nature of a selective process. Autocatalysis occurs whenever the product of a chemical reaction has the property of influencing catalytically the rate of its own formation. There follows a progressive build-up of products in a sequence of in- creasingly complex compounds formed from simpler ones. An early selection of this nature (for complexity) must have gone on in the organic soup. Autocatalytic reactions are only partly analogous to a self-replicat- ing living system. No presently known substance, when isolated, will replicate itself. Only systems have the ability to replicate. The living systems familiar to us are composed of proteins and nucleic acids, together with some means of energy mobilization. Polypeptide chains, the backbones of protein molecules, are formed by the link- age of amino acids in linear series. The linkage is accompanied by the elimination of water as the amino acid chain lengthens. The bonds between the amino acid units are known as peptide bonds. The spontaneous formation of even a small protein in a solution of amino acids requires outside energy and is a very improbable event. But in the absence of free oxygen and predatory organisms, the life of an amino acid "soup" could be extremely long, long enough to turn the improbable into the probable. ( The chance of being struck by lightning in a 70-year life span is very slight, but if one lives for 700 million years it becomes almost a certainty. ) However, as Wald points out, the spontaneous generation of protein molecules is opposed by their tendency toward spontaneous dissolution. Indeed, the equilibrium point in the reversible, spon- The Origin of Life 9 taneous protein-generation reaction lies on the side of dissolution rather than synthesis. Wald suggests, nevertheless, that molecules seem to be able to resist dissolution both through large size and through aggregation with other molecules. Proteins may be an un- stable mid-point, subject either to dissolution into their component amino acids or to the formation of more stable aggregates. The first "organisms" may well have been the result of the formation of larger and larger aggregates. ENERGY SOURCES Ultraviolet light usually is considered the chief source of energy for early synthetic processes. With simple molecules, only very short wavelengths are absorbed, but as more complex molecules appear, absorption of longer ultraviolet wavelengths takes place. As the earth evolved its thick atmospheric layers, ultraviolet of short wavelengths no longer could penetrate to the earth's surface to be used as an energy source. The appearance of colored pigments (e.g., porphyrins mentioned below) made possible the absorption of energy in the visible spectrum. However, whatever the source of energy, there is a considerable gap between the absorption of a quantum of energy and its mobilization for use in biological proc- esses. The problems of utilizing energy for protein synthesis and the conditions under which it may occur are particularly vexing ones. In present-day biological systems the enzymes responsible for the mobilization of energy and for the synthesis itself are proteins. Thus if one is to postulate the functioning of such systems in the forma- tion of the first proteins, he becomes embroiled in a "chicken or egg" dilemma. It has been suggested that, in the absence of proteins, other substances (e.g., clays) may have served as catalysts, since many of the known enzymatic phenomena are fundamentally mole- cule-surface reactions. This raises the question of how proteins sub- sequently came to assume this function. At least it can be said that the surface phenomena of clays and surface configurations of pro- teins have certain aspects in common. Chemical energy for synthesis in modern biological systems in- volves organophosphate bonds that yield exceptionally high energies upon cleavage. The energy released upon cleavage or transfer of these bonds is regulated by a complex system of catalysts ( enzymes plus their coenzymes); the characteristics of these sets of reactions are unique to hving systems. They change velocity in response to changes in concentration of product; are dependent upon physical 10 I The Process of Evolution conditions (temperature, pressure) in a fashion distinct from non- living reactions; and demonstrate a degree of conservation of energy not often equaled in inorganic reactions. Such reactions in living systems are referred to as biological oxidations. The energy needed by the heterotrophic organism is almost universally mediated by a single type of organophosphate bond, that in the energy-rich com- pound called adenosine triphosphate (ATP). When ATP releases energy in a biological reaction, it releases one phosphate and be- comes adenosine diphosphate (ADP). The latter still includes one energy-rich phosphate bond and is particularly susceptible to being rephosphorylated into ATP. But the rephosphorylation (called oxidative phosphorylation) is mediated by the system of enzymes referred to above. These enzymes in their turn depend upon energy from the cleavage of ATP bonds, but the important point is that, for each ATP bond that releases its energy for these enzymes to function, more than one ATP bond is formed. The extra energy is derived from the energy stored in the glucose (or other) molecule upon which the enzymes are acting directly. By this interlocked series of reactions, chemical energy supplied to the organism as molecules of carbohydrate, lipid, or protein (which the organism cannot use, as such) is transformed into ATP-bond energy that the organism can use. In nonliving systems, energy transfer by molecular degradation yields smaller molecules plus much heat. In living systems, the products are high-energy organophosphate bonds, smaller mole- cules, and surprisingly little heat. In fact, one of the salient features of the living energetic machinery is the closeness of the coupling between energy-yielding and energy-storing reactions and the result- ant conservation of energy. Of course the ultimate source of energy in existing organisms (except chemosynthetic bacteria) is that of the sun trapped by photosynthetic organisms and eventually stored in phosphate bonds by a related process involving photophosphoryla- tion in the photosynthetic organism or by synthesis into carbo- hydrate with phosphorylation. Many important enzymes or catalysts in both photosynthesis and biological oxidations are colored com- pounds involving metal ions (Fe, Mg) and the organic substances known as porphyrins. Calvin has diagrammed (Fig. 1.2) how such important biological materials might have arisen in the course of chemical selection involving autocatalysis. In the sequence from simpler to more complex molecules, later stages are catalysts for succeeding stages. Since the use of porphyrins by nonphotosynthetic organisms is widespread, Calvin feels that the presumably random The Origin of Life 1 1 variation involving small changes in the porphyrins led eventually to the construction of chlorophyll and the invention of photo- synthesis. In addition, Granick believes that all the colored com- pounds in the sequence that leads to chlorophyll might have had the same function as chlorophyll. In the early stages, metallic ions, present as constituents of minerals, might have served to catalvze the same reactions that they now catalvze as metalloenzvmes. Photosynthesis is the result of a complex scries of reactions. Some of these can take place in the dark, whereas others can occur only with illumination. Most of the many reactions usually included under the rubric photosynthesis, in the broad sense, are actually dark reac- tions, involving the addition of CO2 to — C — C — C — chains. These dark reactions can be carried out by most cells. It seems likely that many of these reactions evolved independently, perhaps earlier than Random synthesis from C, compounds by radiation ■ Succinic acid CO2H I CHj I CH2 1 CO;H CHj— CO,H / NH2 "^^ Glycine CO2H CH2 C = CO2H CO2H CH2 ■CO, CH-CO.H / NH2 a-NH2-j3-Keto- adipic acid I H2C-0=C I I /C,, /CH2 NH,— CH, II "N ^0 H2 8-Amino- levulinic acid Fe Porphyrins n-steps Possible points for catalytic function of Fe CH2 CH H CH3 C HC {Fe-. CH 2 H CHj I CO2H CHj CHj COjH Protoporphyrin No. 9 Fig. 1.2 I Possible steps in the synthesis of porphyrin compounds from molecules produced randomly under the influence of radiation. Iron may act as a catalyst at the points marked by arrows but is a much better catalyst when combined with porphyrin. Thus the production ot protoporphyrin 9 is facilitated by the presence of protoporphyrin 9. {From Calvin, 1959, Evolution 13.) 12 I The Process of Evolution strictly photosynthetic reactions. The light reaction leads to the production of a reducing agent and some type of high-energy phos- phate, probably adenosine triphosphate (ATP). The reducing agent usually is hydrogen but occasionally is a phosphorus compound. These substances then operate the carbon reduction cycle, and hexose sugar molecules are produced. This energy is mobilized, in ways which are imperfectly understood, so that excited chlorophyll transforms other molecules to produce the reducing agent and the ATP. Thus, typically, photosynthesis involves photophosphorylation, i.e., the transformation of light into the "energy currency" of phos- phate bonds. It probably never will be possible to say with certainty whether or not the coupling of colored compounds with biosynthetic proc- esses took place before or after the appearance of what today would be called living organisms. Calvin believes that the final step in the development of modern photosynthesis, the production of oxygen, did not take place until relatively late in the sequence of events. Therefore reactions like those of some modern organisms, which are photosynthetic but do not emit oxygen, were prior. ORIGIN OF STRUCTURE In the light of the above discussion, it is not overwhelmingly diffi- cult to imagine how the substances required for the processes we think of as metabolism could have arisen. However, living systems are not fluid structureless entities. Generally they have a character- istic and complex organization of the matter comprising this energy- conversion mechanism. Now the factors involved in the evolution of structure as well as of function must be considered. In the sea the original molecules probably were dispersed as a rather uniform colloidal suspension. However, in colloids of different substances, semiliquid colloidal gels or coacervates are formed, and it might be expected that these may have arisen as the organic soup became in- creasingly complex. From the work of physical chemists, much is known about the behavior of coacervates. They often do not form as a continuous layer but rather separate out of the equilibrium liquid (thus left colloid-poor) in the form of discrete droplets. These droplets not only concentrate organic molecules of high molecular weight but also possess a definite internal structure as well as a highly developed surface separating them from the equilibrium liquid. In the coacervate droplet one can see the first distinct sepa- ration of a structural complex of organic material from its environ- ment. The Origin of Life 13 Oparin suggests that, in a sensp, coacervate droplets competed with each other for materials-that some, which by chance had a favorable composition or internal configuration, grew more rapidly than others. These successful droplets were then first to reach a size at which they became unstable and broke apart into smaller par- ticles. These then enlarged, subsequently divided, and thus con- tinued the sequence. One can also imagine that accidental fusion of droplets might have carried them beyond the stability point, causing breakdown into smaller units. Should the droplets have dif- ferent compositions, a sort of protosexual recombination process would occur. Thus one can see in coacervates many properties that would qualify them as hnks in a chain leading to the structure of life as now knowTi, and, since the matter carries on the function, to living systems of the familiar sort. They are clearlv separated from their environment, have internal structuring, absorb matter from their environment, and have sufficient multiplication and "recombi- nation" to permit the operation of natural selection. It seems clear that, in the vast stretches of geologic time, mech- anisms such as those outlined above (and others as vet undiscov- ered) produced the ancestors of the hving systems we know todav. Indeed, when one pictures the vast oceans, lakes, and hot springs rich in organic compounds and presenting a wide variety of con- ditions of temperature, light, salt concentration, and phvsical sub- strate (crystals, clavs), it is difficult not to believe that living sys- tems developed more than once. It is not unlikely that modern organisms are the descendants of the victor in a fierce energy war among the early "organisms." ORIGIN OF THE GENETIC CODE The level of complexity of the hypothetical ancestral organism dis- cussed to this point does not involve a system by means of which the entity could be replicated as a unit. Splitting by fission may or may not result in the formation of equal parts; in fact, it might be imagined that occasionally one of the parts would lack a component essential for the maintenance of life. At this stage there was no sys- tem of heredity, no genetics, which would ensure the continued production of functional entities. The first principle of genetics is "like begets like." This is not the result of a great immutable "law of nature" but rather the functioning of a complex system for trans- mitting genetic "information," the information needed to construct a new organism. Without such a system, it seems certain that life would not have evolved beyond the level of coacervate droplets. 14 I The Process of Evolution The efficiency of the transmission apparatus has been a major factor in determining the hmits of intricacy of Hving entities. In the 2 or 3 bilHon years of chemical evolution that preceded the evolution of life, many systems of transmitting information may have been tried and discarded in a selectional process. It is clear that the hereditary system must have been coupled to the synthetic and energy-converting systems; therefore it is no surprise to note that the substances involved in the hereditary system, the nucleic acids, have adenosine phosphate as a building block. The system found in most cellular organisms is based on coding information in two macromolecular nucleic acids, ribonucleic acid (RNA) and deoxy- ribonucleic acid ( DNA ) . The assumption usually drawn is that the giant ordered molecules of which living systems are composed are the evolutionary end result of some process of aggregation of smaller molecules and that selective forces controlled the process. With the spontaneous ran- dom occurrence of a sequence of ordered molecular subunits of nucleic acid, for example, a template against which further se- quences are replicated becomes possible. This leaves essentially unanswered and unanswerable the question of how the original sequence arose. This is not the only possible interpretation, however, as Pattee has pointed out. He suggests that the precursors of biological macro- molecules were not random sequences but naturally ordered crystal structures. These result from various restrictions found in crystal- lization processes in general. In the growth of polymers the con- figuration at a particular stage may determine which subunits are added. Thus there may be feedback control of the growth of macro- molecules. By using a computer model (Fig. 1.3), Pattee has shown that, with feedback, simple configurations can be assembled to produce elaborate, repeating, and well-ordered sequences. It is not necessary to postulate, as most authors have done, a statistically highly improbable preexisting sequence that must be copied. In Pattee's view, the present genetic mechanisms themselves are the evolutionary result of the natural occurrence of ordered macro- molecular sequences. The exact functioning of genetic mechanisms has not been elucidated entirely, but in general outlines it is as follows: The units of information, known as genes, are coded into the structure of giant self-replicating molecules of DNA. These molecules, reproduced and passed from generation to generation, are the master blueprints from which all living organisms are produced; they maintain the continuity of life. Slight changes in these blueprints are also repro- The Origin of Life 15 duced and are responsible for the variation that permits evolution. DNA molecules are chains made up of four nucleotide units: deoxyguanylic acid, deoxycytidylic acid, deoxyadenylic acid, and deoxythymidylic acid. Similarly, RNA molecules are made up of four nucleotides: guanylic, cytidyhc, adenylic, and uridylic acids. Each nucleotide group consists of a pentose sugar molecule with an attached base and an attached phosphate group. The backbones of the DNx\ and RNA molecules are made up of the pentose sugars linked by the phosphate groups. Attached to each sugar residue in Fig. 1.3 1 Mechanical computer model for the production (by feedback) of ordered sequences. The beam of the balance operates the gate permitting an A ball to enter when the pans are balanced and a B ball to enter when the pans are unbalanced. A balls are heavier than B balls. (From Pattee, 1961, Biophysical Journal 1.) Sequence generated by the balance mechanism BBABAABABBAAABBABBBBABBABABBABBAABAAB A A A B B B A A B B B B A A B A B A B A A B B A B A B B B A B B B A A A B A A A A A BABBBBBAABABA BAAAABBAAAAAB AAA ABA BBAABBBA ABBAABAA BBABAAAB ABBBBBBB BBBAABBABAAABA AAAAAABBBBBBBA BBBABAABABBAAABBABBBBA BAAABBBAAAABABBBBBAABA AA BBBBAAABABAAAABBAAAA ABABABAABBAABBBABBBABA -(m(^ 16 I The Process of Evolution Fig. 1.4 I The structure of DNA. Upper left, complementary chains of nucleotides. Letters, bases; five-sided figures, pentose sugars; circles, phosphate groups; double lines, hydrogen bonds. Lower left, detail of segment of one strand. Right, model of double helix in which the nucleotide chains are arranged. Ribbons, phosphate-sugar backbone; bars, paired bases joining backbones. This double-helix configuration is known as the Watson-Crick model. (After Sinnott, Dun7i, and Dob- zhansky, 1958, Principles of Genetics, McGraw-Hill.) A^^T C=G T = A G = C Base (e.g., thymine) Base (e.g., adenine) Base (e.g., guanine) - Sugar - Sugar - Sugar Base - Sugar (e.g., cytosine) Phosphate Phosphate Phosphate The Origin of Life 17 this regular chain is one of five bases: either a purine (adenine, guanine) or a pyrimidine (cytosine, thymine in DNA, or uracil in RNA). The configuration of the DNA molecule appears to be a double helix of the sugar-phosphate backbones held together with cross-linkages of paired bases (Fig. 1.4). Because of their chemical properties, these bases line up the following pairs: adenine with thymine and cytosine with guanine. This helical structure best ex- plains the results of X-ray diffraction and other studies to determine the physical properties of the molecule. When chromosome duplication takes place in preparation for cell division, the complementary helices probably "unwind," and each helix forms a chemical template on which DNA precursors attach to re-form the complementary strand and reestablish the double helix. This is the mechanism by which the genetic code is passed from cell to cell and (through the gametes) from organism to organism. The genetic code has been shown to be the actual se- quence of nucleotides in the DNA strand. READING THE CODE Protein synthesis takes place largely in the ribosomes ( microsomes ) of the cell— cvtoplasmic structures physically separated from the nuclear DNA. The ribosomes contain the vast majority of the RNA in the cell. DNA serves as a template against which can be as- sembled another identical strand of DNA or a strand of RNA. Thus the code may be transferred to "messenger" RNA molecules which presumably carry it to the ribosomes where protein formation occurs. (It is not known how the cell "tells" the DNA whether to make more DNA or RNA. ) There are several possible explanations of how the DNA master blueprint and the RNA messengers control the assembly of various proteins. It is necessary for the code, transferred from the DN.\ to the RNA, to be able unambiguously to control the sequential positions of 20 common amino acids which may go into the com- position of proteins. One suggestion was based on the mathematical demonstration that 20, and only 20, different sequences that will not be subject to the confusion of overlapping can be constructed by taking the four nucleotide units three at a time. For example, if the nucleotides are numbered 1, 2, 3, and 4, the sequences 131 and 312 would be overlapping; if they were placed adjacent to each other, the sequence might be 141131312, in which 131 occurs in two oxer- lapping positions. The nonoverlapping triplets would be 112, 212. 18 I The Process of Evolution 131, 132, 133, 231, 232, 233, 141, 142, 143, 144, 241, 242, 243, 244, 341, 342, 343, 344. Recently several groups of workers have been able to show that the RNA code consists of nonoverlapping triplets, each of which determines the position of an amino acid. At this writing, codes for 19 of the 20 amino acids have been partially worked out. For in- stance, the amino acid alanine is coded as some sequence of uracil, cytosine, and guanine, and the amino acid serine as a triplet con- taining two uracils and a cytosine. It now appears likely that the nonoverlapping nature of the code is determined not by the structure of the triplets themselves but rather by the existence of some device for designating starting points for "reading" the code. For instance, in the sample sequence above (141131312) there is no ambiguity if the left end is designated the starting point and the code exists only as triplets; it becomes clearly 141-131-312. If the code works in this manner, there are then 4^ = 64 different possible sequences, a plethora for determining only 20 items. It is quite possible that each amino acid can be coded by more than one triplet combination and that certain combinations indicate "capital" triplets (those starting a sequence). Work on the decoding problem is now proceeding so rapidly that it seems in- evitable that some of these questions will be settled before these words are published. Whatever the answers, they will surely contain fascinating hints as to the evolution of the code itself. Now, how do the amino acids "read" the messenger RNA sequence code so that they condense into proteins containing the proper order of amino acid residues? Amino acids become bound to relatively small soluble RNA molecules (transfer RNA) before the acids are linked together into proteins. Further data are very suggestive of the following pattern of protein construction. There is a separate transfer RNA molecule for each amino acid. In one part of the molecule is a sequence of nucleotides that determines with which amino acid it may react, and in another part is the sequence (a triplet?) that determines the position on the RNA template to be assumed by the transfer RNA unit. The compounds of RNA and amino acids are formed with energy supplied by ATP. This enzyme- mediated reaction is diagrammed in Fig. 1.5. The transfer RNA- amino acid units then find their places on the long RNA template, presumably pairing with complementary sequences on the template. The amino acids are still in an activated state; when they are brought into close proximity in the manner outlined above, they condense (probably with the aid of an enzyme) to form a protein with the proper sequence of amino acid residues. The Origin of Life 19 One can well imagine that this system is a far cry from the first system of transmitting genetic information. For example, its sophistication may be seen in the fact that the DNA remains as a master template, reducing the possibilities for error that would be inherent in a system in which copies are made from copies. Similarly, the short transfer RNA molecules are highly specialized to accom- plish the proper positioning of the protein precursors. The manifold interactions of DNA and RNA in the organism are not confined to processes concerning transmission of genetic information and protein synthesis. There is speculation that RNA functions in those higher- organism systems that involve training and memory. The mental properties of primates may eventually be described in terms of fundamental chemical properties recognizable in the simplest cells and organisms. In viewing the complexities of function found in the cells of present-day organisms— the highly specialized organelles, the very efficient system for utilizing high-energy phosphate bonds, the pre- cise mechanisms for distribution of genetic information and for cell division— one may find it hard to believe that such complexity ever arose from the coacervate stage previously described. It is like look- ing at a unicellular organism and a man and trying to imagine one as the ancestor of the other without knowledge of any intermediates. It should be remembered that the time available for the evolution of the coacervate into the complex cell was of the same magnitude as that available for the journey from protistan to man. SUMMARY Life is a complex energy-matter nexus whose origin can be ex- plained logically in general terms. Important events in the origina- tion of life certainly were the development of organic compounds, their segregation into structural entities, the origin of energy-mobiliz- ing cycles, and the development of systems for self-replication. These events presumably must have been partially synchronous and were controlled by a sort of protoselection. The present system for self-replication utilizes information coded in macromolecules of nucleic acids that control protein synthesis. Life may be considered to be an aspect of the matter-energy con- tinuum characterized by incessant replication. Perfect replication is impossible, and therefore natural selection is inevitable. 20 I The Process of Evolution I The Origin of Life 21 REFERENCES Calvin, M. 1959. Round trip from space. Evolution 13: 362-377. A good brief discussion of the problem of the origin of life. Miller, S. L. 1951. The formation of organic compounds on the primitive earth. Ann. N.Y. Acad. Sci. 59: 260-275. See this for details on the atmosphere experiment. This article is in a number entitled Modern Ideas on Spontaneous Generation which contains several interesting papers. Needham, A. E. 1959. The origination of life. Quart. Rev. Biol. 34: 189- 209. A stimulating discussion in very broad terms. Oparin, A. I., A. E. Braunshtein, A. G. Pasynskii, and T. E. Pavlovskaya [eds.]. 1959. The Origin of Life on the Earth. Pergamon Press, New York. This symposium volume contains a large number of important papers, most of which are highly technical. Oparin, A. I. 1961. Life: Its Nature, Origin and Development. Academic, New York. The latest revision of the classic work on the origin of life. Sagan, Carl. 1961. On the origin and planetary distribution of life. Radia- tion Res. 15: 174-192. A recent summary paper with extensive bibliography. Fig. 1.5 I ( see opposite page ) Diagrammatic theoretical representation of the process of protein synthesis under the control of messenger RNA. (1) ATP molecule. (2) Amino acid (tryptophan). (3) Enzyme medi- ating high-energy bonding of tryptophan residue with adenylic acid ( AMP ) . ( 4 ) Phosphate groups previously bonded with AMP dropping away from enzyme substrate. ( 5 ) Same enzyme mediating transfer of tryptophan residue and high-energy bond from AMP to proper transfer RNA molecule. (6) AMP molecule dropping away from enzyme. (7) Glycine-charged transfer RNA unit dropping away from glycine- activating enzyme. (8) Messenger RNA template. (9) Start of synthesis of protein strand. "Capital" methionine-charged transfer RNA unit aligned with "capital" methionine triplet ( UAG ) on RNA template. ( 10) "Lowercase" methionine-charged transfer RNA unit approaching "lowercase" methionine triplet ( UGA ) on template. ( 11 ) Enzyme "zipper" which assembles amino acid residues into protein. ( 12 ) Newly tormed protein that has dropped away from template, freeing the transfer RNA units involved in its synthesis. Only a small sample of presumably many simultaneous reactions is shown in this diagram. units of replication One of the most dramatic results of modern scientific technology and of the development of the electron microscope has been the revival of interest in cytology. The increased resolution of the elec- tron microscope has revealed structures of amazing complexitv where none was known to exist and, indeed, virtually has brought form and function together at the level of macromolecules and their aggregates. From our point of view, these results are particularly interesting; from them we can hypothesize that the membrane sys- tems of which cells are largely composed may have been the in- evitable result of the mixture of large and complex molecules, such as lipids and proteins, before the origin of life itself, as suggested in Chap. 1. These macromolecular structures, originating bv chance, may be similar to the membranes seen to be combined in cells in a variety of ways. The basic cellular constituents are common to plants and animals, providing a structural ground plan for all life except in the most highly specialized cells or organisms. A brief review of cell structure is given below to provide background for the later discussion of the evolution of genetic mechanisms and systems. Whatever may have been the origin of cells, both plant and animal cells show such striking similarity in structure as to suggest that either there is a common ancestral type or, with life as we know it, only one basic type of structure (Fig. 2.1) is compatible with func- tion. The chemical composition of cells is relatively easy to deter- mine, and many physical properties of cells and their constituents can be measured. However, it is in the organization of these chem- icals that the unique property of life and the cellular state is achieved. The chief structural units of cells are large molecules of proteins, carbohydrates, and lipids. Interspersed among, and some- times actually associated with, the physical framework that results from the aggregation of these molecules are the myriad types of smaller molecules: soluble proteins, amino acids, vitamins, inorganic constituents, etc. STRUCTURE OF CELLS Both plant and animal cells appear to be bounded by a membrane, called the plasma membrane, which has the important propcrtv of being differentially permeable. Physical, chemical and biological studies of this membrane indicate that it is a complex structure com- posed of protein and lipid molecules associated in layers. The bipolar lipid molecules are arranged in two layers, with their hydrophobic 23 24 I The Process of Evolution tails together and their hydrophilic heads pointed toward the hydrated protein strands, which have their long axis at right angles to that of the lipid molecule?. Particularly in free-living cells and cells in culture, the property of pinocytosis may be seen: By the rapid and constant extrusion and withdrawal of minute pseudopod- like extensions of their surface, the cells may ingest water and other molecules. Free cells and cells in tissues seem always to have additional layers outside the plasma membrane. In the animal cell, these layers are composed largely of molecules of proteins and sugars, and their integrity depends upon the calcium balance of the cell en- vironment. Plant cells are rather different in two respects. First of all, the outer layers (or cell wall) of plants are composed mainly of carbohydrate molecules. Glucose residues arranged in long chains form the most important constituent, cellulose. Other carbohydrates, as well as fatty acid substances ( suberin, cutin, etc. ) , also are asso- ciated with this wall. Contiguous cells in tissues are cemented to- gether by a middle lamella, which is pectate in nature and also dependent upon calcium for rigidity. The second important diflfer- ence between plant and animal cells is that cells in plant tissues are in organic connection through strands of cytoplasm called plasmo- desmata. By means of electron micrographs, most instances of so- called plasmodesmata in animals have been shown not to involve continuity of protoplasm. That is, across the strands that look like Fig. 2.1 I Diagram of a general- ized cell with parts as seen under the electron microscope ( some components enlarged or simplified). Units of Replication 25 plasmodesmata, there is a pair of plasma membranes. It is as if the tissues of Metazoa were composed of cells stuck together, whereas those of plants result from the more or less incomplete partition of "protoplasm." The boundary between animal cells may be exceed- ingly complex. Structural and chemical properties of the intercellular region suggest that specialization of the periphery of the cell may play an important role in cellular differentiation. The most conspicuous structure within most cells is, of course, the nucleus. Recent electron micrographs show that the nucleus is bounded by a pair of membranes; the term nuclear envelope may be used to refer to both. The inner membrane appears to surround the nuclear contents like a sack. The outer membrane, however, is clearly continuous with a membrane system that permeates the cytoplasm to a greater or lesser extent. Thus the cell appears to be penetrated by a system of tubes, canals, vesicles, and cisternae (the amount and type depending upon the nature of the cell and its state of ac- tivity) called the endoplasmic reticulum. In a sense, the nuclear contents are outside the cell, for the cytoplasmic membranes are continuous also with the plasma membrane. The inner and outer membranes of the nuclear envelope are connected, however, since they are both perforated by pores distributed rather regularly over the surface of the envelope. These pores lead to the cytoplasm sur- rounding the endoplasmic reticulum. Electron micrographs of the cytoplasm around the endoplasmic reticulum show it to be far from homogeneous. The tubes of which the reticulum is composed are associated with small dense granules in synthetically active cells. These are absent where the tubes are continuous with the plasma membrane. Occasionally these granules, which are rich in ribonucleo-protein, are found in the intervening cytoplasm. They are known as ribosomes and, as mentioned in Chap. 1, are thought to be concerned with protein synthesis. Particles known as microsomes which have been studied by physiologists appear to be artifacts: aggregations of ribosomes and endoplasmic reticulum that appear when the cell is fractionated. Ribosomes do not occur where the endoplasmic reticulum is continuous, with more or less flattened, concentrically arranged cisternae making up the Golgi material of both plant and animal cells. The Golgi complex is difficult to isolate from the other cell organelles, and its precise function is not yet known. Scattered among the tubes and vesicles comprising the endo- plasmic reticulum are found the mitochondria of plant and animal cells. These spherical or tube-shaped structures also have a double- membrane boundary, the inner membrane being thrown into a series 26 I The Process of Evolution of convolutions forming lamellae or villae. The mitochondria are the site of most of the reactions involved in cellular respiration, includ- ing the formation of adenosine triphosphate (ATP). There is some evidence also that mitochondria may play a role in cytoplasmic in- heritance in the sense that they may be self -replicating. Closely related to the mitochondria structurally, but found only in plant cells, are the plastids. Like the mitochondria, they are the site of important reactions providing energy for the cell (indeed for nearly all life). Plastids have a lamellar structure, and upon the alternating layers of lipid and protein molecules are found layers of special pigments such as the various types of chlorophyll, carot- enoids, and others ( depending upon the plant group studied ) . Light energy absorbed by the plastid is converted to chemical energy; in a series of steps, energy from oxidation is utilized to phosphorylate ADP to ATP. This photophosphorylation obviously is related to the phosphorylation carried out by the mitochondria. The functional units of chloroplastids in higher plants are minute particles called grana. In the green bacteria and blue-green algae the grana are not organized into plastids. The cytoplasm of most animal and many plant cells has a struc- ture, known as the centrosome, adjacent to the nucleus. Within this relatively clear area of cytoplasm there are one or two granules, the centrioles. These organelles are important in the origin and function of flagella and cilia and in nuclear and cell division. The nine strands making up the outer portion of a flagellum or cilium are continuous with nine tubelike or filamentlike structural components of the centriole or basal granule. This remarkable similarity of centrioles, basal granules, and cilia (which is preserved even in such highly specialized cells as retinal photoreceptors ) is found through- out the animal kingdom. The centriole and obviously related struc- tures (such as basal granules) apparently have properties that lead to the organization of fibrous protein molecules in special ways, for example, in the formation of the spindle tubules (fibers) during mitosis. The genetic information in the cell is located mostly in the chro- mosomes within the nucleus. The precise state of the material is not known. In the so-called resting or metabolic stage, chromosome material in the nucleus usually is difficult to view. In some instances, portions of chromosomes that have not undergone the usual trans- formations accompanying mitosis may be seen. Often one or more of these chromosome regions are associated with the nucleolus, a usually conspicuous feature of the metabolically active nucleus. Presumably during the metabolic stage, material is exchanged be- Units of Replication 27 tween the cytoplasm and the nucleus, and the role of messenger and transfer RNA in translating the DNA code into protein struc- ture is carried on. The mechanism of this exchange is not under- stood, however. Evidence clearly indicates movement of nucleic acid from nucleus to cytoplasm. Actual particles usually ha\e not been found in the pores of the nuclear envelope, as seen in the electron microscope. Cytologists have reported that portions of the nuclear envelope may pinch or bud off into the cytoplasm, where the pieces have the appearance of endoplasmic reticulum or mito- chondria. CELL DIVISION: MITOSIS When cells divide (Fig. 2.2), the first conspicuous change usually is in the appearance of the nucleus. The chromosomes become visible within the nuclear envelope in living or stained cells and, usually concomitantly, the nucleolus decreases in size. This first of the arbitrarily designated stages of mitosis or nuclear division is pro- phase. Toward the end of this stage, it can be seen in the cells of many organisms that the chromosomes are double, each consisting of two half chromosomes ( chromatids ) . In most organisms the dis- appearance of the nuclear envelope marks the beginning of promet- aphase. During this period, or somewhat prior to it, a spindle-shaped bundle of fibers (now known to be microtubules) is organized in the cytoplasm. Toward the end of prometaphase the chromosomes become arranged in a group at the equator of this structure. The spindle in those organisms in which it can be isolated and studied chemically is composed of fibrous protein molecules con- taining many sulfhydryl linkages and apparently oriented by the centrioles at either end. In animals ( and in some plants ) the centrioles also are surrounded by a pompon of fibers, the aster. In the somatic cells of most plants no asters or centrioles are visible, but they may be conspicuous in the reproductive cells. During the brief stage called metaphase, the chromosomes are arranged across the equator of the spindle with at least their spindle attachment points or centro- meres in essentially a plane at right angles to the long a.xis of the spindle. Very shortly thereafter the centromeres appear to divide (they may actually have split at an earlier period) and the chroma- tids— now daughter chromosomes— move to the poles. The phase of chromosome movement is called anaphase, and its mechanism is still not understood. None of the current theories satis- factorily explains the behavior of chromosomes and cells in all organ- isms. In some animals, for example, certain chromosomes behave 28 I The Process of Evolution with remarkable autonomy. Sex chromosomes may appear as pre- cocious or tardy in comparison with the autosomes. Speciahzed chromosomes may be confined to the germ hne and become ehmi- nated in later divisions. In the fungus gnat Sciara a monopolar spindle is formed at one division, and one group of chromosomes moves to the "nonpolar" end. Occasionally in animals the nuclear envelope does not disappear, and chromosome division takes place within the membrane, which eventually is pinched in two. As more work is carried out on the little-known invertebrates, algae, and fungi, other examples of unusual behavior undoubtedly will be found. Indeed, when proper perspective is reached as a result of systematic study, the higher plant-vertebrate mechanisms may seem unusual. When the chromosomes have reached the poles of the spindle, a new nuclear envelope, which may arise from one of the membrane systems of the cytoplasm, is formed about each group of daughters. During this stage, telophase, animal cells usually divide by furrow- ing and plant cells by cell-plate formation. (A new cell wall parti- tions the old cell.) With this formation of two daughter cells, the process of cell division ends. Thus two cells, each with the same genotype, have been produced as a result of equational division of the chromosomes (mitosis) and division of the cytoplasm, during which the cytoplasmic organelles are apportioned roughly equally. During the course of mitosis, the chromosomes (Fig. 2.3) go through an interesting and important series of changes. If a prophase chromosome is compared with an anaphase chromosome, striking differences are seen. The anaphase chromosome is not only easily visible and stainable but fatter and much shorter. By use of appro- priate treatments, it can be shown that the anaphase chromosome is in the form of a tight spiral, the gyres ( turns of the coil ) of which behave as if they were invested with a stainable substance usually called matrix. The basic thread or chromonema of an anaphase chromosome often can be seen to be coiled in a fine series of gyres called minor coils. The easily seen major coils are imposed upon these by whatever process causes the shortening of the chromo- somes. When there is more than one chromatid, as in metaphase, these may be twisted about one another. Mitosis provides for the equational division of the chromosomes so that, barring a mutational event, each daughter cell receives the same genetic information. In many instances, as cells become spe- cialized in form and function, division of the chromosomes may occur without division of the nucleus or of the cell. The result is cells with more than one nucleus or nuclei with more than the zygotic Units of Replication 29 Fig. 2.2 I Mitosis. A, early prophase; B, late prophase; C, prometaphase; D, metaphase; E, early anaphase; F, early telophase; G, late telophase; H, daughter nuclei. (Adapted from De Robertis, Nowinski, and Saez, 1954, General Cytology, 2d ed., W. B. Saunders Company.) H G t y^ \ B C V 30 I The Process of Evolution Fig. 2.3. I Spiralization cycle of the chromosomes. A, interphase; B, C, D, prophase changes, appearance of matrix; £, prometaphase, chroma- tids visibly double; F, metaphase; G, anaphase; H, telophase changes beginning in daughter chromosomes, disappearance of matrix. {Adapted from De Robert is, Nowinski, and Saez, 1954, General Cytology, 2d ed., W. B. Saunders Company. ) B D E G H Units of Replication 31 number of chromosomes. The latter appears to be the more com- mon; it is referred to as endopolyploidy. In some organisms (e.g., insects) each tissue has its own characteristic degree of polyploiciv. In the salivary glands of water striders (Gerris) the highly special- ized cells may be 2,048-ploid. Tissues of other organisms contain a population of cells of varying degrees of ploidy (usually with a norm at a level above diploidy, e.g., at the tetraploid or octaploid level). This is the situation in the human Hver, for example, or in tissues in the roots and stems of the flowering plants. The significance of the phenomenon of endopolyploidy is not well understood. Certainly it correlates with secretory activities of the cells, and it may play a role in development as well as in differentia- tion. It should be emphasized, however, that no qualitative changes in the genetic material have been demonstrated. Germ-line cells in animals and cells producing spores in plants usually do not become endopolyploid. Reproductive cells therefore retain the zygotic or gametic number of chromosomes, while somatic cells may experi- ence successive increases. If vegetative reproduction in plants takes place by means of budding or suckering from somatic tissue, off- spring with increased chromosome number may arise. Such situa- tions are discussed in Chap. 9. CELL DIVISION: M E I S I S In addition to these mechanisms of preserving the existing chromo- some number or of increasing it, organisms obviously must have a mechanism for reducing it. Mechanisms for reducing high endopoly- ploid numbers are poorly understood, but they have been reported in insects (Ciilex) and plants { Allium). The great majority of organ- isms share the mechanism for reducing the zygotic number of chro- mosomes to the gametic number; this mechanism is called meiosis (Fig. 2.4). Meiosis occurs in tissues that have not undergone endo- polyploidy, such as germ-line cells in animals or sporogenous tissue in plants. Again, as with mitosis, its outlines are the same in all organisms, although in animals it results in gametes and in most plants it results in spores. A cell about to undergo meiosis may be called a meiocyte. The results of meiosis are nearly always four daughter cells with half the number of chromosomes of the meiocyte. In the formation of eggs in animals and in the production of mega- spores in plants, three of these may be much smaller and e\entually disappear. Meiosis achieves these results by two cell diN'isions but only one division of the chromosomes. The following description refers to the 32 I The Process of Evolution generally observed behavior of the chromosomes as seen with a light microscope. ( Variations or exceptions have been noted in some organisms or with special techniques.) When a cell becomes a meiocyte, it usually enlarges somewhat, and its nucleus stains more faintly than before. When the chromosomes become visible in pro- phase, they often can be seen to be single strands instead of being double-stranded, as in mitosis. Their subsequent behavior is so complicated that the first meiotic prophase is prolonged in time and has been divided into a series of substages, the names of which need not concern us here. The first occurrence is synapsis of the chromo- somes ( present, of course, in pairs ) with their homologues, precisely point for point along their length. After pairs or bivalents have been formed, the chromosomes then appear double-stranded. (It will be remembered that, in mitosis, prophase begins with the chromosomes double-stranded.) Each bivalent thus comprises four chromatids, two of the chromosome that arrived in the maternal gamete and two paternal chromatids. Any chromosome that does not have a homologue, a sex chromosome for example, remains as a univalent but undergoes doubling at about the same time as the others. Apparently at about the time the chromosomes double, the slender chromatids break and rejoin in the bivalents. Intimately associated, coiled, and twisted, they often reunite in nonsister combinations; that is, instead of sister chromatids rejoining, maternal and paternal chromatids may be connected following a break. This is the phe- nomenon of cytological crossing-over. In some organisms crossing- over does not occur in one sex, e.g., male Dwsophilo and Callimantis. When the chromosomes have become double, they behave as if they now repel one another. Bivalents become widely spaced in the nucleus, and members of bivalents are held together only where crossing-over has occurred. (If crossing-over has not occurred, the chromosomes in a bivalent frequently separate at this stage. ) As a result of the repulsion ( this term is used only descriptively ) of the chromosome arms, the bivalent assumes forms that depend upon the number and position of crossovers; the latter now become visible as chiasmata or cross-shaped configurations. At this stage of meiosis a chiasma indicates a crossover. Subse- quently, as the chromosomes coil and shorten and become more stainable, the chiasmata (but not the points of crossing-over) are pushed to the ends of the chromosomes. This process, known as terminalization (Fig. 2.5), also produces characteristic configura- tions of bivalents, adjoining loops lying at right angles. At the end of the first meiotic prophase, the nucleus contains the gametic number of bivalents ( plus any univalents that are present ) . At the first metaphase a spindle is formed, presumably precisely as Units of Replication 33 Fig. 2.4 I Meiosis. A, single-strand stage; B, pairing to form bivalents; C, four-strand stage; D, opening of bivalents to show chiasmata; E, first anaphase, disjunction of bivalents without centromere division; F, daughter nuclei with crossover chromatids; G, second anaphase, centromeres divide; H, four haploid daughter cells. (Adapted from De Robcrtis, Nowinski, and Saez, 1954, General Cytology, 2d ed., W . B. Saunders Company.) B D \n'; H 34 I The Process of Evolution in mitosis, and the bivalents become arranged on its equator. Dur- ing first anaphase, instead of the centromeres dividing as in mitosis, the two centromeres of a bivalent move to opposite poles. Thus the chromosomes do not divide (for a chromosome is defined by its centromere) but disjoin, and disjunction results in two daughter nuclei that undergo the usual telophase transformation (or the latter may be much abbreviated). The centromeres of univalents similarly do not divide, a univalent going to one pole or the other. The distribution of maternal and paternal chromosomes is com- pletely at random. During the second division of meiosis, the behavior of the chro- mosomes is like that in mitosis, the difference being that crossing- over has taken place so that the chromatids attached to a centromere are not identical. This second division, in which the centromeres divide, results in the formation of the four daughters of the meiocyte. Each has the gametic chromosome number, but the chances of one daughter being genetically like any other are extremely small. Ma- ternal and paternal chromosomes have been segregated at random, chromatids have been segregated at random, and finally, as the result of crossing-over, the genetic material in the parental genomes has been partially exchanged. Fig. 2.5 I Terminalization of chiasmata. From left to right, chiasmata move to ends of ehromosomes. ( Note that the point of crossing-over does not change. ) Far left, cross section of bivalent. Far right, rotation of chromosomes has occurred. {Adapted from De Robertis, Nowinski, and Saez, 1954, General Cytology, 2d cd., W. B. Saunders Company.) o o Units of Replication 35 This then is the tremendous significance of mciosis and crossing- over. It provides a continual reshuffling of the genetic material in reproduction. New gene combinations are continually being pro- duced, and essentially random union of gametes makes it unlikely that any two individuals will have the same genetic make-up. This cytological mechanism of the organism is part of its genetic svstcm, the system that determines the amount of recombination a popula- tion will produce and that will be available for the operation of selection. The organisms most familiar to us are diploid, sexual out- crossing organisms such as cats and dogs, oaks and pines. In later chapters (8,9) other genetic systems will be discussed as examples of the ways in which the amount of recombination produced bv this familiar genetic system may be modified (usually decreased, per- haps to zero ) . SUMMARY The cell is a metabolic unit composed of large and small molecules associated in specific ways, commonly as membrane svstems, to form subunits or organelles of specialized function. The nucleus of the cell initiates and controls protein synthesis through the function- ing of its chromosomes. When somatic cells divide, the cvtoplasmic organelles are apportioned between the daughters in roughlv equal quantitv. By means of mitosis, the chromosomes are di\ided equa- tionallv between the daughters. Meiosis reduces the ninnber of chro- mosomes in cells that will produce gametes. In the first division of this two-stage process, homologous chromosomes first synapse and then disjoin without division of their centromere. Cytological cross- ing-over takes place during the first division, and, when the centro- meres divide in the second division, four daughter cells with re- combined chromosomes and chromosome segments result. Meiosis provides the recombination that results in the variation upon which selection acts. REFERENCES Swanson, C. P. 1957. Cytology and Cytogenetics. Prentice-Hall, Engle- wood Cliffs, N.f. A rather detailed and excellent discussion of c\to!ogy and its bearing on genetics and evolution. The author's introductory The Cell (1960. Prentice-Hall, Englewood Cliffs, N.J.) provides an elementary description. Wilson, G. B., and J. H. Morrison. 1961. Cytology. Reinhold, New York. An excellent modern cytology text which relates structure and cell physiology. The illustrations are generally first-rate. 3 genetics It seems reasonable to assume that the selective forces invoKed in the evolution of the genetic mechanism of early organisms must have been concerned with stabilization of what was at first an almost infinitely variable system. The mechanisms for replicating genetic material generally ensure that it will b'> exacth' duplicated and that, in the offspring, proteins similar to those of the parents will develop. In more complex multicellular organisms, self-rcgulatorv developmental mechanisms are combined with the nuclear and extranuclear genetic material; together they provide a svstem that usually results in what is thought of as a normal, functioning, wild- type organism. As stated before, the basic phenomenon of genetics is that "like begets like." VARIATION AND MENDELIAN GENETICS Nevertheless, errors in replication occur; they result in the variability that permits selection. In general, analysis of the nature and trans- mission of variability, from generation to generation, is the onlv means of studying the mechanism of inheritance. If the patterns of variation in organisms are examined, it will be seen that some organ- isms appear to be more variable than others. Furthermore, the type of variation pattern diflFers with respect to organisms and the traits studied. In some instances, variation occurs in discrete steps and may be termed discontinuous. In other cases, variation appears to be continuous, individual organisms not falling into easily character- ized discrete classes. Galton attempted to study continuous variation when he made his classic investigations of the inheritance of intel- ligence and other traits in human beings. Other workers, even before and including Linnaeus, had studied continuous variation by mak- ing crosses between varying plants and animals. The science of genetics was not really born, however, until the inheritance of char- acteristics that varied discontinuously was studied. Organisms hav- ing these characteristics could be classified as one or another of a very few distinct types. By observing the distribution in these classes of offspring of an experimental cross of parents with different char- acteristics, Mendel was able to describe the basic rules of behavior of nuclear hereditary units. The importance of Mendel's studies was not generally appreci- ated; indeed, Mendel was urged to suppress his results by other scientists who felt that he was considerably off the beaten jxith of scientific research. In 1900 Mendel's papers were discovered by three 37 38 I The Process of Evolution biologists who recognized their significance. Almost overnight, genetics became an important and rapidly developing field of biol- ogy. However, there were many scientists who felt that Mendel's work had little application to evolution in populations in nature or to the prevailing type of continuous variation found in wild and domesticated populations of both plants and animals. It was only after many years of work that the evolutionary significance of Men- del's laws was established. THE UNITS OF HEREDITY The units of heredity postulated by Mendel and subsequently termed genes were identified as specific regions of the chromosome; they are the segment between two closest points of crossing-over. More re- cent work, especially in the biochemical genetics of microorganisms, has led to other definitions of the gene. For example, it is generally accepted that specific genes control the formation of specific en- zymes. By growing microorganisms on media of known composition, it is possible to show that gene mutation results in the loss of ability to carry out some cellular reaction. For example, a mutant bacterium may lose the ability to synthesize a particular substance, such as tryptophan. The number of mutant alleles of genes has been found usually to be quite large. It is necessary to think of the gene, as re- vealed by these studies, as a region of the chromosome that is muta- tionally complex. Benzer has referred to the possible mutational sites within a gene as mutons. Evidence suggests that a single muton may consist of only one or a very few nucleotides. At this level of study, experiments have shown that recombination may occur within the limits of a single gene, i.e., within a functional unit. The smallest unit that is interchangeable is called a recon. Recombination studies show that the recon also is about the size of a nucleotide and that the mutons in the functional unit are arranged in linear fashion. Since it appears to make no diflFerence pheno- typically how the genes are arranged in an organism heterozygous for two factors, i.e., whether the genes are arranged ~^ or — ^, it is interesting to ask the same question about parts of genes. Will enzyme synthesis take place just as well when the mutants within one gene are distributed between the two chromosomes (the so- called trans state) as when the mutants are on one chromosome (the cis state)? The answer is not clear, but occasionally parts of the gene may be divided between the homologous chromosomes, al- though the smallest group of mutants that must be in the cis posi- Genetics 39 tion is rather large. These examples of complementation between mutants are somewhat equivocal, however. The cis-trans test may be used to specify the functional gene units, which have been called cistrons. In the study of populations, the unit of heredity must be given a strictly operational meaning within the context of the study, as, in fact, it must be in all biology. In evolutionary studies this unit can- not be the same as that in biochemical genetics. This must be kept clearly in mind when discussions of genes in an evolutionary con- text are compared with those based upon studies of microorgan- isms in the laboratory. In studying inheritance in populations in nature, the unit of heredity in most cases becomes a statistical one, for the factors controlling the expression of continuously varying traits are numerous and complexly interrelated. At present, only the methods of the statistician can sort out the interactions of the he- redity units (which are assumed to be similar to those affecting dis- continuous variation), the developmental systems through which they are expressed, and the effects of the environment on both these systems. The environment of an organism at any particular time or place is unique and not repeated or repeatable. This means that in experimental studies it is important to make replicate experiments in space and time— an unfortunately expensive and time-consuming process. In the following pages the basic facts of the inheritance of discontinuous and continuous variation will be summarized, to- gether with a discussion of those aspects of gene beha\ior that are particularly important to evolutionary studies. MENDEL'S LAWS The basic rules of heredity deduced by Mendel are familiar to any- one who has had an elementary course in biology. In crossing peas, Mendel found that, when differing parents were mated, the first generation offspring (Fi) resembled one or the other parent. The trait expressed in the Fi he referred to as dominant to that which did not appear (the recessive). Crossing the essentially uniform F, plants to produce a variable F2 generation in which individuals with the recessive trait appeared, showed that the factors responsible for the appearance of the traits were not lost but merely hidden. By a study of the types of progeny in the Fi and Fo, Mendel deduced that each offspring contains two homologous factors, one received from each parent, affecting the expression of the traits studied. .\n Fi offspring from a cross between differing parents must contain two different but homologous factors, one for the dominant trait and 40 I The Process of Evolution one for the recessive. (In other words, the Fi is heterozygous.) In the formation of the F2, these factors are segregated, and the off- spring are produced in the approximate ratio of three with the dominant trait to one with the recessive. The individual showing the recessive condition is homozygous for the factors. Further crossing (inckiding backcrosses to the parental types) shows that, of the three with the dominant trait, one will have like factors, and the other two, difiFerent factors, as in the Fi individuals. When parents differing in, and homozygous for, several characters were crossed, Mendel found that the factors for the different traits he was studying behaved independently. In the Fi both dominant traits were observed, and in the Fo each trait segregated by a 3:1 ratio. If the homozygous parents differ in two traits, for example, the proportion of types in the Fo is the square of a 3:1 ratio or 9:3:3:1. By backcrossing offspring to the parental types, verification of the number of factors and their independence may be obtained. It is clear that the behavior of these factors parallels the behavior of the chromosomes now known to bear them. The factors affecting the traits in peas studied by Mendel were on nonhomologous differ- ent chromosomes. Later studies showed that factors on the same chromosome were linked (tending to occur together more fre- quently than would be expected if assortment were independent). When numerous traits are studied, their factors are found to fall into as many linkage groups as the haploid number of chromosomes. Within a linkage group the amount of recombination varies from a very low percentage for genes close together to 50 percent for genes far apart (which genetically is indistinguishable from independence —occurrence on different chromosomes). RECOMBINATION The cause of genetic recombination of linked genes is cytological crossing-over in meiosis (diagrammed in the previous chapter). In studies of inheritance at a gross level, the factor presumably affect- ing a particular characteristic is the minimum distance between two points of crossing-over. (This is the operational definition of a gene in this instance.) Crossing-over occurs in all organisms in which meiosis and sexual recombination have been found. The basic mech- anism appears to be the same wherever it occurs, and some workers have postulated that meiosis cannot properly take place in the ab- sence of crossing-over (or a specialized substitute). The precise mechanism of cytological crossing-over is not known. Presumably when the chromosomes are synapsed and twisted to- Genetics 41 gether in the first meiotic prophase, the chromatids break and the broken ends subsequently join. If nonsister chromatids are joined, then crossing-over has occurred. Crossing-over does not occur with' equal frequency along the length of the chromosome. It is rare or absent near the centromere and at the very ends of the chromosomes. Near large blocks of heterochromatic material, crossing-over also is reduced. In some organisms, crossing-over is effectively restricted to certain parts of the chromosomes; in others, it seems to occur rather evenly throughout the length of the chromosome arm. Per- haps the diflFerence lies, in part at least, in the amount and distribu- tion of heterochromatin. Occasionally crossing-over is suppressed entirely, as in male Drosophila and female silkworms (Bomhyx mori). The occurrence of a crossover also interferes with the forma- tion of a crossover immediately adjacent. Interference may be meas- ured by studying linkage, and it can be shown to vary along the chromosome and between diflFerent chromosomes. Probably inter- ference is also a structural phenomenon. If two factors are located some distance apart on the chromosome, it is possible that more than one crossover will occur between them. If double crossing-over takes place, the effects on recombination depend upon which chromatids are involved (Fig. 3.1). If the same two chromatids of the four associated in the bivalent are involved in both crossovers, the occurrence will not be detected unless a third factor located between the original two is also observed. Should the other two chromatids experience the second crossing-over, each chromatid will have one crossover. This crossing-over may be re- ferred to as two-strand and four-strand exchange, respectively. Three- strand exchange results in the formation of a noncrossover chromatid and three chromatids with a single crossover. In general with mul- tiple crossing-over between two factors, the resulting chromosomes with an even number of crossovers and those with no crossovers will appear as parental types. Chromosomes with an odd number of crossovers between the factors in question will be recombinant types. Since the number of chromosomes with recombinations equals the number with no or an even number of crossovers, recombination cannot exceed 50 percent. Crossing-over is influenced by environmental factors, such as temperature, and is also under genetic control. This genetic-control mechanism is rather complex and not well understood. Apparentlv genes determine the length of time available for synapsis (which affects crossing-over) and the localization of crossovers; there are genes that have the effect of preventing synapsis altogether. Genetic disturbances of other components of meiosis would be expected also 42 I The Process of Evolution to affect crossing-over. In evolutionary terms, it would appear that these factors directly influencing crossing-over per se have not been important in the control of recombination. When the amount of re- combination is reduced in certain organisms, presumably by the operation of natural selection, other mechanisms usually are in- volved, such as inversion, translocation, or elimination of meiosis altogether (in apomictic organisms; see Chap. 9). THE EXPRESSION OF GENES The action of genes in an individual organism has proved to be quite variable. Genes in a population are rarely found in only two alterna- tive states or alleles. Rather, a system of multiple alleles appears to be the common system governing most characteristics, e.g., blood groups in animals, incompatibility systems in plants, coat color in mammals, flower color in plants, etc. Interaction among the genes in the genotype or of the developmental pathways resulting in the phenotype produces complex genetic ratios. For example, where pigment systems leading to the formation of a particular color are involved, several genes may control different steps in the elaboration of the pigment. If any one is missing, color is lacking. Such genes are called complementary genes. On the other hand, there are many situations where the expression of the gene at one locus masks the expression of another gene. This is known as epistasis; the epistatic gene masks or prevents the ex- pression of a hypostatic gene. In chickens, for example, the Leg- horn white color is epistatic to many genes affecting color and pattern. If it is present, no color but white will be expressed. It is obvious that complementary effects and epistasis, as well as other sorts of modified expression, are related developmentally. A similar phenomenon occurs where there appears to be one gene or a group of genes, each with relatively small effect, that operate to alter the action of a gene with major effect. These "minor" genes are known as modifying factors. It often becomes necessary to specify the type of expression of genes because, for many factors, this is variable. For certain factors, all or almost all individuals with the same genotype develop a characteristic phenotype that distinguishes them from individuals with other genotypes in a certain range of environments. The genes in such cases are said to have high penetrance, since most indi- viduals carrying the gene possess the trait. Other genes do not always produce a detectable phenotypic effect in the individuals that carry them in a given environment. These are genes of low Genetics 43 Fig. 3.1 I Different results in double crossing-over. A, two-strand exchange; B, four-strand exchange; C, D, three-strand exchange. (Adapted from De Robertis, Nowiruski, and Saez, 1954, General Cytology, 2d ed., W. B. Saunders Company; ami White, 1954, Animal Cytology and Evolution, 2d cd., Cambridge University Press.) (a) ib) (c) id) 44 I The Process of Evolution penetrance. It is possible also that the phenotypic expression of a gene may be variable even though it is completely penetrant. If it is relatively uniform in an essentially "normal" environment, the gene has constant expressivity, but if there is interindividual varia- tion in the trait, expressivity is variable. Studies of the manifold or pleiotropic action of genes, as well as of systems of genes controlling the expression of particular character- istics, have suggested that probably no character of an organism is controlled by only one gene, and, conversely, every gene in the genotype of an organism affects a great many ( if not all ) characters in the complex process of producing the phenotype. But it seems likely that the amino acid sequence of a specific polypeptide is de- termined by one and only one gene. MUTATION It is often possible in controlled crosses to identify specific genes affecting particular characters that show discontinuous variation. The most obvious characteristic of such major genes is that they change or mutate— indeed, that is the only way in which their exist- ence may be detected. Mutations occur spontaneously at varying rates; they also may be induced by treatment of the organisms with ionizing radiation, ultraviolet light, or various chemicals. These treatments appear to affect the DNA more or less specifically. The discussion of spontaneous mutation rate is difficult because there are several ways in which the rate may be expressed. As far as is known, the mutational event is random; it is not possible to specify what gene will be affected or to assign the cause of a given mutation to a specific mutagenic agent. It would be desirable to know the chance of occurrence of a mutation per cell per division, which expresses change with respect to time. This is very difficult to determine in other than microorganisms. Even with bacteria, what is measured is phenotypic change which may involve more than one gene or mu- table unit. Rates of from 10"" to 10 ~^ have been measured in micro- organisms. In multicellular organisms, rates must generally be measured dif- ferently (except where tissue culture is possible) since the criterion available is the number of gametes producing mutant individuals per generation of the organism. Thus individuals, not gametes, are counted. In the gonads a mutation occurring in a gamete-producing cell may have many or few mutant daughters, depending upon when in gametogenesis the mutation took place. The mutation rate per generation varies with the gene studied but averages about 10~'^. Genetics 45 When rates per cell division are measured by tissue-culture studies of bone marrow cells, they average about lO"*'. Mutation rate appears to be under genetic control and is therefore subject to change in the course of evolution. Genes whose major effect seems to be affecting the mutation rate of other genes are known. One would expect that in most organisms selection would have resulted in the genotype that maintains an optimum level of mutations in the population. This is difficult to study, and few data are available. The problem is closely connected with that concerning the effects of heterozygosity in developmental buffering or homeo- stasis and in genetic homeostasis and is discussed in later chapters. EVOLUTION OF DOMINANCE There are several other interesting aspects of mutation about which little is known. Most mutant alleles occurring in the organisms that have been studied in detail are recessive to the wild-type gene. This raises the problem of how dominance-recessiveness arose. It is clear that most mutations that take place will be deleterious to the organ- ism, since they alter a functional system. If they have a major effect, almost certainly the complexly interrelated developmental pathways will be grossly upset and the organism will die. Even if the gene has only a relatively minor effect, however, the integration of the geno- type is such that the mutant organism will be less fit than its parents, provided that there is no environmental change. (The chances of improving the operation of a radio receiver by making a small ran- dom change in its circuits are slim indeed.) Recessive mutations, when they occur in a diploid organism, are stored in the organism's reservoir of variability. When they are (rarely) combined in the homozygous state, they will be eliminated unless the environment (in the broadest sense) has changed sufficiently to give them posi- tive selective value. How then does recessiveness arise? There are several hypotheses among which it is difficult to discriminate at present, although all probably have elements of truth. Fisher has suggested that muta- tions will not necessarily be completely recessive on their first occur- rence. They will thus, in general, be disadvantageous unless other modifying genes at different loci reduce the deleterious effects of their expression. Since the homozygous condition for the mutation will rarely occur, selection will operate on the more common hctero- zygotes to build up systems of modifiers that will result in the hctero- zygote resembling the homozygous dominant. Wright and Haldane have discussed the problem of the origin of 46 I The Process of Evolution dominance in terms of the relation between the gene-produced enzyme and its substrate. In their view, recessive genes are those which are less active than the wild type in the production of a par- ticular enzyme. Selection presumably will have built in a safety factor so that there is an excess of enzyme over substrate, and a mu- tation reducing the amount of enzyme will have little effect in the heterozygote. Biochemical genetics is supplying answers to the ques- tions concerning the quantitative aspects of gene function in enzyme synthesis. Wright and Haldane have also suggested that the selective value for the modifying factors might be so low that dominance would arise too slowly for it to have a large chance of appearance when other factors are taken into consideration. In some organisms there is evidence that the selective coefficients for modifying genes may be considerably higher than those postulated by Wright and Fisher. In either event, selection has played an important role in the evolu- tion of the behavior of genes. Hybridization experiments in organ- isms as diverse as cotton plants and butterflies have clearly shown that the functioning of a gene may change when it is moved from one genetic background to another. Thus selection altering the back- ground (e.g., "modifiers") can affect the expression of a given gene. There is considerable evidence that this is exactly what has happened during the development of industrial melanism in the moth Bistort bettilaria ( see Chap. 7 ) . Early samples of heterozygotes for the melanic allele were quite distinct from the homozygous melanics. By the middle of the twentieth century, the heterozygotes were almost identical to these homozygotes. Clearly, dominance has evolved in this case. CHROMOSOMAL MECHANISMS The existence of means of artificially inducing mutations in easily grown organisms with relatively short life cycles ( such as Drosophilu, Zeo, and Neiirospora) has led to careful studies of linkage and the linkage groups or chromosomes. If it is assumed that the amount of crossing-over between two factors is proportional to the distance between them, then the spacing and arrangement of the genes along the chromosome can be determined. A genetic-linkage chromosome map, based upon recombination data, can be made. In such work it must be kept in mind that there may be interference between ad- jacent crossovers; that, with factors that are far apart, multiple cross- ing-over may occur between them; and that only parts of the chro- mosome with easily studied major phenotypic effect can be mapped. Genetics 47 There are other means of producing chromosomal maps, liowever, and these give a check on the method. For example, it is possible to map chromosomes by studying the effects of induced deletions of small portions of the chromosome and of chromosomal changes such as inversions and translocations, as well as other techniques. Tliese have confirmed the linear order of the genes mapped by crossoNcr studies, but these maps vary from genetic maps, often strikinglv, iu spacing and other details. Regions of the chromosome that are heterochromatic and that seem to lack genes with major effect are not easily studied by the recombination analysis; in the main these are responsible for the differences. Progress has been made in local- izing specific genetic effects at visible regions of the chromosomes in Drosophila (with its giant polytene salivary-gland chromosomes) and in Zca (where the chromosomes have characteristic chromo- meres visible under the microscope). Sex Chromosomes A specialized sort of linkage occurs in animals and plants with differ- entiation of sexes. In these organisms where there are special sex chromosomes, as opposed to the other chromosomes (known as autosomes), one sex usually has two homologous sex chromosomes. The other sex has only one chromosome homologous with these and either lacks the second or has another only partiallv homologous chromosome. In Drosophila and man the female is the homogametic sex with two X chromosomes (everv gamete contains an X), while the male is heterogametic with an X chromosome and its partial homologue Y (there are two kinds of gametes). It is clear that the transmission of genes that are located on the sex chromosomes will be different from those on the autosomes. Furthermore, the charac- teristics affected by these genes will show genetic linkage with sex. Sex chromosomes in these organisms differ from the autosomes in that they are specialized into two different regions. A portion of the two different sex chromosomes in the heterogametic sex will synapse and crossing-over may occur. In contrast to these pairing segments, there are the differential segments of the X and Y that do not pair. The differential segment of the Y usually contains few if any genes with detectable effect, and it is heterochromatic and smaller than the differential segment of the X. When it does carry genes, they are passed from father to son and females never show the traits in- volved. The heterochromatic portion of the Y is necessary for fer- tility; therefore it cannot be completely without effect. In some groups the female is the heterogametic sex, while the male 48 I The Process of Evolution is homogametic. In many species the Y chromosome is lacking and the male is then designated as XO. More complicated sex-chromo- some mechanisms have evolved; these are discussed in Chap. 9. For example, there are many sex chromosomes in some organisms. It should be emphasized that the precise mechanism of sex deter- mination varies from group to group, even though the chromosome condition may appear the same. The evolution of sexuality as an aspect of the storage of variability and its release through genetic recombination is discussed in Chap. 8. Alterations of the Chromosomes In addition to the mutations discussed above (called gene or point mutations), changes in the structure of the chromosomes take place spontaneously and may be induced by the same agents that cause gene mutation. Although these chromosomal alterations are sometimes termed mutations, it is perhaps less confusing to restrict the term mutation to gene changes. Chromosomal alterations also are frequently referred to as aberrations or abnormalities. This is because they are compared with an arbitrarily selected standard chromosomal phenotype (usually the wild type); they should not be taken to represent some unusual or deleterious phenomenon that is inevitably disadvantageous to the organism. As with gene muta- tions, chromosomal alterations usually have low or negative selective value when they appear but may become stabilized in the population or replace the standard type if their selective value increases as a result of environmental change (if the nucleus and the genes are included as part of the environment ) . Practically any accident that can be imagined as happening to the chromosomes, during the course of cellular life and division, has been found in laboratory organisms and organisms from the field. Often such modification of behavior can be shown to have become established as a regular feature of particular organisms. A simple classification of chromosomal alterations would include deletions (loss of a segment), duplications (repeat of a segment in a con- tiguous or remote portion of the karyotype ) , inversions ( reversal of a segment), and translocations (transfer of a portion of a chromo- some to a nonhomologous chromosome, usually reciprocally). Changes of chromosome number, often thought of as "chromosomal mutation, " are discussed in Chap. 9. Deletions and Duplications. The role in evolution of loss or addi- tion of chromosome material is very poorly understood. When homo- zygous, deletions usually are lethal. They are a useful tool for map- Genetics 49 ping chromosomes but of unknown significance in populations. Duplications are of importance because they represent a possible cytogenetic mechanism for an increase in the total amount of genetic material. How the total amount of genetic material has changed in the course of the evolution of life is not known. Nor is it possible at present to decide whether it is necessary that the amount of genetic material increase concomitantly with the increase in complexity that has taken place. Inversions and Translocations. Inversions and reciprocal trans- locations are more conspicuous and better-understood chromosomal alterations, and their effects are well known. Organisms in which these changes have become a regular feature of the genetic system are discussed in Chap. 9. Here only the cytogenetic aspects of such changes are considered. Unless pairing behavior is studied, an in- verted region of a chromosome ordinarily cannot be detected visu- ally except in organisms with polytene chromosomes or conspicuous chromomeres. During the first meiotic prophase, when homologous chromosomes pair, heterozygosity for an inversion is revealed bv the fact that one chromosome must twist in order for svnapsis to be accomplished (Fig. 3.2). This characteristic inversion loop is also seen in the salivary-gland chromosomes of DrosophiJu and other Diptera where the polvtene chromosomes are somatically paired. If the inversion is a short one, crossing-over may not take place within the inversion loop. If the inversion is sufficiently long and has the proper relation to the centromere, crossing-over will occur and the result will be the formation of a dicentric chromosome and an acentric fragment. The acentric fragment will not behave properly on the spindle at anaphase, and the dicentric fragment will be broken. Gametes that are formed will include, in addition to bal- anced ones, those in which whole chromosome regions are lacking, and these will be nonfunctional. Looking at the genetic results, it will appear as though crossing- over had been suppressed. However, only the products of crossing- over have been lost. The number of crossovers within the inversion loop and their distribution among the chromatids of the bi\alent determine what effects there will be. Because the genes within an inversion loop are effectively linked in a heterozygote under certain conditions, inversions are discussed in more detail in relation to re- combination in Chap. 9. Organisms that are homozygous for a chromosomal inversion can be recognized as such only by matmg them with a different type and then observing the pairing beha\ior of the chromosomes in the hybrid (except in organisms with polv- tene chromosomes, where they can be detected by careful examina- 50 I The Process of Evolution tion of the banding patterns). Since synapsis will be unafiFected in the homozygote, recombination will not be reduced, but the linear arrangement and linkage relations of the genes will, of course, be changed. Reciprocal translocations involve nonhomologous chromosomes. Here a portion of one chromosome is transferred to another, and vice versa, so that linkage groups are changed. The size of the seg- ments exchanged may be the same or different, large or small. Sometimes one chromosome will exchange a heterochromatic region (with little specific genetic activity) for a euchromatic region (with typical genetic behavior). Heterozygosity for large reciprocal translocations is visible in meiosis, as well as in organisms with somatically paired polytene chromosomes. Synapsis will result in the association of four chromosomes, two standard and two with trans- located regions. At metaphase the appearance of this quadrivalent will depend, among other things, upon the distribution of chiasmata. Usually the chromosomes separate to form a ring. Chromosome ends are held together bv terminalized chiasmata. Disjunction may occur so that adjacent centromeres go to the same pole. Examination of Fig. 3.3 will show that there are two different possibilities for this sort of disjunction but that either will lead to the production of un- balanced gametes (i.e., those with duplications and deficiencies). Only if alternate centromeres go to the same pole can balanced gametes result. The fusion of such gametes, randomly, will produce standard homozvgotes, translocation heterozvgotes, and transloca- tion homozygotes in the ratio of 1:2:1. Organisms that are homozv- gous for a reciprocal translocation exhibit no meiotic peculiarities since svnapsis is undisturbed. The linkage groups are changed, how- ever; this can be detected bv genetic analvsis. It is possible for more than one inversion to occur in a nucleus or in a chromosome. Inversions in a chromosome may be in one arm only or may include the centromere. If there are several, they may be independent or overlapping, or one or more may be included within another. Detailed studies of these conditions have been made in Drosophila and are discussed in Chap. 9. With more than one trans- location, the result depends upon which chromosomes are involved. If a different pair exchanges segments after the first translocation, the result will be the formation of two quadrixalents at metaphase of meiosis. If one of the chromosomes experiencing the first transloca- tion exchanges a segment with a third, a ring of three chromosomes is found. Finally, in some organisms, all the chromosomes exchange arms and all are attached in a ring at meiosis. Examples of this are considered in Chap. 9. Genetics 51 Fig. 3.2 Result of crossing-o\er in the loop of a bivalent heterozygous for a paracentric inversion. ,'-xr-'- Fig. 3.3 Result of distal crossing-over and disjunction in an organism heterozvgous for a reciprocal translocation. Chromosomes or chromo- some sesiments that are homologous are indicated by same type of line. J : : I!! 52 I The Process of Evolution CONTINUOUS VARIATION When one comes to study the genetics of continuously varying char- acteristics, the problems become much more difficult. Usually it is not possible to identify specific genes controlling specific traits. As has been mentioned above, there is thought to be a continuous spec- trum of characters ranging from those which vary qualitatively to those which vary quantitatively. Presumably there is no basic differ- ence between genes with easily detected major eflFect (often called oligogenes or switch genes) and those with only minor efiFect indi- vidually but which operate as part of a system of an indefinite num- ber of factors (often called polygenes, although this term has been used in a more restricted sense). Operationally, it may be said that the difference between the two types of characters depends more on the relative importance of the genetic material and the environment in determining the phenotype than upon the size of individual gene effects. However, there are polygenic characters in which the role of the environment is relatively unimportant, such as number of abdominal bristles in Drosophila. In studying variation in a quantitative character, as large a sample as possible of differing individuals is measured. All the individuals are unique but their measurements may be grouped into size classes. When these measurements are plotted as a frequency distribution, the nature of the variation may be studied. For instance, the arith- metic mean or average may be calculated for all the individuals in the sample. The amount of variation in the sample may be estimated by the standard deviation S or its square, the variance S-. It is usually difficult to separate genetic and environmental com- ponents of variation and to study the genetic portion independently. Various laboratory techniques and rather complex mathematical formulations have been developed to study and separate these com- ponents. For example, one way to estimate the size of these two components involves the reduction of the genetic component until it is negligible. The variance in the character measured is observed in a population in a "normal" environment. Then the variance of the same character is measured in individuals of a highly inbred line raised in the same environment. Since these individuals may be con- sidered to be essentially identical genetically, the variance observed may be attributed entirely to the effects of environment. The differ- ence between the two variances is then an estimate of the genetic variance, since total variance (phenotypic variance S^) is, in this Genetics 53 case, equal to genetic variance S^. plus environmental variance S%. Thus S-^- 5-;^= S|. This procedure for estimating the variance components rests on the assumption that the environmental variance is independent of genotype, an assumption that is often incorrect. Even if the genetic variance can be determined, further com- plexities exist. It cannot be assumed for all characters that the effects of genes are additive in a simple cumulative fashion. The genetic variance itself must be broken down into components. There is the additive component representing the differences between the homo- zygotes and heterozygote(s) for each locus. Also to be taken into account are a component resulting from interactions of alleles, i.e., a dominance component, and a component resulting from inter- actions of nonalleles, an epistatic component. In many situations the additive genetic component is the only one that may be estimated conveniently. Then the phenotypic variance is partitioned into addi- tive genetic and remainder variances. The latter is a catchall term for the nonadditive components of the genetic variance plus tlie environmental variance and gene-environment interactions. The proportion of the phenotypic variance attributable to additive genetic effects is known as the heritability {h^ = S-^/Sj,). Some- times heritability is defined in a narrower sense as Sy'S},, where S^ is the additive genetic variance. Heritability is a good estimator of the degree of resemblance between offspring and parent and as such is of great value to the plant and animal breeder. The evolutionist must deal with these complexities since the great majority of traits found to be variable in organisms vary quantita- tively and are under the control of multiple factors. \\'here crosses can be made between races, species, or even genera, the F] offspring generally prove to be more or less intermediate and the Fo show the continuous variation characteristic of polygenic inheritance. (This is an overgeneralization of a complex situation; those wishing further information should consult Falconer, especially the sections on in- breeding depression and heterosis in chap. 14.) Control of a characteristic by many genes provides a stability of phenotypic expression that may not occur when only single genes are involved. For instance, a single mutational event is unlikely to disturb seriously the expression of a character dependent upon, say, the additive effects of 35 loci. However, a single mutational event f -> / in the color-inhibitor gene of an onion will result in a white onion rather than a red or yellow one. In view of the possible drastic effects of changes in "major" genes, it is not surprising that most characteristics of organisms are controlled multigenically. Selection would have favored the development of such systems since they tend 54 I The Process of Evolution to reduce the possible deleterious effects of minor events such as a single base-pair substitution. Polygenic systems that express relatively little variability may store tremendous potential variability simply because they have the ability to respond to selection by producing genotypes which, in the absence of selection, would never be produced. Let us suppose that a character is controlled by 40 loci, at each of which there are -|- and — alleles, and that the effects of the genes are additive (e.g., the most extreme phenotypes have all loci homozygous -\ — h or homo- zygous ) . If the gene frequency at each locus were +.50 and — .50, then, in the absence of selection, the probability of a single diploid individual having the extreme + phenotype (being homo- zygous -| — h at each locus ) would be ( V2 ) ^", a number infinitesimally smaller than one divided by the number of electrons in the universe —for all practical purposes, zero. However, this potential could be realized in perhaps 8 or 10 generations by selection favoring indi- viduals with a maximum of -j- alleles. Multiple-factor systems of inheritance provide, then, an important mechanism for maintaining balance between fitness for the immediate environmental situation and flexibility for response to long-range change in the environment. SUMMARY In the majority of organisms, genetic material, DNA, is associated with long protein strands forming chromosomes. The chromosomes are linearly differentiated into functional units called genes, exist- ing in numerous allelic states, which control the formation of spe- cific enzymes. Mutation of genes to diflFerent allelic states occurs spontaneously with a frequency of from 10~''' to 10^*^ per generation. Meiosis and crossing-over result in recombinational units, usually equivalent to the functional genes. Except for chromosome linkage, genes segregate and recombine independently in the zygotes. Intra- allelic interaction or dominance and interallelic interaction or epis- tasis occur. Some characters are aflFected by genes with conspicuous major effect, although modifying factors also may be found. Most characters are controlled by a very large number of nonhomologous genes, each with relatively small effect. Study of the resulting quan- titative variation is complicated by the diflBculty of separating the various fractions of the genetic component of variation from each other and from the environmental component. The basic source of variation is gene mutation. In populations of sexual higher organ- isms, recombination is more important as a source of immediate variability in the short-term analysis. Genetics 55 REFERENCES Falconer, D. S. 1960. Introduction to Quantitative Genetics. Ronald, New York. A clearly written modern text dealing with both (juantitative and population genetics. Sager, R., and F. J. Ryan. 1961. Cell Heredity. Wiley, New York. Excel- lent for biochemical genetics but making no attempt to integrate this subject with the rest of biology. Sinnott, E. W., L. C. Dunn, and T. Dobzhansky. 1958. Principles of Genetics. McGraw-Hill, New York. A fine general text. For develop- ments in biochemical genetics since this book was published, the pre- ceding source and issues of The Scientific American may be consulted. development The genetic mechanisms described in Chap. 3 presumably evolved because they preserved successful combinations of genetic material. Some protoorganisms may merely have continued growth until acci- dents led to their disintegration. Many may have died because changing surface-volume relationships disrupted their inefficient internal organization. Some may have fragmented into smaller en- tities, with chance alone determining whether the offspring frag- ments would have the organization to continue growth. Any mech- anisms arising by chance that would tend to ensure that subsequent fragmentation products retained the capacity for growth (and further successful fragmentation) would automatically be perpetu- ated. Thus evolved the mechanisms that led to a stabilization of the marked variation which must have occurred in early division and development. The origin of these mechanisms is, in a sense, the basic problem in the origin of living systems, as has been discussed in Chap. 1. It has been facetiously suggested that human beings are merely one means that DNA has evolved for making more DNA; it may also be said that DNA is merely one device used by human beings to keep from having nonhuman offspring. Genetic material does not replicate without other components of living systems. The course of evolution has involved increasingly complex systems, in- cluding the genetic one. GROWTH AND HOMEOSTASIS Presumably the earliest organisms were unicellular (or noncellular). In such organisms only one or two cell divisions (and possibly one fusion of cells ) produce separate functioning entities. Here the dis- tinction between heredity and development or differentiation that we are accustomed to draw for multicellular organisms is often difficult to make. Each cellular component is a hereditary unit that is replicated with greater or lesser accuracy during cell multiplica- tion. With the development of multicellularity and increased com- plexity, other problems arise. There is eventually a separation of germ-line cells and somatic cells. Nuclear and nonnuclear replicable components of cells that were present in unicellular organisms now appear to diverge somewhat in function. Greater stabilization and control are characteristic of the nuclear material, which we think of as the genetic information (genotype). The nonnuclear material plays a major role in development and differentiation, changmg 57 58 I The Process of Evolution its properties through time and interacting with the nuclear mate- rial and the environment (including other cells). A unicellular or- ganism is, in a way, immortal. The "end" of a cell usually is the result of accidental destruction (including the result of predation) or the division of one cell into two. Death comes eventually to the somatic cells of multicellular creatures and may be considered a part of the genetic system. Increase of size, or growth, is inherent in the idea of continuing reproduction. The term development refers to the changes that take place during the life of an organism. Simple changes in surface- volume relationships, which may have constituted development in a protoorganism, seem a far cry from the life cycle of a monarch butterfly (egg-larva-pupa-adult), but the difference is one of degree, not of kind. Organisms change size in growth, and what is a working design at one size may be completely nonfunctional at another. Given the physical limitations of the size of mammalian cells (imposed by such factors, among many others, as the size of protein molecules and rates of diffusion), it is easy to see that a perfect miniature human the size of an ovum, or a sperm cell 6 feet long, would be impossible. A genetic mechanism thus does not ensure the production of duplicates of the parental multicellular organism but rather the production of entities that, within certain limits of varia- tion, will develop into replicates of the parental type. This regulation may be termed developmental homeostasis and is closely tied to the concept of "wild type." Most kinds of organisms seem to vary greatly only in rather superficial characteristics. The Drosophila wild type has been extensively described genetically. The human wild type does not have each eye of a different color or six digits on each hand. Redwood trees may vary in height and branch number, but they have characteristic green leaves of distinc- tive arrangement and rough red bark. One is more readily struck with variation in large organisms than in small organisms, but this does not necessarily mean that small organisms are less variable. In all organisms, it seems to be true that critical developmental systems are relatively immune to genetic alteration. It is advantageous for an organism to avoid reproductive waste by producing optimum phenotypes from a number of minor variant genotypes. In many organisms the processes of development of a specific form have be- come canalized, leading to a uniform phenotypic expression of in- dividuals in a given population in spite of the genetic variability among them. With this mechanism, genetic variability (of long- range importance) can be present with a minimum of reduction of fitness. Development 59 How is this buffering accomplished? Using the model of Lerner let us assume that gene A produces substance o, which is modified by the action oi b {a product of gene B ) into r, and that r interacts with c (produced by gene C) to give substance t. ( After Lerner, Genetic Homeostasis, Wiley, New York, 1954. ) Now if mutation removes B or if the environment lacks the substrate from which b can be manufactured, then a will accumulate. A svs- tem in which high levels of a interact with d (product of gene D) to make s, which in turn can be transformed by c into t, is a buffered system. < ^^ ( After Lerner, Genetic Homeostasis, C — > C ^ Wiley, New York, 1954. ) This is a true feedback system, since the exact course leading to normal character expression is determined by the "information" that the organism has with respect to the level of a. This does not mean that all buffering is genie, however. It is common practice to draw a sharp line between genotype and phenotype. As a pedagogic device, this is useful for emphasizing the relative permanence and continuity of the genetic information, and, although an oversimplification, it has led to considerable progress in the science of genetics. However, it has also led to the impression that the genotype is somehow the basic entity and that the pheno- type is merely a crude reflection of the genotype (the image of which has been distorted by the environment). One might well wonder why selection has not done away with the phenotype alto- gether, permitting the genotype to evolve unsullied. The answer is, of course, that what can be separated in textbooks or in theory cannot be separated in living organisms. If the genetic material were dissected from a fruit fly, one would obtain a long meaningless string of nucleotides, itself an aspect of the "phenotype." It is clear that at this level of study the distinction between genotype and phenotype is meaningless. The genetic information becomes mean- 60 I The Process of Evolution ingful biologically only when it is translated through contact with the environment. Indeed, the value of the information is judged only by the translation, not the original. Natural selection operates on the phenotype, not directly on the genotype, which merely determines the responses of the developing organism to the environment. Only in recent years have evolutionists given proper attention to the processes of development that result in the production of an adult functioning organism from a fertilized egg or zygote. These processes are interrelated to form a system which Waddington has termed the epigenotype. This may be visualized as a branching system of developmental pathways, each of which leads to one of the compo- nents of the adult form. Because the biochemical reactions deter- mining each path are so interlocked with one another ( as discussed above, there is a strong tendency for the normal end result to be produced even when there is considerable disturbance at early stages. Thus the paths are canalized or buffered as a result of feedback or cybernetic mechanisms interconnecting the paths. This epigenetic system must have been the result of natural selec- tion acting upon the genes that afiFect more or less directly the ex- pression of particular characteristics of organisms. However, selec- tion also must have involved the many genes that have as their only obvious phenotypic effect the modification of the expression of other genes. Waddington has pointed out that, in a population of organisms in a given environment, each individual will have its own genotype, and therefore its own epigenotype, which will eventually result in the adult phenotype. Selection to preserve fitness in this particular environment may act to eliminate genotypes that produce deviant phenotypes. It may also act to eliminate individuals that are imperfectly buffered against environmental eflFects. There would thus be selection for a well-canalized epigenetic system. Should the environment change, some well-buffered individuals would be likely to respond by producing fit phenotypes without the necessity for immediate genotypic change. After a period of time in the new environment, however, genotypic change is inevitable, and it is to be expected that selection would lead to the stabilization of the new developmental paths. When the organisms are returned to their original environment, it would be found that, as a result of this change in the genotype, they no longer produce their original pheno- type. Thus what was originally a phenotypic (actually epigenetic) response to environmental change becomes incorporated into the genotype, as a result of selection for a well-buffered developmental Development 61 system in the new environment. What appeared to be an "acquired characteristic" becomes hereditary through the effects of natural selection. This process, known as genetic assimilation, is discussed in Chap. 7. LIFE CYCLES Cyclic growth is characteristic of all organisms. Yeast cells go through sequences of fusion and fission, including both a haplophase and a diplophase (Chap. 3). In most higher plants, development occurs both in the haplophase and in the diplophase, although the haplophase (male and female gametophytes ) is usually much less conspicuous and of shorter duration than the diplophase ( the sporo- phyte). In most animals there is virtually no development in the haplophase, which usually is restricted to the gametes. (Male hy- menopterans, which are haploid, constitute a conspicuous exception; see Chap. 9. ) The simple growth-fragmentation-growth cycle hvpothesized as the most primitive form of development has been altered by selection in diverse ways. As an example of a complex developmental svstem, consider the protozoans that cause malaria. The sporozoites of Plasmodium, which in a mosquito environment migrate to the sali- vary glands, will, when injected into the blood stream of Homo sapiens, invade specific cell types. Here they may reproduce asex- ually producing merozoites which infect other cells or invade the erythrocytes. Those in the erythrocytes may reproduce asexually, producing merozoites which will infect other erythrocytes, or they may develop into gametocytes and eventually produce gametes which will fuse in the gut of another mosquito. The motile zygote thus formed migrates to the gut wall, and develops into a sporocyst. Sporozoites are formed in the sporocyst by cell division. The same genotype responsible for the efficient feeding machine that we call a caterpillar also contains the information needed for the manufacture of the highly dissimilar reproducing-dispersing machine called a butterfly. The zygote that develops into a giant sequoia also contains the information necessary for the manufacture of its tiny pollen grain ( few-celled male gametophyte ) . The single cell of the human zygote, through division, gives rise eventually to such diverse descendants as erythrocytes, muscle cells, and ner\e cells. These deviations from the simplest cycle of development ha\e been in response to selection operating on the entire life cycle of the organism from zvgote formation till death. 62 I The Process of Evolution The details of how selection operates and has operated to produce these systems will become clear only when the mechanics of the systems themselves are elucidated. Therefore it will be necessary to consider briefly developmental systems. DIFFERENTIATION AND MORPHOGENESIS Mitosis has been described as a means of ensuring the equal alloca- tion of genetic information to the daughter cells in the course of cell division. That mitosis can accomplish this is easily demonstrated in a number of ways, as previously discussed. For example, if the zygotic nucleus of the dragonfly Platijcnemis is permitted to divide seven times (to the 128-cell stage) and then all but one daughter cell are killed with a narrow beam of ultraviolet light, a complete embryo still will develop. Obviously all the necessary genetic in- formation has been passed on from the original nucleus to its de- scendants. In view of the complex mechanism that seems to exist for the purpose of ensuring this successful transfer of necessary genetic information (and considering the demonstrable success of this sys- tem), it is pertinent to ask how cells and tissues become differenti- ated and arranged into a functional organism. Why is a nerve cell so diflFerent from an erythrocyte when both are descended from the same zygote? One answer might be that the two cells were exposed to difi^erent environments during development. Even in very early cleavage stages, when few cells are present, the differences in cellular en- vironment may be striking. DiflFerentiation of animal cells may be influenced by such things as their positions relative to the animal and vegetal poles, the outside or inside of the blastula, and proximity to the blastopore in the gastrula. Position may aflFect the amounts of vital nutrients reaching the cell, the amount of oxygen available, the rate of accumulation of excess metabolites, etc. Once differentia- tion has begun, the effects multiplv exponentially. Various combina- tions of differentiated elements add to the heterogeneity of the cellu- lar environment, and complex interactions could provide the basis for the development of the entire organism. (The complexity of the developmental system, of course, varies greatly from organism to organism.) It can thus be said that development of the organism is controlled entirely by interactions within the cluster of dividing and growing cells. Each cell possesses the same information but uses it differently because it is operating in a difiFerent physiological en- vironment. Development 63 This picture of development is supported by a vast array of data from experimental embryology. Interactions of cells may be seen in cultures of microorganisms in which density of the culture mav affect rate of growth ( or determine whether growth is possible at all ) . The literature on induction (by contact or at a distance) and organizers testifies to the potency of effects of cellular environment and to the complexitv of the systems that have evolved. None of these data, however, demonstrates that the genetic information takes merelv a passive role in development. That mitosis does not parcel out por- tions of genetic information to the proper parts of the developing organism seems certain. Experiments such as those mentioned above on Flatijcnemis have demonstrated that in most organisms, at least, the earliest cleavage cells are totipotent, retaining the information necessary for the development of the entire organism. In addition, cells taken from various parts of the body and examined micro- scopically do not seem to be deficient in their chromosome content, as would be expected if gross partitioning took place. In those in- sects that have polytene chromosomes with distinct banding in more than one body tissue, it has been reported that the banding does not change from tissue to tissue. The study of these giant chromosomes has provided other critical data for the interpretation of development. During the course of ontogeny certain bands become enlarged tremendously and are known as pujfs. This puffing process is reversible. At the same stage in other tissues, different bands are in the puff condition. Clearly there is differential behavior of the chromosomal material, but the mechanisms controlling this behavior are unknown. It is assumed that the presence of puffs is an expression of gene action. The num- ber of puffs that may develop on a chromosome is much lower than the number of bands; therefore the activity of other parts of the genetic code must be "invisible." If the genetic material participates more actively in differentia- tion than outlined above, it must do so in a very subtle manner. In- teresting recent experiments of Briggs and King have shown that the nuclei in the cells of some embryonic tadpoles are in some way altered in the course of development. When the nucleus of a frog egg was removed and replaced with one from a frog blastula, normal development ensued. When it was replaced with a nucleus removed from a gastrula or neurula, deformed embryos resulted in which the only normal tissues were those derived from the germ layer from which the donated nucleus was taken. While this does not necessarily indicate that the genetic information has been altered in the course of development, it certainly does not militate against that hypothesis. 64 The Process of Evolution Of course, other nuclear constituents may have changed, altering the translation of the code rather than the original information. Perhaps the greatest challenge facing embryologists today is the exact elucidation of the mechanisms controlling differentiation. The question is of more than casual interest to the evolutionist. If por- tions of the genotype can somehow be turned on and ofiF ( as is sug- gested by the "puffing" process ) , the operation of selection might be quite different from that in a situation in which the entire genotype always is operant. If a portion of the information that controls, say, the color pattern of a caterpillar is somehow inactivated when the adult tissues are differentiating, it might be possible for selection to alter the larval color but not affect the adult in any way. Equally, if genes affecting hair were inactivated in endoderm tissue, the form and color of the hair could be changed without any effect on the gut. If, on the contrary, such differential activity does not exist, then all changes in genetic information would, to one degree or another, be reflected throughout the life of the organism and at all stages in the life history. In many cases the effect might be so small as to be lost in the normal developmental "noise." In view of the known physical dispersion in the genome of the genes affecting the same character (e.g., the genes controlling wing characters in Drosophila melano- gaster are not concentrated on any one chromosome), the high fre- quency of pleiotropy, and the lack of a known mechanism to act as an off-on switch for major portions of the genotype, it seems most reasonable to assume that the genetic material available in most cells of an organism is essentially identical (except in quantity in endo- polyploid tissues). It is known that the cells in different tissues of the same organism (each cell presumably containing identical genetic information) do not have the same complexes of proteins. For protein synthesis, then, there must be specific control mechanisms that regulate the quan- tities of various gene products. An interesting model describing such a mechanism has been pro- posed by Jacob and Monod. This model concerns the transcription of the DNA code as discussed in Chap. 1. It is suggested that genes may be classed as either structural genes or regulator genes. The primary product of structural genes is messenger RNA, the synthesis of which is a sequentially oriented process initiated at certain regions of the DNA strands. These regions of initiation are called operators. An operator may control the transcription of more than one struc- tural gene. The adjacent genes controlled by one operator form a unit of transcription, the operon. Development | 65 The product of regulator genes is a cytoplasmic repressor sub- stance, perhaps the RNA transcription of the gene. It is postulated that the repressor substance tends to associate reversibly with a particular operator, and the combination of operator and repressor prevents the transcription of an entire operon. Protein synthesis is thus blocked. The repressor is also viewed as reacting reversibly with small molecules, called effectors, in the cytoplasm as well as with the operator. In certain systems, only the unaltered repressor can associate with the operator and block the operon. Presence of the effector will then eliminate the effect of the repressor and release the operon from repression. In other systems, only the reactant of the repressor and the effector can combine with the operator. Transcrip- tion of the structural gene is thus prevented by the presence of the effector. A simplified diagram of this model is given in Fig. 4.1, which should be compared with Fig. 1.5. The details of the Jacob-Monod model are beyond the scope of this book. It is necessary only to add that microbial genetics pro- Fig. 4.1 j A model of gene function. R, cytoplasmic repressor substance; E, effector substance; R', repressor modified by association with effector; E', effector modified by association with repressor. ( Adapted from Jacob and Monad, 1961, Cold Spring Harbor Symp. Quant. Biol. 26.) Operon Regulator gene Operator Structural gene 1 Structural gene 2 / ' / / / / / / F / Effector E Either y^ or / Ribonuc leotid es ' ' ' i / Messenger RNA Messenger RNA F / fro structure m \\ gene 1 fro structura m 1 gene 2 H E V \ ' ' Proteins made Proteins made by ribo somes by ribo somes 66 I The Process of Evolution vides considerable evidence for the various postulated processes and systems. It seems clear that some feedback mechanism of this sort must operate at the level of transcription of the DNA code and pro- tein synthesis, just as such mechanisms are believed to be respon- sible for developmental homeostasis at later stages. MODIFICATION OF THE DEVELOPMENTAL SYSTEM Of all the phenomena of morphogenesis, none has received more attention from evolutionists than so-called recapitulation. It was soon observed by embryologists that early developmental stages of vertebrates resembled one another ( at least superficially ) to a much greater degree than did the adults. This has been interpreted by some workers to mean that, in the course of development, each organism goes through a condensed version of its phylogenetic history— that man, for instance, goes through a one-celled stage (zygote), fish stage (when gill pouches appear), a mammal stage, etc. This generalization was originally called the biogenetic law by Haeckel and is often stated as "ontogeny recapitulates phylogeny." This crude interpretation of embryological sequences will not stand close examination, however. Its shortcomings have been almost uni- versally pointed out by modern authors, but the idea still has a prominent place in biological mythology. The resemblance of early vertebrate embryos is readily explained without resort to mysterious forces compelling each individual to reclimb its phylogenetic tree. It first should be emphasized that an early mammalian embryo resembles a fish embryo, not an adult fish. Virtually all organisms begin development as a single cell. The great diversity of life forms is the result of different courses of develop- ment determined in large part by the sets of genetic information that cause alterations of the course of development. However, each change does not mean transformation of the developmental system. These tremendously complex integrated systems may be successfully modified only through accumulation of minor changes, with con- comitant readjustments of balanced interactions of the various fac- tors. By careful examinations of living and fossil organisms, we can infer these gradual changes of developmental pattern. A good ex- ample is the slow change in the vertebrate jaw structure, with the articular and quadrate, which were parts of the jaw in mammal-like reptiles, having been utilized as the ossicles of the hearing apparatus in mammals. All such changes have involved the modification of a preexisting developmental sequence and were possible only when Development | 67 this sequence could be modified without throwing it lethally out of balance. For example, gill pouches (embryonic precursors of gill slits in fishes) became altered into other structures, such as eusta- chian tubes and thymus glands, in higher vertebrates. This course of evolution avoided the possible complications which might have resulted from altering the entire set of processes producing the pouches themselves. Such alteration might well have caused great disturbance in the inductive systems responsible for, say, the devel- opment of the aortic arches. The idea of recapitulation involves resemblance of developmental stages to ancestral forms. However, there are some cases in which adult forms appear to be similar to embryonic stages of their puta- tive ancestors. For example, the females of some moths and beetles are larviform; certain salamanders do not metamorphose into adults but reproduce as larvae. Many characteristics of adult human beings (relative hairlessness, large head, etc.) are reminiscent of those of young anthropoid apes. The milk teeth of Australopithecus, the earliest known fossil man, resemble the adult teeth of Homo sapiens, while the permanent teeth of Australopithecus are like those of apes. In these and in a great many other similar cases, evolution seems to have altered the developmental system so that an intermediate ancestral growth stage becomes the terminal form in the descendant. This phenomenon is known as neoteny. The sequence of stages in the development of an individual organ- ism often is thought of as merely steps toward a final goal: the adult. It is surely more realistic biologically to think of ontogeny as the continuallv changing response of a given body of genetic mate- rial to a given environment. The various processes of the epigeno- type regulate in varying degree the expression of the initiating geno- type. Evolutionary change may involve any of the arbitrarily de- limited stages of development. SUMMARY A line of descent does not consist of a straight-line sequence of individuals but of a series of cyclic phases. Each complete cycle is a developmental sequence, traditionally thought of as extending from the beginning of one diplophase (zygote) to the beginning of the next. Changes in the genetic information cause a variation in de- velopmental sequence, and the accumulation of these genetically initiated changes constitutes evolution. It is important to remember that the entire life cycle evolves and that all stages of any given cycle are essential to survival and thus equally important from the 68 I The Process of Evolution standpoint of evolution. Many students of evolution, viewing the process from the end of a diplophase, have tended to ignore this fact of life. REFERENCES Cellular Regulatory Mechanisms. 1961. Cold Spring Harbor Symposia on Quantitative Biology, vol. 26. See especially the articles by Jacob and Monod. Darlington, C. D. 1958. The Evolution of Genetic Systems. 2d ed. Basic Books, Inc., New York. The relationships between heredity and devel- opment are boldly explored in this stimulating book. Waddington, C. H. 1957. The Strategy of the Genes. G. Allen, London. The steps (epigenotype) between genotype and phenotype are dis- cussed in one of the few synthetic works in the field. 1 2 populations: properties Biologists working at the popiihtion level of organization have been oriented in large degree by the characteristics of the organ- isms studied. For instance, cytological features of genetic systems are more readily studied in Drosophila and Oenothera than in Papilio or Sequoia. Unusual combinations of circumstances have presented opportunities for studying the operation of natural selection in certain organisms, organisms about which there may be little or no cytogenetic information. Much of our knowledge is gleaned from work on organisms of economic importance, such as crops, domestic animals, and pests. Thus circumstances have made it impractical to produce a unified description of all aspects of evolution within populations. The theory of population genetics has been created largely to treat diploid, outcrossing organisms. It is therefore convenient to present this body of theory and related examples from nature before discussing the cojiiplexities of systems controlling recom- bination in various kinds of organisms. It is hoped that eventually a theory may be constructed which will consider the interactions of the genetic system of an organism and the evolutionary forces acting upon the organism. In the meantime, the warning of Norbert Wiener inust be kept in mind: It is very difficult to study the inter- actions of two systems with very different rates of time course. This is true when we attempt to understand history on the basis of day-to-day human behavior or when we try to understand phylogenetic history on the basis of individual gene changes in contemporary organisms. populations In a sense, every phenomenon is unique. No two objects can occupy the same space and time. Sets of energy relations, if recurring with exact precision, at least differ in time. However, the perceptual uni- verse is one of ordered uniqueness. The human mind is an apparatus that functions by imposing relationships upon unique events. A col- lection of objects having characteristics in common are grouped into a class (e.g., table, race, butterfly), and this group concept is use- ful for communication. Indeed, the existence of virtually all organ- isms depends upon their ability to generalize in some sense from collections of unique events. A completely unique event, one for which there could be perceived no relationship with any other event, would be totally without tneaning. All human understanding is based upon populations of things and events and the patterns of interrelationship thought to exist among them. In order to understand the workings of cells, a bio- chemist studies the populations of chemical constituents and proc- esses within the cell. For insight into the organization of organ- isms, physiologists and embryologists study populations of cells and tissues and the interactions among them. At the highest level of biological organization, the population biologist investigates popu- lations of organisms and the relationships within and among them. In this book the term population will be restricted to aggregations of individual organisms, the sense in which it customarily is used by evolutionary biologists. Population biology deals, then, with the properties of aggregations of organisms, particularly those emergent properties not possessed by the individual constituents of the popu- lations. Populations rarely can be studied in their entirety but must be sampled at one or more points in time. Unfortunately, it is not possible to sample the same population twice. INDIVIDUALS AND COLONIES The first problem arises with the definition of an individual organ- ism. At first sight this appears to be easy, since familiar plants and animals exist as discrete units. However, the situation is compli- cated by the existence of forms such as lichens. These plants consist of a fungus now known to be parasitic upon algal cells included in its thallus. Different lichens have different morphological and bio- chemical characteristics, but these fail to appear unless the correct combination of alga and fungus occurs. The alga and fungus repro- duce separately, but the lichen reproduces as well, with propagules | 73 74 I The Process of Evolution consisting of both alga and fungus. Often the alga can be grown without the fungus, but the latter does not survive without its algal host. Complex colonial organisms also present difficulties. The colonies of social insects present analogies with organisms, but usually such colonies are referred to as quasi organisms. The Portuguese man-of- war, a colonial hydrozoon, can be analyzed into its constituent polyps, which exhibit a striking division of labor. Among the algae and protozoa there are less specialized aggregations of individuals, in which what appear to be units may exist separately or as part of the colony. Even such forms as yeast (Saccharomyces) may show diJBFerent behavior, depending upon environmental conditions. In liquid culture, yeast cells (plants?) are small ovoid cells that re- produce most frequently by budding. Short chains of cells may occur. When grown on a solid medium, however, yeast forms a giant "colony." This structure is a flattened object, several centimeters in diameter, with characteristic color and surface texture as well as bio- chemistry. Cells from the outermost layer, from the center, and from the portion adjacent to the medium are very different in form and presumably in function. Nevertheless, cells from any region may be used to start a new colony or liquid culture. In the higher organisms there also may be difficulties in defining individuals. Many plants reproduce vegetatively (see Chap. 9), and if the "offspring" remain attached to the parent, the whole is con- sidered an individual. Should they become separated, each plant usually is thought of as an individual even though it is genetically identical with its "parent." The self-sterile triploid day lily Hemero- callis fulva is one genetic individual throughout its range in much of the eastern half of the United States. Populations of hydra derived from a single budding individual likewise genetically constitute an individual, but ecologically and functionally they consist of many individuals. Complexes of individuals belonging to what are called different species may also occur. Many scale insects form amazing compound colonies in symbiotic association with a fungus (Septobasidium). Forest trees commonly become grafted when their roots touch in the course of growth. It has been found that, if a root-grafted tree is cut down, the stump may survive for many years. Although without photosynthetic tissue of its own, it may produce new bark from the cambium so that the stump is completely covered. Individual organ- isms are genetically different in these situations, but they are united closely into an ecologically meaningful unit. In the same way, a clone of viviparous onions (see Chap. 9) that are genetically iden- Populations I 75 tical constitutes an ecologically meaningful assemblage as it forms part of the environment of other organisms. An individual is a set of operations (or machine) programmed in advance to do particular things. In organisms, of course, the program is established by the coded genetic information. A group of ge- netically identical individuals is one individual reproductively. Eco- logically they represent a population of individuals with different epigenotypes. If we had, historically, begun to think about biology in ecological terms rather than taxonomic terms, we would probably now deal with biological "facts" very differently. It is obvious that the concept of "individual," like other concepts in biology, can be given only operational meaning. To make the definition clear, one must specify whether he is concerned with taxonomy, genetics, or ecology. In what follows, a genetic definition of individual will be employed. Most of the evolutionary work on populations has been done with organisms among which the discrete individuals are the result of sexual reproduction and thus are, usu- ally, genetically diverse. In sexually reproducing organisms the most inclusive populations are generally considered to consist of those individuals sufficiently alike that, given the opportunity, successful reproduction will occur. The criteria for just what sort of assemblage may be labeled a population are hard to establish, and the degree of conformity with these criteria in natural aggregations is usually only guessed. In this chapter, examples will, in general, replace definitions. A butterfly and a bison obviously do not belong to the same popula- tion; a pair of robins raising a brood in the garden obviouslv do. Near the center of the continuum, problems arise: Could the Euro- pean brown bear and the American grizzly be part of the same population? Are the eastern and western sycamores part of the same population? They have been geographically separated since the Miocene, but their hybrid is a vigorous and much-used street tree. Since our interest is primarily in the process of evolution, rather than in making arbitrary decisions, no answers will be sought to these questions. SPATIAL DISTRIBUTION One property possessed by populations, but not ( in the same sense ) by their constituent organisms, is distribution. At any instant in time, checkerspot butterflies (Euphydryas edifha) are distributed along an outcrop of serpentine rock on Stanford University's Jasper Ridge Biological Experimental Area. The distribution of adults in 76 I The Process of Evolution two successive years is shown in Fig. 5.1. The distribution in the second year is somewhat different from that in the first. Such colonies of Euphydryas editha occur throughout the San Francisco Bay area; indeed, they are found along the West Coast from Baja California to British Columbia. It is difficult to specify the limits of the most inclusive population in which the Jasper Ridge individuals could be placed. Most biologists would place in this most inclusive group- ing individuals from colonies as far away as Montana. Similarly, clusters of individuals in various-sized aggregates are found in plants. Clematis fremontii var. riehlii, which occurs on limestone glades in the midwestern United States, is a perennial plant that has been studied in some detail by Erickson. Individuals are grouped into aggregates of several hundred plants, many such aggregates occupying a single glade. The outcroppings of limestone are clustered and aggregated geographically with respect to the mountain systems and rivers. In the Midwest, the plant has a much wider distribution that represents the most inclusive population (Fig. 5.2). In its loosest usage, distribution generally means the smallest geo- graphic area that will enclose all the area normally occupied by the organisms under discussion. On a small world map of the distribution of Homo sapiens, the entire United States would be shaded to indi- cate its occupation by man. (Oceanic areas and most of the Green- land ice cap would be left blank. ) In contrast, if we were mapping the occurrence of man on a large-scale map of Colorado, many high mountain peaks and some other areas would be left blank. The problems of such a mapping are obvious. Organisms are mobile at some stage of their life history, and their distributions are con- stantly changing. Furthermore, no known organisms are uniformly distributed over large areas. Thus the more resolution one strives for in describing a distribution, the more difficult the task becomes. ECOLOGICAL DISTRIBUTION The nonuniformity of geographic distributions can usually be ex- plained by the relationships of the organisms with their living and nonliving environments. Gross examples of ecological factors con- trolling distribution are easily understood; the factors controlling the fine points of the distribution of a given organism virtually are never fully understood. In the San Francisco Bay region Euphydryas editha larvae feed on Plantago erecta, a small native plantain espe- cially abundant on serpentine outcrops. In this area the butterfly occurs only where both P. erecta and serpentine are found, but the Populations [ 77 Fig. 5.1 I Distribution of individuals of the butterfly Eup/jf/c^rj/as cditha on Jasper Ridge in two successive years. The colony occurs in an island of grassland surrounded by chaparral. Each dot indicates the place of first capture of an individual. Letters refer to areas into which the colony has been arbitrarily divided for study. {From Ehrlich, 1961, Science 134 and unpublished.) 500 1,000 1 I I I I I \ 2,000 3,000 4,000 5,000 — I I I I Feet 78 I The Process of Evolution Fig. 5.2 I Hierarchy of aggregates of Clematis fremontii var. riehlii. (After Erickson, 1945, Ann. Missouri Bot. Card. 32.) Aggregate Populations 79 presence of both plant and rock does not guarantee that the butter- fly also will be there. On Jasper Ridge, areas of serpentine with abundant Phntago remain unoccupied although they are imme- diately adjacent to the colony. Not only must all environmental conditions be suitable for a habitat to be occupied, but chance must supply access to the suitable area. Thus some suitable areas for E. editha may not support colonies simply because no fertilized fe- males have ever reached them. Man has provided many organisms with access to previously uninhabited but suitable regions, as star- lings, English sparrows, cabbage butterflies, honeybees, dandelions, and mustard constantly remind us. STRUCTURE The structure of a population is considered here to be the totahty of all the factors that govern the pattern in which gametes from various individuals unite with each other. The structure can vary from situations in which combinations might seem to be essentially random (e.g., certain marine animals that release gametes into the sea, some wind-pollinated plants) to those in which the probability of certain combinations is much higher than others. The latter case is certainly the rule, if for no other reason than that close neighbors usually have higher probabilities of mating than more distant ones. Such factors as length of generation and size of individuals also are important. If the variable to be measured is the number of new gene combinations produced in a given area per unit of time, then small organisms will differ from large ones. In any place there are fewer large organisms than small ones and thus less recombination. Organisms with a short life cycle produce more gene combinations than those with long generation time, and their mutation rates also differ. Especially in higher animals, there have evolved many behavioral systems that profoundly affect the structure of a population. Many animals are effectively sedentary in spite of great dispersal potential. Birds often return from long migrations to exactly the same breeding location as was occupied in previous years. Twitty has shown that California newts have incredible perseverance and navigating ability, returning precisely to a particular segment of a stream to breed. In- deed, displaced individuals have returned to their home pool over several miles of mountainous country. Specificity within a stream is clearly shown in Fig. 5.3. Butterflies often use their powers of flight merely to patrol a restricted area. In the Jasper Ridge colony of Euphijdryos editha, 625 out of 647 recaptures of marked adults 80 I The Process of Evolution (98.6 percent) were made in the area of previous capture. Similar behavioral restriction of physical-dispersal ability seems to be the rule rather than the exception in nonmigratory butterflies. Few animals seem to be truly nomadic. Most (including most human beings) stay close to their birthplaces, occupying a home range which was "good enough for their parents." Many animals defend all or part of their home ranges from intruders of their kind —the well-known phenomenon of territoriality. This behavior, com- mon in birds, mammals, reptiles, fishes, and some invertebrates, results in a nonrandom distribution of individuals in the population; they are dispersed more evenly than one would expect in a distribu- tion governed solely by chance. Among other things, this often keeps the population size at a level where the supporting resources of the environment (food, nesting space, etc.) are not strained or entirely consumed. Individuals, often young adults, that do not successfully occupy and defend a territory must find greener pastures or starve; thus a dispersing component is added to the population. Statistically, the opposite of territoriality is aggregating behavior which results in more "clumping" than if individuals were randomly distributed. Animals showing this behavior may have little or no known social organization, as in the case of prehibernation aggre- gations of ladybird beetles or snakes. At the opposite extreme we have the highly social insects, among which there are morpho- logically differentiated castes and the reproductives in a colony may consist of a single pair of individuals. At this extreme, selection operates largely through differential reproduction of colonies, not individuals. In many vertebrates a social hierarchy (peck-order) is established in which some individuals dominate others, obtaining perquisites ranging from first choice in mate to first place in line going through the barnyard gate. Dominant males may control large harems and, in contrast with their less aggressive brethren, make a large contribution to the pool of genetic information of succeeding generations. Often, social groups (colonies of social insects and bands of howler monkeys, for example) exhibit territoriality. NUMBERS OF INDIVIDUALS One of the most obvious attributes of any population is the number of items, events, or individuals that it contains at a given time. The number of individuals in biological populations is of great interest, but unhappily it usually is difficult or impossible to ascertain with accuracy the size of any natural population. The most inclusive populations may include billions of individuals (e.g., man, house Populations 81 Fig. 5.3 I Homing behavior of newts, (a) Recaptures of individuals displaced in 1956 to a point about one-half mile downstream from place of original capture, (b) Recaptures of individuals displaced in 1956 to a point about one mile upstream from place of original capture. Area of original capture and point of release shown by arrows. Station numbers refer to arbitrary subdivisions of stream course. All individuals leave the stream after each breeding season. {From T witty, 1959, Science 130.) 10 8% recaptured 1957 1 fl I M I I I I I I I I 1 10 20 ■ ' I 1 1 1 I I 1 I I I I r I I I I I I I I I I I I I I I I ( 30 40 50 15% recaptured 1958 (a) 20 Station numbers 10 - 3% recaptured 1957 10 11% recaptured 1959 -m-n- 10 -r-rfl- I i I W I I I I H I I I I I ■ 30 40 ib) 20 Station numbers 50 82 I The Process of Evolution flies, some microorganisms, various algae) or less than 100 indi- viduals (whooping cranes or certain rare endemic plants such as Pedicidaris dndleyi and Tetracoccus ilicifolius). In the strictest sense, whenever the number of individuals in a population changes, the dis- tribution of the population changes; often a change in distribution also means a change in numbers. Study of Fig. 5.1, on which the position of first capture of E. editha adults for two consecutive years is plotted, will give some idea of the problems of dealing separately with distribution and abundance. In area C there was little increase in numbers between 1960 and 1961 but some change in the distribu- tion pattern. In area G the numbers decreased, and there was a concomitant shrinkage in the area occupied. In area H the numbers increased greatly, and the population occupied an area that was virtually devoid of individuals in the previous year. It is important to note that the figures, like virtually all representations of distribu- tion, are a stylized, static representation of a dynamic situation. The numbers and distribution of individuals in biological populations are constantly changing, the speed of the changes varying greatly from organism to organism. Population dynamics, the study of changes in population size, is a very complex subject which cannot be pursued here. It is of con- siderable interest to the evolutionist, for, as will be seen, changes in population size affect the evolution of a population in diverse ways. This can be understood intuitively, since each individual in a population is part of the environment of every other individual. Therefore any change in population size is automatically a change in the environment of the population, and populations evolve in re- sponse to environmental changes. The factors that control population sizes are diverse and in many cases poorly understood. There is little doubt, however, that usually feedback mechanisms operate to regulate the size of populations; that is, the size of the population influences its growth rate. Basic references from the abundant literature on this subject are cited at the end of this chapter. ENVIRONMENT An individual organism, when such can be recognized, is in a sense the morphological resultant of the physiological processes of which it is composed. Each of these processes is interrelated with the other functions of the organism, and the complex of processes cannot be separated from the environment, except artificially. The function- ing of an individual is determined by the relationship of its constitu- Populations 83 ent processes to factors of the environment. Each process has a range of tolerance for the environmental factors that must fall within the intensity span of the factors. Organisms do not function unless the ranges of tolerance of all these processes fall within the in- tensity spans of all the environmental factors. These ranges of toler- ance of any organism are, of course, determined by the genotype assembled in the zygote and by the developing epigenotype (see Chap. 4). It is important to note that, until reproduction occurs, there are at most physicochemical, not biological, functions among individuals. Organisms may be considered part of the physical en- vironment of other organisms until they cooperate to reproduce. There is no satisfactory way, at present, of dealing with the com- plex interactions of organism and environment. Usually some very rough classification of environmental factors is employed. For exam- ple, Andrewartha and Birch have divided the environment of a given organism into four components: weather, available nutrition, other organisms, and a place to live. These components may be further subdivided as required. All are continually changing in some degree. Just as the range of tolerance of a particular process changes during the course of development (resistance to desiccation, heat or cold sensitivity), so the intensities of environmental factors change cyclically, as well as in complex and Httle-understood patterns. The soil around an oak tree's roots may become leached of mineral ele- ments, which are restored by leaf fall and disintegration. The food plant of a butterfly dies out in a drought year. The required en- vironmental factors for the establishment of seedlings or for the pollination of flowers may be present for a brief period at only one time of the year, and the behavior of the plant must be closely correlated with the occurrence of these factors. Interactions of amazing intricacy may be seen in natural popula- tions. For example, "other organisms" in the environment may be classified also as nutrition (host, prey, food plants of herbivores) or as a place to live ( host, trees, etc. ) . A young muskrat may find all suitable burrow sites (a place to live) occupied by older stronger individuals (other organisms) and be forced to migrate. During its migration it may freeze (weather), starve before it finds suitable forage (nutrition), or be killed by a coyote (other organism). As part of the environment of an organism, other organisms may change the microclimate (as in the shade, leaf-fall zone, and root range of a tree). They may serve as vectors of genetic material in infection or reproduction, as well as of propagulcs. The flowers, fruits, and seeds of the angiosperms show a great diversity of devices effecting successful pollination and dissemination by specific animal 84 I The Process of Evolution vectors. Everyone knows of the instances of pollinating insects carrying the pollen grains (male gamete-producing plants) from flower to flower on their legs or bodies. Less familiar are those orchids in which the flower resembles the abdomen of a female fly or wasp so closely that males of the mimicked species attempt to copulate with the flower. The pollen is carried, in tiny bags or pollinia, from one flower to another on the end of the abdomen of the male insects. Similar situations are not unknown among animals. An interesting instance is that of the adult human botfly (Dermatobia hominis), which catches mosquitoes and attaches eggs to their bodies before releasing them. The eggs hatch when the mosquito lands on the warm skin of a man, and the larvae burrow in and start to develop. This fly parasitizes a number of mammals other than man. Besides the interactions among plants and animals commonly ob- served in the temperate zone, there are less well-known examples of extreme intricacy in the tropical rain forest. On the branches of the giant trees, plants of various kinds accumulate water and soil among their leaves. In this specialized niche the larvae of mosquitoes and of frogs hatch, grow, and metamorphose. The mosquito fauna is stratified in part because of the distribution of the epiphytic plants in which they grow. In the same forests lives the three-toed sloth, the hairs of which are colored greenish by symbiotic algae. The sloth moth Bradypodicola hohneli spends its entire life on the sloth, its larvae presumably feeding upon the algae. Explaining the evo- lutionary history of associations such as these, and others perhaps even more bizarre, is a challenge to the evolutionist interested in the structure of the ecosystem. Changes in climatic patterns strongly aflFect the kinds of organisms that can exist in an area. Arctic fossils of tropical plants and amphibia testify to warmer times in the past, and long-empty desert cities to changes in rainfall pattern or soil fertility. Years of commercial grazing, with no return of essential elements to the soil from decay- ing plants and animals, have changed the nutritional characteristics of many areas of the Great Plains of the United States, with a re- sultant change in the flora and fauna. English-sparrow populations in our cities have become much smaller since the disappearance of the horse and its seed-laden droppings. Grasslands have diminished in some areas as roads and other sorts of fire control increase. Man's activities in transplanting organisms have provided many striking instances of change in the influence of "other organisms." The imported cabbage butterfly, Pieris rapae, has increased in North America at the expense of our native cabbage butterfly, P. protodice, presumably by out-eating it. Storksbill, mustard, and wild oats Populations 85 (Erodiiim, Brassica, and Avena) have extirpated native plants in some areas of California. Shipborne rats have virtually destroyed the fauna of small vertebrates on numerous islands. Changes in the availability of places to live also constantly occur. Silting and slowing down of streams make them untenable for larval and pupal black flies and other organisms depending on swift-running, oxygen-rich water. Planting of trees across the Great Plains has permitted range extensions by tree-nesting woodland birds. The slow accumulation of humus and disintegration of rocks into soil make homes for orbatid mites, centipedes, fungi, and other lovers of dank, dark places. Often the organisms living in areas sub- ject to frequent catastrophic change have specialized genetic sys- tems affecting their genetic behavior (see Chap. 9). All these examples are of relatively spectacular variation in the environment. Important also are the smaller, more frequent changes: the day-to-day temperature variation that affects the plankton popu- lation of a shallow pond or the yearly precipitation changes that determine the condition of a butterfly's food plant. None of these changes can be viewed as an isolated event. Increased moisture may improve the condition of the larval food plant of a butterfly, and a large adult population may result. Among other things, this may mean more food for nestling song birds and, in the long run, more food for hawks. Large numbers of butterflies may mean more cater- pillars next year, a year when little moisture means a poor crop of food plant. Thus few adults survive, almost no food plants survive, and topsoil is lost through erosion. Large numbers of song birds from the previous season may find no suitable substitute food, with resultant starvation, emigration, and unsuccessful reproduction. The hawks go hungry. This somewhat overdrawn example illustrates only a few of the many permutations of effects which might be hypothesized as result- ing from a simple change in precipitation. Actually such gross effects are relatively infrequent, for most ecological systems are made up of a great many elements and have a historical dimension. Complexity leads to less one-to-one dependence. Drought which reduces butter- fly populations may lead to an increased supply of grasshoppers, and the song birds readily shift their diet. Long-term associations have presumably experienced most of the usual variations in climate, and their members presumably can respond to it. Thus the butterfly food plant will probably survive a drought (perhaps, if it is an annual, as ungerminated seeds ) and be ready to return to abundance when moisture reappears. If erosion has not proceeded too far, it may be stopped. Doubtless, however, many organisms would be permanently affected. 86 I The Process of Evolution COMMUNITIES Even if a single interbreeding population were found in an environ- mentally diverse area, it would be expected that, in time, genetic processes would lead to diversification (Chap. 6). The processes in- volved in the formation of complex communities of organisms are virtually unknown. It is clear that a denuded area will become re- populated. During the early stages of repopulation, the aggregations of plants and animals are short-lived. Several different aggregations can be distinguished over a period of time before a relatively stable community develops. These stages make up what is referred to eco- logically as succession, and the terminal stage often is called a climax community. Ecologists do not agree on the best methods of studying succession or terminal communities. Modern workers feel that communities are part of a continuum and that they can be distinguished as units only artificially. Indeed, since there are no biological functions operating between reproductively isolated populations, one would not expect communities to be discrete units. Each interbreeding population behaves according to its own cytogenetic processes, producing indi- viduals whose genotypes determine ranges of tolerance that enable them to function. Each entity has had an evolutionary history de- pendent on, among other things, its own genetic processes. Because of these diflFerences one would not expect any two populations to follow the same historical pattern for even a short period of time. The community, however defined, results from the overlapping ranges of tolerance of the individual organisms for the various fac- tors of the environment at a particular place. If studied for relatively short periods, the terminal communities in a successional series usually appear to be stable or in a steady- state equilibrium. Within such communities, cycling of energy and matter is constant and regulated by feedback mechanisms. Much energy is stored in organic materials (plants, animals, humus, etc.). Such a community is disturbed by outside influences only with difficulty. It is thought that organisms from other environments find it difficult to migrate into the community. Energy relations appear to be clearly established, and primary producers (green plants), primary consumers ( herbivores ) , secondary consumers ( carnivores ) , and decomposers (microorganisms) can be distinguished. Natural selection has resulted in an ecological unit of great complexity. Organisms have evolved with respect to their position in this com- plex (or in successional stages) as the environment of the earth has changed through time. Populations 87 It is obvious that the different populations of organisms found in a given place are not a random sample of organic diversity. Caribou are found with Cladonia (reindeer moss), and wolves with caribou. If a butterfly collector seeks the larvae of Battus philenor in the Arizona desert he must find the decumbent pipe vine, Aristolochia watsonii. He soon learns that this often grows in the shade of other desert plants, especially along the edges of depressions. From the searching behavior of adult female butterflies, it seems obvious that they use similar associations in their search for an oviposition site. Oaks and hickories are often found together, and oak-hickory forests are a good place to hunt the Virginia deer. Wheat, dogs, house flies, body lice, and Treponema pallidum, as well as many other organisms, are often found in association with man. Most of the ecological analyses that have been made are descrip- tive. The results, on the whole, have been disappointing, especially for the evolutionist. A recently developed ecological school is at- tempting to formulate mathematical descriptions of the structure observed in communities. It is perhaps still too early to decide whether these efforts will be successful, but the results thus far are encouraging. At any rate, it seems certain that no special mech- anisms are necessary to account for "community evolution." Ex- tremely complex interactions of those processes described in this book seem to explain all community phenomena that have been ob- served. Some of the complexities of dealing with aggregates of popu- lations are considered in the final chapter. SUMMARY Although for some organisms the concept of individual is difficult to define, in most organisms aggregations of individuals referred to as populations arise. These, or aggregations of these, may form interbreeding populations; the latter are variously combined to form taxonomic groupings. The functioning of individuals is physiological and is determined by the genotype and epigenotype that set the ranges of tolerance of the organism to intensity spans of complexly varying environmental factors. The organic functions of the popula- tion are genetic and determine the genetic constitution of the zy- gotes. Communities are aggregations of diverse populations which form part of each other's environment, and they become structured with respect to energy relations. There are no biological functions be- tween populations in a community. Communities owe their existence only to the mutuality of the tolerance ranges of the constituent organisms at a particular period of time. 88 ! The Process of Evolution REFERENCES Andiewartha, H. G., and L. C. Birch. 1954. The Distribution and Ahiin- dance of Animals. Univ. of Chicago Press, Chicago. A comprehensive modern treatment of certain aspects of animal ecology (mostly "au- tecology"). Ehrlich, P. R., and R. W. Holm. 1962. Patterns and populations. Science 137: 652-657. Problems of dealing with the properties of populations are discussed and recent literature cited. Slobodkin, L. B. 1962. Growth and Regulation of Animal Populations. Holt, Rinehart and Winston, New York. An excellent short exposition of the ideas of "the new ecology." Mathematical treatments of population growth, cycles, predator-prey interactions, communities, and the like. Stebbins, G. L. 1950. Variation and Evolution in Plants. Columbia Univ. Press, New York. See especially the first chapter. Wynne-Edwards, V. C. 1962. Animal Dispersion in Relation to Social Behavior. Hafner, New York. This book presents a mass of evidence to support the author's contention that the size of animal populations usually is kept far below the starvation level by what is described as "conventional competition" (competition for such things as territories or high places in the peck-order) . Q the theory of population genetics < In the first part of this book, the origin of Hfe, the coding and trans- fer of genetic information, the development of organisms, and some features of populations have been discussed. We shall now begin to deal with the very core of evolutionary theory— changes not in individuals but of populations. This chapter will be concerned with the theoretical aspects of the genetics of mendelian populations. A knowledge of the basic ideas of population genetics is absolutely essential to an understanding of how the mass of inherited informa- tion possessed by a population changes from generation to genera- tion. Familiarity with the simple mathematical ideas presented here will permit the reader to comprehend the more complex situations discussed in ensuing chapters. Although nonmathematical descrip- tions accompany the various algebraic examples, a firm grasp of the material will be facilitated by working through the simple algebra. The examples in this chapter are gross oversimplifications. The integrative aspects of the genotype, multiple alleles, simultaneous operation of diflFerent evolutionary forces, and other complicating phenomena are largely ignored. For the moment, it is assumed that a single locus can be torn from its substrate and subjected to condi- tions of our choice; complex interactions are left for later considera- tion. MENDELIAN POPULATIONS Only sexual organisms comprise mendelian populations, which can be defined loosely as aggregates of interbreeding individuals. A more precise definition is neither possible nor desirable, for the word "interbreeding" may refer to any situation from panmixis to almost complete isolation. One might consider the potato beetles on a single potato plant as a mendelian population, or the definition might be broadened to include those in a single potato field, those in a group of adjacent potato fields, or indeed those in a county or larger area. It is therefore important to indicate the scope of a population under discussion and to state what is known of its structure. Panmixis A population is panmicfic if the individuals within it mate at random. Each individual is equally hkely to mate with every individual of the opposite sex within the population as defined. The expected fre- quency of any given kind of mating is the product of the frequency | 91 92 I The Process of Evolution of the type of the male participant and the frequency of the type of his female partner. For example, consider a hypothetical animal which has both black and white forms and exists in a panmictic population consisting of 100 males (90 black, 10 white) and 100 females ( 70 black, 30 white ) . The expected frequencies of the vari- ous matings are given in Table 6.1. Statistical study of the frequencies of the various matings might show that observed deviations from these expected frequencies are satisfactorily explained by sampling error (i.e., chance), leading to the conclusion that, at least with respect to color, the population was panmictic. To look at panmixis another way, it can be said to occur when the genotypes of the individuals in each mating pair are a random sample of the genotypes present in the population. Complete panmixis seems rare or nonexistent in nature, if for no other reason than that relatives often tend to live close together and thus mate with one another. When this happens, the mating pairs are not a random selection from the population, and a component of inbreed- ing is added to the population-genetic picture. Gene Pool and Gene Frequency The total genetic information possessed by a population may be referred to as the gene pool of the population. If the gene pool could be described completely, one would know not only what kinds of information were present but also the frequencies of the different kinds. This chapter is concerned mostly with the distribution within the gene pool of the information at a single locus. One of the basic ideas of population genetics is that of gene fre- quency. If it is assumed that there are only two alleles at the locus {A,a) under consideration, there are then N diploid individuals of which D are homozygous for one allele (AA) with respect to the Table 6.1 Mating Product Expected Frequency Male Female Black X black .90 X .70 .63 Black X white .90 X .30 .27 White X black .10 X .70 .07 White X white .10 X .30 .03 1.00 ' The Theory of Population Genetics 93 locus studied, H are heterozygous (Aa), and R are homozygous for the other allele (aa). Then D-\-H ^R = N, and there are three types of individuals earrying two types of genes. The N individuals have 2N genes at this locus. Since each AA individual has two A genes and each Ao individual has one A gene, the total number of A genes in the population is 2D + H. The proportion p of A genes in the population is _ 2D + H _ D + 1/.H ^~ 2N ~ N The quantity p, the proportion of A genes in the population, is known as the geiw frequency of A. By convention, the gene frequency of the other allele (a) is q. Since these are the only two alleles at the locus, p -h ^ = 1, and q = I — p. Hardy-Weinberg Law If there is random mating in a population and if the gametes pro- duced by the mates combine at random, there is complete random union of all the gametes produced in the population. As each gamete contains only one of the alleles, the frequency of the two different kinds of gametes (A and a) and the gene frequency are the same. Combining the gametes at random to produce zygotes gives us [p (A sperms ) + f/ ( a qoerms ) ] )< [p (A ova ) -\- q (o oxa ) ] = (p-^ q)- = p- (AA individuals) -\-2pq (Aa individuals) + 9- (aa individuals) Populations with this distribution of genotype frequencies are in an equilibrium condition. This equilibrium is described by the Hardy-W^einberg law, which may be stated briefly as follows: If alternate forms of an autosomal gene are present in a large panmictic population, then in the absence of mutation, selection, or differential migration the original proportions (gene frequencies) of these alleles (pu p^, p-A, • • • p„) will he retained from generation to generation, and after one generation the proportion of genotypes will also reach an equilibrium. The genotype equilibrium frequencies are given by the terms of the expansion (pi + ;?2 + Pa + " • ' + P»)'~- Further discussion of population genetics will center around this law, which is one of the fundamental concepts of biology. An alge- braic demonstration of the maintenance of Hardy-Weinberg equi- librium is given in Table 6.2. 94 I The Process of Evolution Table 6.2 | Matings and Offspring in a Population in Hardy-Weinberg Equilibrium P] oportions Frequency of Offsprir 'S Type of Mating * of Mating AA Aa aa AA X AAip'' X p^) P' P' AA X Aa{p- X 2pq) 2p-q p^q p^q Aa X AA{2pq X p~) 2p\, p-q p-q Aa X Aa{2pq X 2pq) Ap\f p-q- 2f-r rq' AA X aa(p2 x q^) P T f~q^ aa X AA(p2 X q^) p~q- P'T Aa X flrt(2p7 X q^) 2pq' pq^ pq- aa X Aa(2pg X q^) 2pq' pq^ pq' aa X oaiq'^ X q^) 1.00 f pH q. The change at 1 (all a) is one-half as great as at (all A) since A-^a = u = .00004 anda-^A = v= .00002. Thus we can see that, in a population meeting all the requirements of Hardy-Weinberg equilibrium except the absence of mutation, the equilibrium value for the frequency of a gene is determined by the mutation rate and back mutation rate at the locus in question. 102 I The Process of Evolution Table 6.5 | Probability of Extinction of a Mutation Appearing in a Single Individual Probability of Extinction Generation No advantage 1% Advantage 1 0.3679 0.3642 3 0.6259 0.6197 7 0.7905 0.7825 15 0.8873 0.8783 31 0.9411 0.9313 63 0.9698 0.9591 127 0.9847 0.9729 Limit 1.0000 0.9803 From The Genetical Theory of Natural Selection, Second Rev. Ed., by Ronald A. Fisher, 1958. Published by Dover Publications, Inc., New York, N.Y., and reprinted through permission of the publisher. SELECTION Selection is the nonrandom (differential) reproduction of genotypes. One might regard the streams of hfe of a population as made up of continually varying, dividing, fusing, and disappearing particles flowing from the past to the present through a series of immensely complex screening sieves. Selection can be said to have occurred when the stream at a lower point differs from the stream at a higher point to such a degree that it is highly improbable that the observed difference is due to sampling error (drift) or mutation. In looking for selection, one must be sure that the stream neither branches (emigration) nor receives a tributary (immigration) in the stretch observed. If a population is genetically heterogeneous, the probability of success of some genotypes will be higher (with possible rare excep- tions) than the probability of success of others. Thus certain kinds of genetic information will become more and more common in the gene pool of the population and other kinds will become less and less common. The gene frequencies p and q will change with time rather than remaining constant, as would be expected under the conditions of Hardy-Weinberg equilibrium. It is popular to speak of selection as a great "creative force" in evolution, the "cause" of observed trends. In fact, it is a phenomenon observable only a posteriori— a description of occurrences. When a The Theory of Population Genetics 103 nonrandom set of genotypes leaves more oflFspring than others, selec- tion has occurred. In the broadest view, selection reduces the diver- sity of living organisms; organisms containing certain types or com- binations of genetic information are inviable or do not persist. This process "creates" what we recognize as a certain order in nature, in the same way that the order we see in a team with seven heavy, muscular linemen is "created" by the game of football. It is only in this restrictive sense of combating a trend toward increasing entropy that selection is "creative." Critics of the theory of natural selection have claimed that selec- tion can in no way be creative since it functions merely to eliminate certain types. They would point out that, in the common analogy of natural selection with a sieve, if large and small rocks are screened, the end result is a pile of the smaller rocks from the original pile. Nothing new has been created. What is forgotten is that the analogy is too simple, for it ignores the potential mutational and recombina- tional variability of biological entities. It is as if two small rocks could mate and their offspring included rocks smaller than either parent. If a sufficiently fine sieve were used, screening would create a heap of rocks smaller than any in the original pile. There are two types of selection: natural selection, in which the environment determines which genotypes are the most "fit," and artificial selection, in which man determines which genotypes are the most "fit." It is important to note that, while natural selection must always operate on the genotype through the phenotijpe, it is some- times possible for artificial selection to act directly on the genotype. For example, let us suppose that a moth has a melanic form pro- duced by a dominant gene M. If the dominance is complete, only two distinguishable phenotypes will be presented to the environ- ment, melanic and nonmelanic {M— and vim). As far as natural se- lection is concerned, the homozygous dominant {MM) and hetero- Fig. 6.3 I Equilibrium with mutation and reverse mutation. See text for explanation. {From Wright, 1940, The New Systematics, Oxford University Press. ) 104 I The Process of Evolution zygous (Mm) genotypes are indistinguishable. In the laboratory the situation would be quite different; appropriate test crossing would permit the selection of either genotype. Fitness or Adaptive Value In any given environment, the product of the relative survival value and relative reproductive capability of a given genotype consti- tutes its fitness or adaptive value. Fitness has many components, ex- amples of which are the relative fertility, duration of reproductive period, ability to find a mate (in animals), efficiency of pollination mechanism (in plants), and general hardiness of individuals of the genotype in question. By convention, the continuum of adaptive values runs from 0, as for a zygote homozygous for a lethal gene, to 1, for the genotype that donates the largest number of gametes to individuals of the next generation. This maximum number de- pends, of course, on the organism under consideration. Thus, in this book, the term fitness indicates the success of a genotype in trans- mitting genetic information to the next generation, this success being measured relative to all other genotypes along a scale run- ning from (no information transmitted) to 1 (the most informa- tion transmitted). Some authors include within the concept of fitness such things as long-range fitness, the ability of a population to meet hypothesized future changes in the environment. There can be no doubt that, as noted under genetic systems and later in this chapter, certain types of populations are better able to adjust to environ- mental changes than others. However, because of the difficulties of working with this aspect of fitness, it seems best to utilize the con- cept in a restricted time sense, as above. Further discussion of these and related problems will be found in the last chapter. It should be noted that these definitions of selection are a long way from the popular "bloody tooth and claw" picture usually painted by the uninformed. The creative aspect of the process consists almost entirely of the environment, through selection, af- fecting the genetic structure of populations so that they produce the fittest phenotypes. Types of Selection There are three basic types of selection operating within popula- tions: directional, stabilizing, and disruptive. Directional selection has occurred when there is a shift in the position of the population mean for the character considered. Stabilizing selection is lowered The Theory of Population Genetics , 105 fitness of extreme individuals and the concomitant reduction of the variance of the character, resulting in a more uniform population. Disruptive selection has occurred when two or more different types have been favored but intermediate types were at a disadvantage. It usually increases the variance and, under some conditions, mav lead to fragmentation of the population. Examples of all three types will be found in the next chapter. Extensive work has been done to describe various types of selec- tion mathematically. Most of this work lies outside the scope of this book, and the interested reader is again referred to Li. However, a few examples of this quantitative treatment follow, both to illustrate the methodology and to relieve the reader of the necessity of accept- ing on faith the results of certain selective processes. Homozygous Recessives Completely Unsuccessful Taking an array of genotypes ( for example, AA, Aa, aa ) , one may, by assigning to each one an adaptive value W, measure the differ- ences in their capacity to contribute to the filial gene pool. As stated above, the most successful genotype is given the value 1, while less fit combinations have lower values. Thus we might find the following situation: Genotype AA Aa aa Adaptive value W 1 1 1-* In this case the homozygous recessives would be adaptively inferior to either the heterozygote or homozygous dominant. The degree of disadvantage is measured by s, the selection coefficient, \ — s being the fitness or adaptive value. In this situation the coefficient could vary from (no disadvantage, making the comparison pointless) to 1 (homozygous recessives lethal). The consequences of complete removal of the recessives from the population ( homozygous recessives lethal, 5=1) are shown in Table 6.6. The relationship between the gene frequencies of any two con- secutive generations is qn + i - ^ where the subscripts indicate the generation numbers. Thus 9o = 9o 106 1 The Process of Evolution 9o+i = 9i qi + i = 92 90 90/(1 + 90) 1 + 90 90/(1 + 90) 1 + 90/(1 + 90) (l + 9o)/(l + 9o)+9o/(l + 9o) 9o/(l + 9o) _ 90 92+1 = 93 (l + 29o)/(l + 9o) 1 + 390 1 + 290 These successive 9's (gene-frequency values) fall into a harmonic series, i.e., one whose terms are the reciprocals of those in an arithmetic series. When the initial gene frequency is known, the gene frequency for any succeeding generation may be found by substi- tuting in the equation 9,, = 90/(1 + n9o). The change in gene fre- quency per generation is again symbolized by A9 and is given by the following equation: A9 1 + 9 _ -r 1 + 7 Note that the rate of change of gene frequency is itself a function of the gene frequency. When the gene frequency is high, the gene is removed from the population rapidly. A few representative values are given in Table 6.7. Table 6.6 { Complete Elimination of Recessives Gener- ation Before or After Selection Genotype Frequencies AA Aa aa Gene Frequency of a Before P' 2pq n' q After » p- 2pq p^ + 2pq q p^ + 2pq 1 + q Before f 1 2q q' q 1 (1 + ?)^ (l + q)^ i^ + qV l + q After 1 1 2q 1 + 2q q 1 + 2q 1 +2q * p- + 2pq represents the total after the aa (q-) genotypes are removed. To find the frequencies of the two remaining genotypes they must be expressed as proportions of the total. These two frequencies are obtained simply as follows: _ pip) 1-9 - 1-9 p^ + 2pq p{p + 2q) 1 - q + 2q l + q The Theory of Population Genetics ; 107 The reason for this change in rate of removal is that the propor- tion of recessive genes in the heterozygotes increases rapidly as the gene frequency decreases. Where q = gene frequency of a, the percentage of a genes in the heterozygotes is as follows: '1 Genotype Frequencies AA Aa aa Percent in heterozygotes .9 .1 .01 .01 .18 .81 .81 .18 .01 .9801 .0198 .0001 10 90 99 The recessive genes in the heterozygotes are "hidden" from selec- tion, since only the homozygous recessives are lethal. The lower the gene frequency, the smaller the proportion of recessive genes ex- posed in homozygotes becomes, and the progress toward removal of the gene from the population slows down accordingly. This result is of particular interest to students in eugenics. If a particular unde- sirable gene (a) had a gene frequency q = .01 in the human popu- lation, so that q- (aa) individuals made up .0001 of the individuals (one defect per 10,000 "normals"), it would take 100 generations (roughly 2,500 years) of a program of sterilization of defective and similarly 2q 2pq 2q p^ + 2pq 1 p-p— + J-— =1 q = gene frequency of a = - X - 2q + q 1 + q p=l-q=l q _ I + q I + q f The genotype frequencies before selection are obtained by iwing the new p and q values and expanding the following: \ l+q l + q) For example, the zygotic frequency of AA= (new p)^-= /^_V= ^—— ^' \l + q/ (l+qV- t Frequencies after selection are calculated as in the first footnote, e.g., 1/(1 + qr- 1 1/(1 + 9)^ + 2(//(l + g)= l+2q 108 I The Process of Evolution Table 6.7 Gene Frequency Decrease per Generation .9 .5 .1 .05 .01 .426 .167 .009 .0024 .000099 individuals to halve the gene frequency and reduce the number of defective individuals to 1 in 40,000. The problem of carrying out such a program without mistakes for such a protracted period makes it highly unlikely that such meager results would justify the effort involved. On the other hand, selection against dominants is rela- tively highly effective. If a dominant gene became lethal, for in- stance, it would be removed from a population in one generation. Homozygous Recessives Relatively Unsuccessful The situation where the dominants are favored over the recessives but the homozygous recessive individuals make some contribution to the gene pool of the succeeding generation is probably more common than that of complete homozygous recessive lethality. If fitness values of 1 are assigned to the two dominant genotypes, and 1 — 5 to the homozygous recessives, after one generation of selection there would be p- AA individuals, 2pq Aa individuals, and q^ — sq~ aa individuals out of a total of p~ + 2pq -\- q- — sq~ = 1 — sq- individuals. Using the same procedure as in the previous example, the change in gene frequency per generation is Pq + qHl-s) -sqHl-q) ' 1 — sq^ " 1 — sq"^ Thus the change in gene frequency under these conditions is small when q is very large or very small and is relatively large when the value of q is intermediate. When q is large, progress is slow be- cause of the relatively large reproductive contribution of the reces- sive homozygotes (in contrast with the dominants). As q becomes small, the sheltering effect of the heterozygotes slows progress, as it does when the homozygous recessives are lethal. A few sample values are given in Table 6.8, where the selection coefficient oper- ating against the homozygous recessives is 5 = .5. The Theory of Population Genetics I 109 Table 6.8 9 A Sgq, then Aq is positive and the frequency of a is increasing, and if SaP < Saq, then Aq is negative and the frequency of a is decreasing. When SaP = Saq, Aq = 0, and the frequency of a is at equihbrium. In this situation, then, the equihbrium value of q (q — "'q hat" ) is determined solely by the magnitude of the selec- tion coefficients: SaP = Saq Sa{1 — q) — Saq = Sa — SAq — Saq = A . A A , . . A Sa Sa = SAq -{-Saq = q{SA-\-Sa) q = — ^r^ Sa -J- Sa Similarly, A Sa p = Sa + Sa If the adaptive values of the three genotypes remain constant, the equilibrium value will also remain constant. If some incident, such as the arrival of a group of migrant individuals, shifts the gene fre- quency away from the equilibrium value, the selective forces will restore the equilibrium. Therefore, we refer to this as a "stable" equilibrium. Balanced Polymorphism and the Retention of Variability The selective system in which the heterozygotes are superior to either homozygote results in the retention of both alleles in the population rather than a trend toward fixation of one or the other. A situation in which two or more forms of an organism persist in the same population, with the rarest form in a frequency too high to be accounted for by mutation alone, is known as pohjmorphism. When heterozygotes are favored over homozygotes, the establishment of a gene-frequency equilibrium creates a balanced polymorphism. This type of polymorphism is important in evolution in part because it permits a certain amount of variability to be retained in the popu- lation. This means that the population may be able to react very rapidly to an environmental change and thus avoid extinction. For example, suppose that a certain locust living in a semiarid environ- ment shows the following array of adaptive values at a locus: BB, .50; Bb, 1.00; bb, .40. The heterozygotes are physiologically superior to either homozygous type, and the BB nymphs are slightly more resistant to desiccation than the bb nymphs. In such a popu- lation the gene frequency would reach a stable equilibrium at ^ ^ Si>^Sb ^ .60 +.50 ^ -^^^ The Theory of Population Genetics [ 111 ( Note that the bb adaptive vahie is .40 but the selection coefficient is .60, since 1 — 5 ecnials the adaptive value. ) The maintenance of the equilibrium at B = .545, b = .455 by this selective system is shown in Table 6.9. Suppose that a climatic change suddenly increases the rainfall in the area occupied by our hypothetical locusts, encouraging the growth of a mold which is fatal to BB and Bb eggs but to which the bb eggs are relatively immune. The adaptive values W are now BB = .00; Bb = .00; bb = 1.00. The survival of the population now depends entirely on the presence of bb eggs. If a prerain (poly- morphic) adult population of 100 pairs and an average egg produc- tion of 100 eggs per female are assumed, there would be 10,000 eggs exposed to the mold. Of these, 2,070 (.207X10,000) would be of the resistant kind, presumably giving the population a reason- able chance of survival. On the other hand, if the prerain population had been mono- morphic (all individuals BB, perhaps because of strong selection against Bb and bb individuals), the outcome would almost cer- tainly be different. If a mutation rate B — > Z? of 10~'' is assumed, only one egg in 10 billion would be of the surviving genotype. ( The chance of both members of a pair of alleles being mutant in a single individual is the product of the chances of either one being mutant: 10-^ X 10-5 ^ 10-1" ^ 1/10,000,000,000.) The advantage of bal- anced polymorphism to the population is obvious. Table 6.9 | Balanced Polymorphism Generation Before or After Selection Genotype BB Bb Frequencies bb 2 Gene Frequencies B b Before * After f .297 .496 .148 .496 .207 .083 1.000 .727 .545 .455 .545 t .455 1 Before After .297 .496 .148 .496 .207 .083 1.000 .727 .545 .455 ..545 .455 Succeeding generations continue this pattern as long as assumptions hold. * Random mating is assumed in the calculation of this row, giving the follow- ing genotype frequencies: B" = .545" = .297; ZBb = 2 ( .545) ( .4,55) = .496; b^ = .4552 = .207. f The selection pressure is included by multiplying each genotype frequency by its adaptive value: .297 (.50) = .148; .496 ( 1.00) = .496; .207 (.40) = .083. tB gene frequency is given by (D + Vs H)/N. Half of .496 = .248, thus (.148 + .248)7.727 = .545. Gene frequency oi b is 1 - .545 = .455. 112 I The Process of Evolution Simplified and overdrawn as it is, this example demonstrates how balanced polymorphism maij be advantageous to a population be- cause it prevents loss of variability due to fixation at the locus. An- other mechanism that tends to slow the loss of variability at a locus is dominance, which permits the "sheltering" of otherwise unde- sirable mutants in heterozygotes. However, viewed in another way, it might be said that both these phenomena tend to keep "unde- sirable" genes in a population. To understand this point of view, the question of "genetic load" must be examined. Genetic Load If one genotype in a population at a given time is superior to all others, it may be assumed that a population consisting only of individuals with that genotype would have the highest possible fitness. Thus, if the most fit genotype is assigned an adaptive value W = 1.00, the population fitness W will also equal 1.00, since W = 2 Wj gj ( where W,- is the adaptive value of the ith genotype and g,- is the frequency of the ith genotype, i being any number from 1 to n and n the number of different genotypes ) . Under this model, any population consisting of a mixture of genotypes will be "less fit" than the ideal monomorphic population. The amount by which a population differs from this ideal is its genetic load L, which may be viewed roughly ^ as the complement of W or L = 1 — W. Given this model, two extreme possibilities may be considered. In one case the "ideal" genotype is homozygous at all, or nearly all, of its loci. At the other extreme all loci may be overdominant with respect to fitness, and the "ideal" genotype in this case is a multiple heterozygote. Considerable controversy has surrounded the question of which of these two extreme possibilities is more realistic. Evi- dence from response by populations to inbreeding is at the moment inconclusive. There is considerable evidence for the importance of overdominance with relation to fitness in animals, and it has been suggested that for many organisms extreme deviant phenotypes may be the result of multiple homozygote genotypes segregating in popu- lations where multiple heterozygotes are the "normal" genotypes. Many plants, however, have genetic systems that seem to ensure ' Actually it is defined as L = —In W. Thus W = e"^, so that L is tlie average number of potential deaths per individual and W is the probability of genetic survival, that is, the probability of an individual not suffering death because of the properties of its genome. (Of course, an individual with the equivalent of five lethal genes in its genome dies only once, although it contributes five "deaths" to the average. ) The reader familiar with statistics will recognize the expression W = e'^ as the first term of a Poisson distribution. The Theory of Population Genetics 1 113 almost complete homozygosity, for which they appear to suffer not at all. As discussed in the preceding section, balanced polymorphism per- mits a population to store variability for future evolution. Inter- population selection thus may have favored populations with balanced polymorphic systems at many loci, in which case the con- comitant increase in genetic load could be viewed as the penalty paid for increased evolutionary flexibility. It should be noted, how- ever, that load is calculated at a given point in time and that the question of future potential for evolution is therefore not germane to the question of which kind of genotype is "ideal" as it is phrased here. A basic problem lies in two of the assumptions upon which the load controversy in large part rests. The first is that an "ideal" population would be monomorphic (that is, made up of only one kind of genotype). This would not be true if there were different niches in the area occupied by the population and if diflFerent geno- types within the population had high adaptive values in one or more niches and lower values in others. It would also not hold if there were some sort of ecological synergism in which the presence of different genotypes added to the adaptive value of each geno- type. _ A second basic assumption is that W is a measure of a biologi- cally important quantity and that one population can reasonably be considered "better adapted" than another. It can be cogently argued that any population that is maintaining itself is just as well adapted as any other and that any standard for comparing "adaptedness" is arbitrary. For instance, there is one population of Dwsophila fropi- calis in which all surviving individuals are heterozygous for an in- version, both homozygous types being lethal. In spite of this huge segregational load, it is difficult to see any reason for considering this population "poorly adapted." Finally, it must be pointed out that the character of the human genetic load is of some practical consequence. We need to know what portion of the load is segregational ( due to unfit homozygotes segregating at balanced polymorphic loci) and what portion is mutational (due to harmful mutations at loci already homozygous for "good" alleles ) . This information would help us to evaluate the long-term efiFects of mutations caused by ionizing radiation. Although the problem is quite complex, it seems that if the load is largely mutational the additional mutations will add proportionately more to the load than if it is largely segregational. (An oversimplified explanation of this is that at loci showing overdominance for fitness the new mutations would make a positive contribution when they 114 I The Process of Evolution occurred in heterozygotes, while at loci not showing such over- dominance they would be "all bad." ) Heterozygotes Inferior to Homozygotes When selection is against the heterozygotes, an unstable equilibrium point exists at p = q = .50. Any deviation from this value leads to extinction of the allele that is made less frequent by the deviation. As shown by the values given in the discussion of selection against recessive homozygotes (p. 107), the rarer an allele is, the larger is the proportion of that allele in the heterozygotes. Therefore when selection is against the heterozygotes, the less frequent allele is at a disadvantage which increases as it becomes rarer. This means that there is no tendency to return to the equilibrium point, once a deviation has occurred; rather, the situation proceeds to fixation of one allele or the other. MIGRATION AND POPULATION STRUCTURE When a population is not completely isolated from other popula- tions, its gene frequencies are subject to alteration through the in- corporation of migrants which, as a group, have gene frequencies deviating from that of the recipient population. At any locus the change in gene frequency per generation is given by the expression Aq = -m(q - q,„) = -mq-]-mq,„ where m is the number of migrant individuals divided by the popu- lation size of the recipient population, q is the gene frequency in the recipient population, and q,„ is the gene frequency in the migrant group. Manipulating the right-hand side of this equation by adding and subtracting the quantity mq^q, we get —mq-\-mq,„q-^mq„, — mq„,q = —mq(l — q,„) -\-mq,„(l — q) = —mq{p,„) -^mplq,,,) This final equation is in the same form as the expression for A^ in the discussion of the action of mutation and back mutation (p. 101); indeed, the situations are analogous. Figure 6.4 shows how the distribution of gene frequencies changes with changes in m or N, where the gene frequency of the migrants is .50. Diagrams such as Fig. 6.4 are known as stationary frequency The Theory of Population Genetics | 115 distributions. Each curve in the figure represents a probabiHty den- sity function of the form where C is a constant making the function integrate to 1, and the other notation is as above. Such a function may represent the man- ner in which the probabihty is distributed over the possible events. The area under each curve is unitv, and the area between the curve and each section of the abscissa (q axis) is the probabihty that the gene frequency will lie along that stretch of the q axis. Thus in Fig. 6.4 one can see at a glance that, under the given conditions, there is a much smaller probability that q will lie between .4 and .6 when m = 1/4N than when m = 4/N. Stationary frequency distributions are a very convenient way of illustrating the effects of various evo- lutionary forces on different kinds of populations and are widely used for this purpose. Readers interested in further information on probability density functions and other subjects relating to the mathematical treatment of probabilities are referred to any intro- ductory text on probability theory. Stationary frequency distributions may be used to represent the distribution of the gene frequency under consideration in a large number of populations under the same evolutionary conditions, the Fig. 6.4 j Distribution of fre- quencies of a gene among sub- divisions of a population, where the gene frequency of the migrants is p = q = .50. For further ex- planation of this type of diagram see text. {From Wright, 1931, Genetics 16.) 9 = 116 I The Process of Evolution distribution of the gene frequencies at a large number of loci subjected to the same pressures within a single population, or as the probability distribution for the chances of a given gene frequency occurring in any one generation. Thus the Nm = 4 curve {ni = 4/N) in Fig. 6.4 may be interpreted in the following ways. It can be said that under the same given conditions the gene frequencies of a large number of populations (or loci within one population) would tend to cluster rather tightly around the value of .50, or that among all loci within a population subjected to the same conditions the proba- bility of any given locus having a gene frequency between .35 and .65 is high. Finally, the curve represents the probability of the gene frequency at one locus having a given value in the generation observed. Thus the chance of observing a value oi q = 1 in any one generation is vanishingly small. The type and amount of movement of genetic information ("gene How") found within and among populations are important factors in determining their evolution. Obviously, a situation in which a group of semi-isolated subpopulations randomly exchange genetic informa- tion among themselves (the "island model") is quite different from a situation in which the gene How is unidirectional along a linear array of subpopulations (the "river model"). In turn, both of these differ from a situation in which a group of organisms is continuously distributed over a large area. In the latter case, although semi- discrete clusters of individuals may not exist, the probability of mating by two widely separated individuals may be very low because of their remoteness alone. The effects of such "isolation by distance" have been dealt with mathematically by Wright, who showed that the amount of local differentiation in a population is largely a func- tion of the size of the panmictic units ( neighborhoods ) of which it is composed. When a population is divided into semi-isolated sub- populations or when some degree of inbreeding is found in the population as a whole, the general result is a reduction in the fre- quency of heterozygotes. An important aspect of this change in genotype frequencies is illustrated by the effects in such a popula- tion of selection against the homozygous recessives. This would be much more effective in a subdivided or inbreeding population because the reduced number of heterozygotes would "shelter" fewer recessive genes. JOINT PRESSURES Up to this point, only single evolutionary forces acting on isolated loci have been considered. However, in virtually all cases studied, The Theory of Population Genetics 1 117 two or more pressures act jointly to affect the gene frequency at a given locus. In addition, the gene frequencies at different loci are not independent of each other, and the gene frequency at one locus may have a profound effect on the gene frequency at another. To appreciate this, one need only recall the phenomenon of linkage. Some progress has been made in describing mathematically the results of various types of interactions in mendelian populations. Whether a completely satisfactory mathematical description of the simultaneous action of all evolutionary forces (varying with the en- vironment) on an integrated genotype will ever be possible is an open question. Progress in the development of computers gives reason for hope, but the extreme complexity of the situation to be analyzed would require a computer of as yet undreamed-of sophisti- cation. For the moment we must be satisfied with combining gross oversimplifications. There is solace in the fact that these simple models seem to approximate some natural situations and have proved quite useful in describing them. As a short excursion into more complex situations, consider Figs. 6.5 to 6.12. Figures 6.5 to 6.8 illustrate the effects of different selec- tion pressures in populations of different sizes. In Figs. 6.5 to 6.7 mutation and back mutation rates are considered constant and equal (ii = v). In Fig. 6.5 the population size is N = l/40t); in Fig. 6.6, N = 10/40u; in Fig. 6.7, N = 100/40o. In all three figures the sohd line is the case with the least selection (s = — d/100), the broken line the case with selection ten times as severe (not represented in Fig. 6.5 since it is practically indistinguishable from the preceding), and the dotted line the case with selection 100 times as severe. Note that selection in the very small population ( Fig. 6.5 ) merely slightly alters the svmmetry of the distribution, the probability of loss or fixation remaining high. As the population size increases (Figs. 6.6 and 6.7) the selection effects become much more pronounced. Fig- ure 6.8 illustrates the distribution when the heterozygotes are favored and there is no difference between the selective values of the two homozygotes. Again u =v, N = l/40t;, and s = lOOu. (Note that in these figures and in Figs. 6.9 to 6.12 the selection coefficient is not used as defined earlier but is given both positive and negative values. Thus s = lOOu is an index of the advantage of the heterozygotes, whereas above s = — u/lOO is an index of the disadvantage of the allele under consideration. ) Figures 6.9 to 6.12 show the distribution of gene frequencies in populations of different sizes and different states of subdivision, under various selection and mutation pressures. Figure 6.9 depicts a small population under virtually no selection or mutation pressure. 118 I The Process of Evolution The majority of alleles are fixed or lost at random. An intermediate- sized population under opposing selection and mutation pressures is shown in Fig. 6.10. There is random variation around modal values established by the opposing pressures. The case of a large popula- tion with gene frequencies at equilibrium points determined by the magnitudes of opposing selection and mutation pressures is covered in Fig. 6.11. Finally, Fig. 6.12 gives the gene frequencies in sub- divisions of a large population fluctuating around modal values established by opposing forces of migration and selection. Figs. 6.5 to 6.8 | Distributions of gene frequencies under different selection pressures and in populations of different sizes. For details see text. (From Wright, 1937, Proc. Nat. Acad. Sci. 23.) 6.5 6.7 6.6 6.8 * The Theory of Population Genetics I 119 6.9 ANs-g'. / f^' r- 0.5 1.0 6.11 4iVs = 4iVw = 6.12 0.5 1.0 4iVm = 16 9m = \ 4iVs=10 j 4Ns = \ 4Ns = 4A^s = Figs. 6.9 to 6.12 | Distributions of gene frequencies in populations of different sizes and different states of subdivision under various selection and mutation pressures. See text for details. (From Wright, 1931, Genetics 16. ) 120 I The Process of Evolution ADAPTATION AND GENE COMBINATIONS As Sewall Wright pointed out, sexual organisms have available to them a tremendous number of possible gene combinations. A species with only 1,000 loci, each occupied by a series of 10 alleles, could, through recombination, produce 10^*^°*^ different genotypes (a num- ber inconceivably greater than the number of electrons in the uni- verse ) . While many of the theoretically possible combinations would be inviable or would yield identical phenotypes, such a species obvi- ously has a very large capacity for genotypic variation. Within this vast field of possible combinations, there must be a large number of highly adaptive combinations and also many less highly adaptive or even lethal combinations. Wright represented this field as a con- tour map in two dimensions, with adaptive peaks and nonadaptive valleys, and stated that "the problem of evolution ... is that of a mechanism by which the species may continually find its way from lower to higher peaks in such a field." A population of mosquitoes selected to avoid insecticides by not landing on poisoned surfaces might be considered as occupying an adaptive peak. A higher adaptive peak might be the development of a method of physio- logical resistance (allowing, perhaps, better access to houses). In order to acquire the more efficient physiological resistance, con- siderable reorganization of the genotype could be required; this reorganization might result in the loss of the behavioral resistance. The species would then have to cross an adaptive valley (little or no resistance) in order to attain the higher peak. Figure 6.13a to / illustrates what might happen to certain kinds of populations occupying the adaptive field under different specified conditions. The field is represented as a topographic map with con- tour lines indicating different levels of adaptation. The heavy broken line represents the initial position of the population, and the arrow the direction of subsequent change. In Fig. 6.13a one sees the effect of increasing mutation rate or reducing selection pressure, a general increase in the variance and lowering of the average adaptive value of the population. If it spreads far enough, a portion of the population may occupy the lower slopes of an adaptive peak that is higher than the initial one; if this occurs, the entire population will move over and occupy the new peak. The effect of increasing the selection pressure or decreasing the mutation pressure is shown in Fig. 6.13b. The average level of adaptation increases at the expense The Theory of Population Genetics | 121 of evolutionary plasticity. The chances of capturing a neighboring higher peak are reduced. Figure 6.13c illustrates the consequences of the omnipresent changes in the environment (adaptive peaks becoming valleys and vice versa ) . Here the result depends on the severity of selection and the speed of the environmental change. A species occupying a small field under strong selection pressure may not have the variability to permit it to move to the emergent peaks and may thus be left in a "pit" and become extinct. A population under less stringent selection will merely move as the conditions change. Figure 6.13f/ shows the effects of great reduction in population size and close inbreeding. Random fixation and loss move the population erratically down from its adaptive peak, and inbreeding (producing homogeneity) re- duces the size of the adaptive field occupied by the population. This process (if unchecked) leads ultimately to extinction. In Fig. 6.13e one sees the results of an intermediate relationship between popula- tion size and mutation rate. The population tends to wander from Fig. 6.13 I Field of gene combinations occupied by various kinds of populations under different conditions (explanation in text). (From Wright, 1932, Proc. VI Congr. Genetics 1.) (a) Increased mutation or reduced selection. ANu, ANs very large (6) Increased selection or reduced mutation. 4Nu, ANs very large (c) Qualitative change of environment. ANu, ANs very large (d) Close inbreeding. (e) Slight inbreeding. ANu, ANs very small ANu, ANs medium (/) Division into local races. ANm medium 122 I The Process of Evolution its peak, though remaining in the vicinity. This is a trial-and-error situation which may lead to the capturing of higher and higher peaks, although under the described conditions progress would be extremely slow. In Fig. 6.13/ a large species is subdivided into numerous semi- isolated populations. The part of the field occupied by each sub- population shifts continually in a largely nonadaptive fashion and at a much faster rate than in the preceding case ( since it is dependent on the amount of intermigration rather than mutation rate). With the rapid movement in the general neighborhood of one peak, sooner or later one subpopulation will cross the lower slopes of a higher peak and ascend it. This subpopulation will then expand in numbers; by migration its genes will flow into the other subpopula- tions, and the whole species will be brought into the field of influ- ence of the new peak. This situation, featuring intergroup selection, permits trials of new combinations with a smaller risk to the species than a situation involving only intragroup selection. Such a sub- divided population, then, provides the best opportunities for low- risk evolutionary change. SUMMARY In this chapter an attempt has been made to give a brief introduction to evolutionary processes from the viewpoint of population-genetic theory. One of the fundamental concepts of biology is the Hardy- Weinberg law which states that, in an idealized population and in the absence of evolutionary forces, the gene frequencies of auto- somal alleles in the population will not change and, after one- gen- eration, the proportion of genotypes will reach an equilibrium. The ways in which mutation, migration, drift, and selection may cause deviations from this equilibrium have been formulated mathe- matically. The effects of these forces depend not only on the inter- actions among them but also on the structure of the population and the feedback effects of this structure on the forces themselves. Theoretical descriptions of possible responses of populations to various combinations of factors have been developed. Considering populations as shifting and interacting arrays of gene frequencies has given the evolutionist tools (however crude) for analyzing his ob- servations with some degree of rigor and precision. Population- genetic theory is extremely useful in describing what may happen in natural populations and in interpreting data gathered from such populations. The Theory of Population Genetics \ 123 REFERENCES Crow, James F. 1958. Some possibilities for measuring selection intensities in man. In J. N. Spuhler [ed.], Natural Selection in Man, Wayne State Univ. Press, Detroit. This paper and those cited in its bibHography deal with the problem of genetic load. Li, C. C. 1955. Population Genetics. Univ. of Chicago Press, Chicago. Every student should be familiar with this clear, concise, and generally excellent text. The bibliography contains references to most of the theoretical literature, including very important general works by Fisher and Haldane and the classic papers of Hardy and Weinberg. Population Genetics: The Nature and Causes of Genetic Variability in Populations. 1955. Cold Spring Harbor Symposia on Quantitative Biology, vol. 20. The papers in this volume are a good sampling of recent thinking in population genetics. See especially the papers in the section Integration of Genotvpes. Wright, Sewall. 1932. The roles of mutation, inbreeding, crossbreeding and selection in evolution. Proc. Sixth Intern. Congr. Genet. 1: 356-366. In this important paper Wright develops his idea of "adaptive fields." . 1958. Systems of Mating and Other Papers. Iowa State College Press, Ames, Iowa. This useful booklet reprints four of Wright's most important papers, including Evolution in mendelian populations {Genetics 16: 97-159). changes in populations Now that some of the theoretical aspects of population genetics have been discussed, it is appropriate to ask if it is necessary to rely on inference for the investigation of evolutionary mechanisms or whether it is possible to study them directly. Especially in recent years, examples of evolution in natural populations have been in- vestigated, and some of these will be considered in the first part of this chapter. The remainder of the chapter is devoted to more gen- eral discussion of some aspects of evolutionary changes in popula- tions. EXAMPLES FROM NATURE Differential Mortality in Sparrows There have been numerous efforts to demonstrate the action of natural selection by comparing the characteristics of surviving and nonsurvdving individuals. A brief summary of one of the earliest of these studies, Bumpus's work on sparrows, will serve to represent them all. In the winter of 1898, after a severe snow, rain, and sleet storm, H. C. Bumpus brought 136 stunned English sparrows into his laboratory at Brown University. Of these birds, 72 revived and 64 died. Bumpus measured the total length, wingspread, weight, length of beak and head, length of humerus, length of femur, length of tibiotarsus, width of skull, and length of keel of sternum on all the birds. His measurements showed that these various characters of the surviving birds generally were closer to the mean than those of the birds that died. This mortality of sparrows with extreme measurements is an ex- ample of stabilizing selection. Many other studies also have demon- strated the correctness of the widespread notion that selection often results in the elimination of deviant individuals. As has been pre- viously mentioned (Chap. 6), this is only one aspect of natural selection. The examples which follow often involve complex inter- actions of the so-called basic types of selection. It is to be expected that stabilizing selection in the form of failure of extreme deviants occurs in virtually all natural populations. Industrial Melanism During the past 120 years, dark forms of numerous cryptically col- ored (camouflaged) species of moths have appeared in certain areas | 1 25 126 I The Process of Evolution of northern Europe and North America. In many of these areas the melanic forms have become predominant, replacing or partially re- placing protectively colored "typical" forms. These changes have taken place primarily in heavily industrialized areas and have been especially spectacular in England where they have been studied extensively. It has been estimated that in the area of Manchester, in 1848, the dark form of the moth Biston betttlaria made up a maxi- mum of 1 percent of the population and that in the same area, in 1898, it made up more than 99 percent of the population. In most of the known cases the melanism is produced by a single dominant gene. The following hypothesis has been developed to account for the phenomenon of "industrial melanism." The spread of melanic forms seems to be intimately connected with the pollution of woods by soot in industrial areas. Apparently in unpolluted areas the dark forms Fig. 7.1 I Two individuals of Bt'sf on fot'iuZaria, one typical, one melanic, resting on an unpolluted, lichen-covered tree trunk. (After Kettlewell, 1958, Proc. X. Int. Cong. Ent. 2.) \ Changes in Populations | 127 are removed from the populations by visual predators (those which hunt by sight ) because they are conspicuous when resting on lichen- covered tree trunks (Fig. 7.1). This disadvantage outweighs a pos- sible selective advantage of the larvae of the melanics which may be physiologically superior. In polluted areas the situation is reversed; the typical (light, mottled) forms, which are nearly invisible on unsooted, lichen-covered bark, are conspicuous on sooty trees where pollution has killed the lichens (Fig. 7.2). Thus the melanics are protectively colored in the polluted area; in addition, any physio- logical advantage they possess is magnified under the stress of eat- ing contaminated food. Selection therefore strongly favors the me- lanics in industrial areas and the typicals in unpolluted areas. In polluted districts a directional selection moved the frequency of melanic individuals toward 100 percent. What evidence supports this hypothesis? Some of the most com- Fig. 7.2 1 Two individuals of Biston betularia, one typical, one melanic, resting on a soot-covered tree trunk. {After KettJewell, 1958, Proc. X. Int. Cong. Ent. 2. ] i'iWP|3'»WP^^WMilpFi'f!^''V''"^'|"i^W 128 I The Process of Evolution pelling is the strong correlation, in time and space, of industrializa- tion and the appearance of the melanic forms. This is so striking as to make virtually certain some relationship between the two phe- nomena. The composition of various English populations of Biston hetularia is shown in Fig. 7.3. The populations in the industrial mid- lands are highly melanic, as are those in eastern England on the downwind side of the industrial areas where pollution fallout has been most intense. The most elegant demonstration of one factor responsible for in- dustrial melanism is the observations and experiments on differential predation by Kettlewell and Tinbergen. In an unpolluted wood in Dorset, equal numbers of melanic and typical individuals were re- leased; predation was observed and photographed from a blind. Spotted flycatchers (Muscicapa striata), nuthatches {Sitta euro- paea), yellow hammers (Emberiza citrinella), robins (Erithacus rubecuh), and thrushes (Tiirdus ericetonim) ate 164 melanic in- dividuals but only 26 typical individuals (P <^ .01).Mn a polluted wood near Birmingham, opposite results were recorded, with red- starts (Phoenicurtis phoenicurus) eating only 15 melanics in contrast to 43 typical individuals (P <^ .01). Very impressive motion pictures were made of these experiments, with repeated sequences of birds searching tree trunks and eating the conspicuous moths without noticing adjacent protectively colored individuals. A series of release and recapture experiments supported the visual-predation hypothesis. Known numbers of marked individuals of both types were released in an area, and the percentage of each recaptured later at a light was recorded. The percentage of recov- ered moths contrasting with their background (melanics in un- polluted and typicals in polluted areas) was considerably lower than the percentage of cryptically colored moths recaptured. For instance, near Birmingham (where the population is 85 to 87 per- cent melanic) 154 melanics and 73 typicals were marked and re- leased. Later 98 marked moths were recaptured, 82 melanics (53 percent of 154 released ) and 16 typicals ( 25 percent of 64 released ) . If experimental error is ignored and the survival value of the favored melanics is set equal to 1, the survival value (1 — s) of the typical ' Probaliility much less than 1 in 100. This means that a statistical test has indicated that a deviation from the expected frequencies of this magnitude would, on the basis of chance alone, be expected much less than 1 percent of the time. The notation (P < .02) means probability less than 2 in 100. Many biologists conventionally consider "significant" any result where the probability of chance alone being responsible is less than 5 percent. An elementary discus- sion of the use of statistical inference in biology may be found in Simpson, Roe, and Lewontin, Quantitative Zoology. 2d ed. Harcourt, Brace & Co., New York. Changes in Populations | 129 phenotype becomes .25/. 53 = .47. Hence, s is equal to .53, coinci- dentally the same value as the percent recovery of the melanics. It is highlv unlikely that these results were due to chance alone (P < .01). Similar experiments in unpolluted areas have given reverse results. The relationship between the release-recapture ex- periments and the observations of visual predation is this: The re- lease-recapture work shows that selection occurs; the observations show what the selective agents are. Fig. 7.3 I Distribution of Biston hetularia, showing proportion of typicals and melanics. {From Kettlewell, 1958, Heredity 12.) m*^-: ^J}/'-^*^ ^^'l^,.-^-Nfl^ O Typical A Melanic 130 I The Process of Evolution There have been numerous experiments testing the viabihty of larvae of the melanic form. Ford, working with the moth Cleora repandata, produced backcross broods by mating melanic hetero- zygotes with typical recessive homozygotes. As in all test crosses, a 1:1 ratio of melanics to typicals was expected, and when the broods were well fed there was no significant deviation from this ratio. However, when the caterpillars were starved every day (put under physiological stress), the ratio found was 51 melanic to 31 typical (P < .02), a significant departure from the expected 41.5 of each type. Kettlewell exposed six backcross broods to stress and found 108 melanic survivors as opposed to 65 typical individuals (P=.01). There can be little doubt that, under certain conditions of stress, the larvae of the melanic moths are better able to survive, but the most recent work on the subject indicates that the situation is more complex than was previously thought. In some recent ex- periments, the expected deficiency of nonmelanic individuals was not found. In other experiments the results showed interbrood heterogeneity: While the offspring from some matings showed a significantly higher proportion of melanics, the offspring from others did not. Furthermore, a study of backcross broods of Biston hetularia raised between 1900 and 1905 showed no surplus of melanics but rather a slight (not statistically significant) defi- ciency of them. It is possible that, early in the evolution of industrial melanism, melanic larvae were not physiologically superior and that this superiority, where it now exists, is a rather recent develop- ment. Perhaps it was only with the easing of the severe selection against melanic adults that melanic individuals increased sufficiently in populations to permit selective reorganization of the melanic genotype to gain the physiological advantage. Just as selection for modifier genes increased the dominance of the genes producing melanism (Chap. 3), so could selection enhance the efiFects of the melanic genes on viability. Kettlewell tried experiments to see whether melanic moths tended to settle on dark surfaces and typical moths on light surfaces. He painted the inside of a barrel with alternate black and white surfaces and then released in it an assortment of moths to see which svirfaces they chose. Seventy-seven moths selected the noncontrasting back- ground, while forty-one selected the contrasting background. It has been suggested that this choice is possible because the moth can de- termine the degree of contrast between the scales around its eyes and the background on which it is resting. In some areas, melanics are becoming predominant where the I Changes in Populations 131 countryside appears to be unpolluted. Two reasons may be given for this. First, pollution is often greater than meets the eye. Smog clouds tend to drift a long distance, and, in spite of its overall green appearance, the countryside may actually have a considerable layer of soot and industrial chemicals. A second reason is that man's other activities alter the countryside, and these changes (e.g., decimation of the predators in an area ) may be enough to shift the balance to the melanics. Although recessive melanics are known, the spread of industrial melanism is due to the spread of dominant genes. The possible reasons for this were discussed in Chap. 3 in relation to the origin of dominance. Industrial melanism is an example of transient polymorphism. Re- member that polymorphism is the occurrence in the same habitat of two or more distinct forms of a species in such proportions that the rarest of them cannot be maintained by recurrent mutation. Tran- sient polymorphism is the situation in which the two forms coexist while one is in process of replacing the other. Another example of what is probably transient polymorphism involving melanic moths has been studied in an old woods in Scotland. In these woods, which are essentiallv free from pollution, one species of moth (CIcora reparulata ) had a population in which 10 percent of the individuals were melanic. At rest on lichen-covered trunks (light background) the typical forms were very inconspicuous. On dark trunks the melanics were inconspicuous, but their protective coloration was (to the human eye) not as eflFective as the camouflage of the light form. In flight, in the dark woods, the light forms were much more con- spicuous. (Three were observed to be taken on the wing by birds in a period during which no melanics were observed to be eaten. ) If the advantages were the same both at rest and in flight, progress toward fixation would be more rapid than in the situation described. Nevertheless fixation will still occur unless conditions change. Mlcroevoiution in British Lepidoptera British lepidopterists have pioneered in studies of other types of evolutionary changes in populations of butterflies and moths. Long- term studies of the gene frequency at a single locus in the scarlet tiger moth, Panaxia domimdn, have been made by Fisher, Ford, and Sheppard. In the only colony (near Oxford) where the gene in question has been detected, its frequency has been estimated every year since 1939, and population-size estimates have been made for all years since 1941. Three phenotypes occur in the colony: dominula 132 I The Process of Evolution (homozygous for the common allele), meclionigra (heterozygous), and bimacida (homozygous for the rare allele). The frequency of the rare gene was .012' prior to 1928, .092 in 1939, and .111 in 1940. After 1940 it dropped rapidly, leveled off around 1947, and since then has fluctuated between .011 and .037. There seems to be little doubt that these changes are due largely to fluctuating selection pressures. Among other things, the gene is known to affect color pattern, mating behavior, fertility of the males, and larval viability. There has been some controversy over the possible role played by drift in this situation; the degree to which random changes interact with selection has not been determined. Dowdeswell, Fisher, Ford, and others have made a long series of studies of the frequency distribution of spot number on the under- side of the hind wings of the satyrine butterfly ManioJn jiirtina. Over most of southern England the spot distributions are remark- ably uniform, in spite of the great diversity of environments. How- ever, the spot distribution in females was found to be sharply difiFer- ent between populations in Devon and Cornwall. Furthermore, the change was found to be extremely abrupt; indeed, in 1956 the border between populations with the two kinds of spot distribution was found to be a hedge which was not a barrier to the passage of indi- vidual butterflies. There was no sign of a gradient between the two types; if anything, the difference was greatest at the point of con- tiguity. In 1957 the border between the two forms was found to be 3 miles east of its 1956 location, and the transition was more gradual. The boundary itself was a strip some 150 yards wide which was occupied by an intermediate population. Spot distributions also have been studied extensively on a small archipelago, the Scilly Isles, off the western tip of Cornwall. Numerous differences were found among the islands, but each island tended to have a characteristic population which remained the same from year to year. Spot distributions certainly are under strong selective control, although it seems clear that the selective value is manifest in other effects of the genes involved rather than in inconspicuous spot changes. It is difficult to formulate any other explanation for the sharp and mobile border between the Devon and Cornwall types than the shift from one highly integrated gene complex to another. The types seem highly successful, as they extend for a considerable distance in either direction from the border (especially eastward) and are relatively undisturbed by the heterogeneity of the habitats they occupy. It does not seem likely that the environmental factors responsible for the change in selection coefficients actually change as abruptly as the spot frequencies; the reason for the sharp border almost certainly lies in the genetics of Maniola. Changes in Populations j 133 Polymorphic Land Snails The microevolution of polymorphic land snails of the genus Cepaea has been studied in detail, mostly with the very variable species C. nemoralis. The shell of this snail may be yellow, brown, or any shade from pale fawn through pink and orange to red. The lip of the shell may be black or dark brown (normally) or Dink or white (rarely), and up to five black or dark brown (rarely transparent) longitudinal bands may decorate it. The genetic basis of many of these characters is fairly well understood, and fossil evidence shows that this poly- morphism has existed since before the Neolithic. The snails have been studied extensively in Europe, where the fre- quencies of different forms vary greatly from colony to colony. The roles played by selection and drift in accounting for the intercolony differences have been the subject of controversy. Lamotte in France originally claimed that genetic drift accounted for the observed diversity. However, Cain and Sheppard demonstrated that, at least in some English colonies of the snails, selective forces were at work. Near Oxford they found that the frequencies of the different kinds of shells were correlated with the microhabitat of the snails (Fig. 7.4). Collections were made in six main types of habitats: downland beech woods, oak woods, mixed deciduous woods, hedgerows, open areas with long coarse herbage, and open areas with very short turf. Analysis of the frequencies of color and pattern types in these sam- ples showed that they were highly correlated with the color and uniformity of the background. For instance, the percentages of effectively unhanded shells in the five localities with the most and the least uniform backgrounds ( uniformity decreases to the right in each series) were as follows: Most uniform 100 100 100 93 79 Least uniform 39 35 34 25 4 The percentages of yellow shells in the five greenest and in the five least green localities (greenness decreases to the right in each series) were: Greenest 76 64 45 43 41 Least green 17 8 4 This association between the frequencies of shell types and the character of the habitat suggested that visual predators eating the most conspicuous snails might be a selective agency causing differ- ences among colonies. One such predator is the song thrush, Turdus ericetorwn. In the summer of 1951 Cain and Sheppard studied a colony of snails in a small hillside bog in Wytham Woods near 134 I The Process of Evolution Fig. 7.4 I Map of Oxford district showing the correlation of Cepaea shell type and environment. In each histogram, the left-hand column represents percent of yellow shells and the right-hand column, percent of effectively banded shells. Woodlands are stippled; all colonies outside of woodlands are in hedgerows or rough herbage. ( From Cain and Sheppard, 1954, Genetics 39.) I Changes in Populations 135 Oxford. Thrushes removed snails from the colony, cracked their shells on stones on a nearby bank, and ate the soft parts. Thus a sample of the predated portion of the population could be obtained by collecting the broken shells from around the thrush anvils, and a sample from the entire population by collecting individuals from the bog. Of 560 individuals taken from the bog, 296 (52.8 percent) were unhanded, while of 863 shells collected around the rocks only 377 (43.7 percent) were unhanded. This significant difference (P < .02) indicates that unhanded individuals in this colony were less likely to be eaten than banded individuals. Similar methods had been used by Sheppard in 1950 to study two other colonies. He found that there was a decrease in the percentage of yellow snails killed as the season progressed and that the rate of decrease appeared to be the same in both localities. There was no evidence that this was due to the thrushes hunting in different areas or to a change in the percentage of yellow shells in the population at large. Apparently the selective value of the yellow phenotype was at least partly a function of the background on which it occurred. Early in the spring, when the woodland floor was predominantly brown, the yellow shells were relatively conspicuous and thus at a selective disadvantage. As the season progressed, the background became greener and this disadvantage lessened. By late April or early May the yellow shells were selectively neutral; by mid-May they were at a selective advantage. These data indicate a rather strong selective pressure. Because of this, one would expect populations living on uniform backgrounds to be composed only of unhanded individuals, and those living in rough tangled habitats to contain only banded individuals. How then is the polymorphism maintained? Shifts of selective values with the seasons would delay, but not prevent, the removal of the less favored varieties. Interchange of individuals among colonies in different habitats would account for some of the variability, but the range of movement of snails is too small to support this hypothesis for more isolated colonies. The answer is that there are physiological factors genetically cor- related with pattern type. Thus experiments with Cepaea nemoralis have shown that unhanded individuals ( especially yellow ones ) are more heat-resistant than banded individuals. Yellow snails are more resistant to cold than pink snails, and unhanded snails are more cold-resistant than banded snails. These and other similar character- istics indicate that color and banding are subject to strong nonvisual selection because they are associated with important physiological advantages. In some cases, heterozygotes may be expected to be 136 I The Process of Evolution more viable than either homozygote, so that physiological selection would tend to establish a stable polymorphism. These nonvisual selective forces, interacting with the selection pressures created by visual predation, seem to be responsible, in large part, for the ob- served pattern of variation in Cepaea. Several other factors may be of importance in some situations. One is predation in which the selection pressure is a function of the rela- tive frequency of the type of individual predated. Certain predators may form search patterns that result in selection against the com- monest type in the population, without regard for which type is commonest. This sort of predation pattern could lead to a stable polymorphism. Random processes, once considered to be the prime factor in differentiating Cepaea populations, may have relatively minor importance. Undoubtedly drift plays some role in the smaller populations, and it may account for some patterns of variation recorded by Lamotte in France. Recently established colonies may not have achieved equilibrium with their environment, and their composition may be strongly influenced by the genetic information possessed by the snail or snails that established them. This influence of the genetic endowment of the individuals involved in starting a new colony is known as the "founder principle" and is discussed further in Chap. 10. One of the arguments used to demonstrate that visual selection does not play an important role in determining the characteristics of Cepaea colonies is that, in mixed colonies of the two closely related species C. nemoralis and C. hortensis, the phenotype frequencies of the two snails were uncorrelated. Clarke has satisfactorily explained this by showing that both species respond to visual selection pressure but in difi^erent ways. It appears likely that the selective values of various pattern and color genes differ in the different genotypes of the two species. As a hypothetical example, a strongly banded pat- tern might be at a selective disadvantage in a certain dry habitat. In one species the genes for strong banding may also be involved in producing individuals resistant to desiccation. In the other species there may be no such system. Thus in the same habitat one species may have a population with a large proportion of strongly banded snails (because of heavy selection for resistance to desiccation), whereas the other species may have no banded snails at all ( because of the selection against banding). Although the genes controlling the various factors seem to be homologous, they cannot be divorced from their genetic environment. It will be recalled that in Chap. 6 genes were treated as if they were independent entities; results such as these studies of Cepaea constantly remind us that this is an over- simplification. I Changes in Populations j 137 Island Water Snakes Camin and Ehrlich have studied microevolution in populations of water snakes (Natrix sipedoii) on the islands in western Lake Erie. The snakes on these islands have variable banding patterns. These have been divided into four classes: A, B, C, D (type A snakes being unhanded and type D snakes being completely banded; see Fig. 7.5). Except in the area of western Lake Erie (and one locality in Tennessee), all known N. sipedon populations are made up of type D individuals. Virtually all the snakes taken from the mainland sur- rounding Lake Erie are type D. On the islands a large proportion of the snakes are of the other types, including numerous individuals that are totally unhanded. The islands have very little inland water, and the snakes are restricted largely to the lake shore. The marginal flat limestone rocks, limestone cliffs, and pebble beaches are the only suitable habitats. Large samples of adult snakes were taken from the islands, and pregnant females were kept alive until their young were born. The distribution of pattern types in the litters was compared with that of the adults. The distribution of the adult and litter pattern types from one group of islands is shown in Fig. 7.6. In spite of the diffi- culties of statistical comparison of the cluster-sampled litter popu- lation with the random-sampled adult population, it was possible to show a significant difference in banding pattern between the two populations. The percentage of relatively unhanded individuals (A and B) was higher in the adult population than in the litter popu- lation. The observed significant differences between the young and adult populations can be accounted for only by differential elimination of pattern types or by pattern changes in the individual snakes. The evidence is overwhelmingly in favor of the former hypothesis, as snakes kept in the laboratory show no evidence of pattern change in ontogeny. In addition, individuals of all pattern types have been recorded from both adult and litter populations; only the frequencies differ. Evolutionary agencies other than selection are easily disqualified. A high proportion of unhanded pattern types might be maintained by migration from other unhanded populations, but the nearest such population is in Tennessee. To maintain the unhanded genotype by mutation alone would require a mutation rate far above that known for any locus ever studied in any organism, even if it is assumed that color pattern is a single factor trait ( which it is not ) . Nor will genetic drift account for the observed differences. There is no sign that the populations on the islands ever approach a size at which drift is 138 I The Process of Evolution Newborn young Differential mortality Native adults Occasional immigrants o ZD Q O cc Cl. UJ a: A A A A A A B B C D Changes in Populations 139 likely to be a significant factor. Seven collectors once captured 400 snakes on an island in 5 hours, and three collectors at another time took 234 snakes in 4 hours. The juvenile-adult shift is toward un- handed individuals on all the islands studied, indicating a system- atic pressure rather than random drift. Therefore, by a process of elimination, selection seems most likely to be responsible for the change in pattern-type frequencies. To prove that selection has taken place, one does not have to dis- cover its mode of operation. However, it is interesting to speculate on the factors producing the observed situation. To the human eve, unhanded snakes are cryptically colored on the flat limestone rocks of the island peripheries and banded individuals are very conspicu- ous. It is likely that banding would help to break up the outline of the snakes in their more typical, less uniform swamp habitat. There are visual predators present which will eat snakes (gulls, herons, hawks, etc.), and man kills many with firearms. The selective force is obviously very strong. If the pattern spec- trum is divided arbitrarily into two halves (banded and unhanded), the snakes in the banded half have only about 25 percent of the chance of survival of the unhanded half ( 5 for the banded phenotype equals approximately .75). This raises the question why, with such heavy selection, any banded individuals at all remain in the popu- lation. The answer appears to be that migration brings a steady in- flux of genes for banding into the gene pools of the island popula- tions. Snakes have been observed swimming far from land on many occasions, and the distance from the shore to the islands is not too great to be spanned by migrating individuals. Thus the mainland populations form a reservoir of banded individuals, some of which periodically reach the islands. The resultant interaction between selection and migration has produced a situation unusually amen- able to analysis. Chromosomal Polymorphism in Drosophila Classic examples of microevolution are found in the work pioneered by Dobzhansky on inversion frequencies in some 30 species of Dro- sophila that show polymorphism in chromosome type. This type of Fig. 7.5 I ( see opposite page ) Natural selection in water snakes on the islands of Lake Erie, as shown by the differential in frequencies of banding types in young and old snakes. 140 I The Process of Evolution N=212 100 90 80 70 60 50 40 30 20 10 N=2U Fig. 7.6 I Comparison of Natrix sipedon litters and adults from the Bass complex of islands in Lake Erie. Ordinate, percent in class; abscissa, banding type. A, least banded; D, completely banded. ( From Camin and Ehrlich, 1958, Evolution 14.) investigation is possible in Drosophila because of the giant polytene chromosomes found in the saHvary glands of the larvae of these fruit flies. These chromosomes show the close pairing usually associated with the zygotene stage of meiosis. Their size and this somatic pair- ing make them very useful tools for research. Inversion heterozy- gotes in Drosophila may be detected by examination of the salivary chromosomes for the characteristic inversion loops (Fig. 3.2). Vernacular names have been given to the different sequences of the banding in the very variable third chromosome of D. pseudo- obscura. The three most widely discussed inversions in evolutionary literature are Standard (ST), Arrowhead (AR), and Chiricahua ( CH ) . When two different kinds of chromosomes occur in the same population, three types of individuals will be present: two inversion homozygotes and one inversion heterozygote. For example, where Changes in Populations 141 only Standard and Chiricahua chromosomes are present, the three different kinds of individuals are the two homozygotes ( ST/ST and CH/CH) and the heterozygote (ST/CH). The homozygotes can be distinguished only by progeny testing or careful study of the pattern of banding, while the heterozygote is easily recognized by the in- version loop. The different kinds of chromosomes in D. pseiicJoohsciira have different geographic distributions, apparently because of their vary- ing adaptive values in different habitats. Figure 7.7 shows the geo- Fig. 7.7 I Frequencies ( in percent ) of different kinds of chromosomes in populations of Drosophila pseudoobscura in southwestern United States. Black columns, Standard; white columns, Arrowhead; hatched columns. Pikes Peak. {From Dobzhansky, 1947, Evolution 1.) 142 I The Process of Evolution graphic variation in frequencies of three different chromosome types in a series of locaHties transecting the southwestern United States. The chromosomal frequencies also change with altitude, as can be seen in Fig. 7.8, which shows the proportions of three difiFerent types at different elevations in the Sierra Nevada of California. Super- imposed on this geographic variation is a seasonal variation in fre- quency. For instance, in the Sierra Nevada, the Standard gene arrangement is commonest at lower elevations, becoming pro- gressively less common with increasing altitude. Arrowhead, on the other hand, has a frequency that is positively correlated with alti- tude, being greatest in the subalpine zone (about 10,000 feet) and least at the base of the mountains (850 feet). However, in general. Standard chromosomes tend to increase in frequency as the season progresses, whereas Arrowhead chromosomes tend to decrease in frequency. Thus the frequencies of these chromosomes in late sum- mer populations at high elevations tend to approach the frequencies of spring populations at lower elevations. These changes may best be explained as the result of rather strong selection pressures. Dobzhansky has tested this hypothesis in Fig. 7.8 I Frequencies of different kinds of chromosomes in populations of Drosophila pseudoobscura at different altitudes in the Sierra Nevada. Black columns, Standard; white columns, Arrowhead; hatched columns, Pikes Peak. The areas of the squares are proportional to the fre- quencies of chromosome types; ordinate, elevation in feet; abscissa, horizontal distance in miles. The elevation of a locality is indicated by the position of the base of the lowermost square. ( From Dobzhansky, 1948, Genetics 33.) 10,000 5,000 Changes in Populations | 143 a series of experiments in which he reared D. pseiidoobsctira in population cages. His experimental populations were started with known frequencies of chromosome types, and then he repeatedly sampled them to determine what changes in frequency, if any, had occurred. He found that in populations maintained at 16.5 °C there was no change in the chromosome frequency. However, in popula- tions maintained at temperatures above 20 °C the frequencies change, usually arriving at an equilibrium point at which all the original types are still present but in frequencies quite diflFerent from the initial ones. The data from one population cage experiment are given in Fig. 7.9. The cage colony was constituted on March 1, 1946, with indi- viduals selected so that the population had 10.7 percent ST and 89.3 percent CH chromosomes. Throughout the remainder of the year the percentage of ST chromosomes increased, at first rapidly and then more slowly, until at the end of the year ( some 15 genera- tions later) it had leveled off at about 70 percent. This pattern of increase and the establishment of an equihbrium strongly suggested a selective advantage of the structural heterozygotes (ST/CH) over both homozygous types. From the standpoint of population genetics, this would be comparable to overdominance for fitness at a single locus. Similar results could be obtained, however, by negative assortative mating (unlikes mating). This latter possibility was ruled out by taking a sample of eggs from the population cage and raising the larvae under optimum conditions so that almost all survived. The different genotypes proved to be present in the expected Hardy- Weinberg frequency, demonstrating that mating was random and that differential fecundity or fertility was not involved. Samples of adult flies taken from the population, however, showed the following deviations from Hardy-Weinberg frequencies: ST/ST ST/CH CH/CH Observed number 57 169 29 Expected number 78.5 126.0 50.5 Deviation -21.5 +43.0 -21.5 These results, along with the establishment of equilibrium at about 70 percent, clearly indicate that there is differential ehmination of the homozygous types between the egg and adult stages. It has been demonstrated that in eggs laid by wild flies there is no signifi- cant deviation from the expected Hardy-Weinberg frequencies but 144 I The Process of Evolution that in adult males heterozygotes are significantly more common than one would expect in a population in Hardy-Weinberg equilib- rium. An extreme case of selective advantage of structural heterozygotes occurs in a population of Drosophila fropicalis where there are two common chromosome sequences. Both types of structural homo- zygotes die early in development, and only the heterozygotes sur- vive to breed. Although only half of the zygotes formed are viable, this population flourishes. Long-term changes in inversion frequencies have been reported. In California before 1941 only four Pikes Peak (PP) chromosomes were found among 20,000 chromosomes studied. In 1957 PP chromo- somes were found in all 10 localities sampled in the state, the mean frequency being about .08 (a 400-fold increase). This increase occurred at the expense of CH chromosomes. The agent behind the change is not known, but it is almost certainly selection. Since 1941 the PP chromosomes have continuously predominated in popu- lations found on the eastern slope of the Rocky Mountains and in Texas. The California increase could not be due to migration from this reservoir of PP chromosomes, as geographically intermediate populations in Arizona and Utah did not change significantly be- tween 1941 and 1957. The source of selection pressure is not known, but widespread drought and increasing smog are two of the more obvious factors which might be considered. The above is only a brief outline of some of the highly interesting work done on the genetics of natural populations of Drosophila. There can be little doubt that the different gene constellations or supergenes in the inversion chromosomes (remaining together as rather stable blocks because of the effect of the inversions in sup- pressing the results of crossing-over) have different adaptive values under different conditions. Many examples of chromosome fre- quency changes correlated with environmental changes have been elucidated. Less success has attended attempts to determine when advantages and disadvantages occur in the life cycle. Describing the exact nature of these adaptive changes also has proved diflBcult. It is impossible to reproduce natural conditions in the laboratory, and very little is known about the life history and ecology of the various Drosophila species. Studies of D. pseudoohscura and D. persimilis breeding in "slime fluxes" (yeast- and bacteria-infected sap exudations on trees) in the Sierra Nevada, and laboratory studies of many species, have yielded valuable information. Much more work is needed. The complexity of the problem may be ap- preciated by considering the factors determining pupation site in Changes in Populations 145 Fig. 7.9 I Frequency of Standard-type chromosomes ( in percent ) in different months in a single population cage. (From Dohzhansky, 1947, Evolution 1 . ) Mar Apr May June July Aug Sep Oct Nov Dec D. mclanogaster, a characteristic known to be of considerable selec- tive importance in laboratory populations. Among other things, temperature, humidity, moisture content of the medium, larval density, and length of larval period all affect the place in which the larva chooses to form the pupa. The ecogenetics of Drosophila will supply a fertile field for research for a long time to come. Examples from Man As one might expect, natural selection plays an active part in shap- ing the genetics of populations of our own species. An outstanding example of this is the selective control of the frequency of the so- called "sickle-cell" gene. Individuals who are homozygous for this recessive gene show distortion of their erythrocytes, accompanied by severe anemia and general, serious and painful disabihty. The condition is usually fatal. Heterozygous individuals may be detected by the distortion (sickling) of their red blood cells, which occurs when the oxygen concentration of the blood is reduced. Sickle-cell heterozygotes, however, apparently are protected to some degree against malaria and thus are favored by selection in malarial areas. Allison found that the frequency of sickle-cell heterozygotes was 146 I The Process of Evolution higher in adults than in young children, indicating that this genotype is at a selective advantage. The advantage of the heterozygotes is about of the magnitude theoretically necessary to maintain the fre- quency of the sickle-cell gene (about .20). This advantage appears to be responsible for the maintenance of a balanced polymorphism at the sickle-cell locus throughout much of Africa, in spite of the very low viability of the homozygous recessives. Glass and his coworkers studied the frequencies of genes con- trolling a number of characteristics (blood groups, mid-digital hair types, etc.) in a "Dunker" religious community in Pennsylvania where the population was less than 100 individuals. The gene fre- quencies at several loci deviated strongly from those found in the surrounding population and in the population from which the group was originally derived (western Germany). Drift is tentatively con- sidered to be responsible for these deviations. Interestingly, there was little or no deviation at difiFerent loci known to be under rather strong selection (e.g., Rh). The one known example of selection against heterozygotes in- volves the Rh locus in man. When an Rh negative mother (double recessive) is fertilized by an Rh positive sperm, the resultant hetero- zygous fetus runs a high risk of death due to antigenic incompatibil- ities between it and the mother. This selection is apparently com- pensated to some degree by a tendency of families with Rh problems to have repeated pregnancies until a number of children are raised successfully. This, however, is not sufficient in itself to account for the continued presence of the polymorphism in the human popula- tion. The gene-frequency equilibrium point, although no longer at .5 ( see Chap. 6, selection against heterozygotes ) , is still unstable. It is possible that migration has helped to prevent fixation in the human population as a whole. Fixation of the Rh positive genes may be approached in some subpopulations, and fixation of Rh negative in others. Intermixing of these populations may then reestablish the polymorphism. Pasture Plants Unfortunately botanists have found few situations in nature that are amenable to the sort of analysis discussed in the foregoing cases. Kemp in southern Maryland studied pasture seeded with a grass- legume mixture and subsequently partitioned. One half was pro- tected from livestock, while the other was used for grazing. Three years later, plants of bluegrass (Poa pratensis), orchard grass {Dacfylis glomerata), and white clover (Trifolium repens) from each half of the pasture were dug up and transplanted to an experi- Changes in Populations 147 mental garden with uniform conditions. It was found that the grazed half yielded a high proportion of genotypes that produced a low prostrate growth. Those from the ungrazed half were erect and showed no tendency to procumbency. This clearly shows that there was a heavy selection in these populations for adaptive growth forms. Mimicry of Flax An extremely complex system of mimicry has been found in the genus CameUna, plants of the family Cruciferae. Various types of Camelina occur as weeds in fields of cultivated flax ( Linum, family Linaceae). It has been hypothesized that, as the cultivation of flax became more efficient, Camelina was subjected to a series of in- creasingly severe selection pressures. For instance, there was a selec- tive advantage for the CameUna seeds to remain with the flax seeds during the winnowing process so that they would be sown along with the flax. Plants that grew were from seeds with the correct aero- dynamic properties; the other seeds were never planted. The more thorough the winnowing, the stronger the selection pressure. This selection produced Camelina seeds that mimicked flax seeds, not in appearance but in distance blown by a given amount of wind. Sim- ilarly, selection favored the production of tall spindly Camelinu plants that would not be shaded out of existence in the dense stands of cultivated flax. Such selection has produced flax mimics not only in the genus Camelina but also in other plants, including Spergula and Silene (Caryophyllaceae). It should be noted that the evidence for selection here is more inferential than in the preceding examples. Disruptive Selection in Mimetic Butterflies When selection favors two or more phenotypic modes, it is said to be disruptive. Experimental work with bristle numbers in Drosophila has shown that a pattern of selection in which extremes are favored over intermediates can produce a bimodal population with increased variance. Apparent examples of the operation of disruptive selection in natural populations are found in arrays of mimetic butterflies. For instance, the widespread and much-studied African swallowtail butterfly Papilio dardamts (Fig. 7.10) has a wide variety of mimetic females, although the males never show mimicry. Presumably this is because "normal" color pattern and wing shape of males are im- portant in making them sexually acceptable to the females. Several different forms of the females commonly occur in the same locality, each one accurately mimicking a different distasteful species of butterflies. 148 I The Process of Evolution There is evidence (summarized by Sheppard, 1961) that certain combinations of characteristics give the best mimicry of different models and are at a selective advantage. Others do not look like any model and are at a disadvantage. In at least some cases, selection seems to have reduced the possibility of the production of poorly protected combinations by increasing the linkage between the loci concerned in producing the pattern. This permits linked groups of loci (supergenes) to be selected as a unit and the superior com- binations to be preserved. (The phenomenon is quite comparable to the holding together of gene constellations in Drosophila popu- lations by inversions. ) In mimetic butterflies disruptive selection may also operate through the accumulation of modifier genes which further perfect the resemblance. This is supported by hybridization experiments in which supergenes are transferred to a new genetic environment. There they do not produce phenotypes that mimic the model as precisely as before. Resistance to Antibiotics and Insecticides No discussion of evolution would be complete without mention of the response of some organisms to man's attempts to reduce their population size or eradicate them. Striking and important examples of the response of natural populations to human endeavors center around the phenomenon of resistance. Indiscriminate application of insecticides to large areas of the earth's surface has constituted a very potent selective force. In the vast majority of cases, the large population sizes of pest insects have contained sufficient residual variability to allow these insects to develop strains resistant to virtu- ally all the compounds that the ingenuity of the organic chemists can produce. The chemists are, of course, limited severely by the survival requirements of nontarget organisms such as man. Even so, there is considerable evidence that man and his domesticated plants and animals have not escaped damage from powerful syn- thetic pesticides. It is interesting to note that insects have met the challenge in diverse ways. There have been examples of behavioral resistance in which insects no longer alight on sprayed surfaces and of many kinds of physiological resistance in which the penetration or action of the insecticide is prevented by various mechanisms. Parallel to insecticide resistance has been the appearance of strains of micro- organisms that are highly resistant to antibiotics. This has been caused by the overuse of these antibiotics by well-meaning doctors when other treatments might suffice or be better. So far, the increase Changes in Populations | 149 Fig. 7.10 I Mimicry involving an African swallowtail butterfly, Fap/Zio dardanus. Upper butterfly, male P. dardanus; right-hand column, three danaine butterflies (Danais chnjsippus, Amauris niavius, and Amauris echeria); left-hand column, forms of female P. dardanus mimicking the danaines. {After Punnett, 1915, Mimicry in Butterflies, Cambridge University Press. ) 150 I The Process of Evolution of these organisms has been countered largely by a scramble to find new chemical weapons to use against them, rather than by the application of methods that are biologically more sophisticated and infinitely more beneficial in the long run. Such alternative approaches are well known. Some resistant insects prove to be less viable than their nonresistant relatives when they exist in an environment free of insecticide. In these cases, moratona on insecticide applications would give time for the forces of natural selection to return the populations to their previously susceptible condition. By intelligent use of insecticides at critical moments, a reasonable level of control may be attained with a minimum danger of creating resistant strains (a danger maximized by "broadcast" application ) . Where moratoria are not feasible, multiple applications of many different poisons may make it impossible for the popula- tion simultaneously to develop resistance to all. Because it is not possible, however, to affect one member of a community without affecting the entire ecosystem, this approach is fraught with un- foreseen dangers. It is well known that accumulation and concentra- tion of chemical poisons take place in members of food chains. And, if the target organisms are not eliminated, their predators and the predator's predators may be severely affected. In addition to this, many pestiferous insects are more readily and economically controlled by interfering with their biology in a non- chemical way than they are by the application of insecticides. For example, the draining of swamps in which mosquitoes breed, the introduction of predators to control imported pests, the releasing of multitudes of sterilized males to compete with wild males for mates, and the dissemination of laboratory-grown pathogens have all proved to be effective. Similarly, classical methods of antisepsis will often deal with highly resistant microorganisms much more effectively than newer and costlier antibiotics. LABORATORY POPULATIONS The term artificial selection pertains to man's control of the geno- types that contribute to the gene pool of succeeding generations. Artificial selection is carried out by both plant and animal breeders and also by scientists wishing to study the effects of selection in the laboratory. It may have a purposiveness directed at a single trait, another respect in which it differs from natural selection, which operates on all the phenotypic characters affecting fitness and has no purpose. Nevertheless, even under the most carefully controlled laboratory conditions, natural selection still operates in conjunction Changes in Populations 151 with artificial selection. A great deal of work in artificial selection has been concerned with so-called quantitative characters. ( Quanti- tative characters are those influenced by many pairs of alleles at many different loci, such as height and intelligence in man, bristle number in Drosophila, color pattern in water snakes, etc. ) The work of Mather and Harrison is a rather typical example of artificial selection. In that case, selection was for changes in the number of abdominal chaetae in Drosophila melanogaster. The diagram of Fig. 7.11 summarizes the results of more than 100 gen- erations of selection for higher bristle number. For 20 generations, selection was extremely effective. At generation 20, reduced fertility and fecundity in the population made it necessary to discontinue selection in order to keep the strain from dying out. In the absence of selection over several generations, the population reached an equilibrium chaeta number higher than the original one but much lower than that achieved at the peak in generation 20. From the line in which selection was suspended, a new selected line was extracted at generation 24, and progress was rapid to a point near that Fig. 7.11 I Artificial selection for abdominal bristle number in Dro- sophila melanogaster. Ordinate, mean number of bristles; abscissa, number of generations; lines under selection are solid, lines not under selection are dashed; T indicates deliberate termination of a line, D that it died out through sterility; circled numbers indicate different lines. From line 1, all the selections were for low number of bristles, except that marked H which was for high number. ( From Mather and Harrison, 1949, Heredity 3. ) 25 30 35 Generations 60 152 I The Process of Evolution achieved with the first selected hne. When selection in this line was relaxed, there was only a slight regression in bristle number. In addition, after some 85 generations in the continuously selected line, further response was achieved. Artificial selection often produces rapid results at first; then a plateau is reached at which further progress is difficult or impos- sible, or the viability of the line reaches such a low ebb that either selection must be discontinued (relaxed) or the line is lost. Gener- ally, if selection is discontinued before a plateau is achieved, the relaxed lines regress toward the control level. If selection ends after the population has reached a plateau, there may be little or no re- gression. Continuous selection of a population that has achieved a plateau often will not produce appreciable results for long periods. However, if selection is continued long enough, progress once again may be made. One reason for these phenomena presumably is the balance be- tween artificial and natural selection. Although the details are neither clear nor uniform, it appears that natural selection must create a balanced system in which the best possible relationship of characteristics determining fitness is produced. In other words, fit- ness must be maximized. The available evidence seems to indicate that, especially in animals, a high degree of heterozygosity in the genotype produces a high degree of physiological fitness. It also seems likely that extremes of quantitative characters often are produced by a high degree of homozygosity at the loci concerned. Therefore artificial selection for high or low bristle number may well be countered by natural selection for fitness if the bristle-number extremes are produced by homozygosity at a series of loci. One might make a crude analogy to an airplane. One could try to improve the airplane by making the motor more powerful, but this would do little good if increased speed would tear off the wings. This problem might be solved by strengthening the fusilage or the structural members of the wings, but this would not help if it made the airplane too heavy to get off the ground. In an organism, as in an airplane, a viable balance of all the various factors that ensure successful functioning must be attained. There is a limit to how much one factor alone can be modified before the "working combi- nation" is seriously disrupted. Lerner (1954) has produced a mathematical model which might explain the establishment of a plateau below the maximum level of expression of a character under selection. He bases this model on a system in which there is an obligate level of heterozygosity de- termining fitness. Crossing-over can convert potential genetic varia- Changes in Populations 153 bility to free genetic variability, permitting further selection without loss of fitness. The whole situation can then be looked at in terms of shifting states of balance. Strong selection at one or a few loci places a stress on the balanced genotype. After the expression of the char- acter has been shifted a certain distance, this stress will result in loss of fitness, followed by either extinction or the attainment of a new balanced state. If selection is relaxed before either of these events, the line tends to regress to the control level. A tremendous volume of literature on artificial selection has ac- cumulated, as work on economic problems (improvement of do- mestic animals and plants, studies of resistance, etc. ) has produced information of great value. Much of our understanding of such di- verse problems as the origin of dominance, the integrative prop- erties of genotypes, and the efficacy of selection under varying conditions has been the direct or indirect result of investigations of such prosaic matters as egg laying in chickens, the weight of swine, rust resistance in wheat, the yield of corn and cotton plants, and the productivity of bovine mammary glands. The reader wishing a well- organized introduction to this vast and complex subject is referred toLerner (1958). GENETIC HOMEOSTASIS At this point it would be well to mention an important steady-state property of mendelian populations, the often-observed tendency of populations subjected to directional selection to regress toward the original mean. Lerner has called this phenomenon genetic home- ostasis. A mendelian population has characteristics above and beyond those of its component individuals. For instance, it would be mean- ingless to say that an individual is in Hardy-Weinberg equilibrium. Populations tend to retain a genetic composition that produces a maxi- mum number of individuals with a high degree of fitness. In short, there is selection in favor of maintaining the balanced "maximum-fit- ness" genotype. This is essentially a stabilizing selection operating against deviant individuals. A genotype showing a high degree of fit- ness is adapted not only to the environment in the classical sense but also to its genetic environment, that is, the gene pool in which the genotype occurs. In other words, the frequency and distribution of genes in the population help to determine the fitness of any genotype within the population. One may summarize the subject of genetic homeostasis by saying that selection organizes the gene pool of a population in such a way that its various components are coadapted and produce a maximum number of highly fit genotypes. Well- 154 I The Process of Evolution integrated genotypes are "winning combinations," and, as demon- strated in the experiments of Mather and Harrison, selection to change them in order to meet a particular environmental stress is countered by selection favoring the retention of the successful inte- grated unit. The unusually sharp break between the two kinds of Maniola jiirtina populations mentioned earlier in this chapter may represent the border between two such highly integrated units. Permanent directional progress is made only when the selective forces operating in favor of change are able to overbalance those operating in favor of retaining the successful combination. There is much to indicate that the phenomenon of overdominance with re- spect to adaptive value (selective advantage of the heterozygote over homozygotes) is one of the fundamental mechanisms contrib- uting to genetic homeostasis in most animals and many plants. How- ever, the term includes all methods of genetic autoregulation of populations. GENETIC ASSIMILATION When individuals of the plant Achillea lanulosa (Compositae) were transferred from localities at various altitudes in the Sierra Nevada to an experimental garden at sea level, the plants did not all grow to a uniform height (Fig. 7.12). In the now classic experiments of Clausen, Keck, and Hiesey, plants from the higher elevations were much shorter than those from lower elevations. Since all were grown under roughly identical conditions, those from the higher localities were shown to be genetically dwarfed; that is, they had genotypes that tended to produce short individuals regardless of the environ- ment in which they developed. However, when low-altitude plants were divided (giving genetically identical stocks) and these divi- sions were grown at sea level and at mid-altitude, the mid-altitude individuals were shorter than their identical twins at sea level. In other words, the low-altitude genotype interacting with mid-alti- tude environment developed into a plant similar to those with a mid-altitude genotype. Such forms, in which a phenotypic change simulates a genotypic change, have been termed phenocopies. There seem to be many situations in nature where such pheno- copying occurs, although rigorous demonstrations of the phenomenon generally are lacking. For instance, many butterflies have spring generations that are smaller and darker than their summer genera- tions, the difference presumably being due to the seasonal variation in the environment. However, in more northern parts of their range, the butterflies have only a single summer generation which is small Changes in Populations 155 1^ C J£ ^ C t « -fi .2 3 o a: - 0) '^■•^ > .5 5i 1-1 ^ c ^ ° • •" 4-1 ■'-' f-S J5 "S -S ^ Oj Ji „ < a ^ O ^ - c ^ ^ S ^ id "I ". t: ^*^ ■g ^ bC 2 ^ ^ ir ti S fij ^ C^r, 3 '^ tn 03 OJ ^ C ^ fee ,a: ? O ^ « c OJ oo" S II « 2 O 3 t/2 j3 ;s 3 =?> ±i cb :3 5 *l 3 . « 3 3^ to:; • 3 CM ■" en 3 li) „ ■<-( CO — 3 tj_i 3 s ° ^ ^ z: jj 4^ O •c 5 ■! t £ •y. 4^ ^- « ^- ^ • 2 r* 3 Im -ti 3 •- O 3 ri 'g X! ^ i~i J-H — -^H _« i^ 3 1^ 156 I The Process of Evolution and dark and resembles the spring generation in southern locaHties. In the northern populations the individuals are presumed to have genotypes that produce the dwarfing and darkening. Although the critical transfer experiments have not been done, the greater con- stancy of the northern forms in the face of environmental changes supports these presumptions. How can one account for the development of the high-altitude races of plants, the high-latitude races of butterflies, and similar phenomena? The environmentally produced changes cannot be directly transmitted to succeeding generations; the Lamarckian idea of the inheritance of acquired characters has long been dis- carded. A method by which such acquired characters could, through selection, become assimilated in the genotype has been proposed by Waddington and is supported by a series of experiments by Wad- dington and others. This genetic assimilation is best explained by a brief example. A strain of a wild-type laboratory population of Drosophila melanogaster was subjected to a high-temperature shock during the pupal stage. A few of the adults emerging from the treated pupae showed an abnormal break in the veins of the wings (the "cross- veinless" phenotype). Only those individuals showing the acquired crossveinless phenotype were used as parents for the next genera- tion. After more than a dozen generations, the frequency of cross- veinless flies from treated pupae was over 90 percent, and a few crossveinless flies began to appear from untreated pupae of the selected strain. Crossing these latter flies produced strains that had a high frequency of crossveinless phenotypes in the absence of heat shock. It would appear that an acquired character had become heritable. Actually, in this experiment, selection seems to have fa- vored those genotypes that had a low threshold for producing the favored phenotype. Eventually the threshold was lowered to a point at which no heat shock was necessary to move the developmental sequence to the crossveinless end point. Similar results have been obtained in studies of other characters in Drosophila. It is most important to remember that the range of possible viable phenotypes is genetically determined and that selection may alter this range so that phenotypes that previously could be induced by the environment become genetically fixed as the only possible result of gene-environment interaction. Selection may favor phenotypic plasticity where butterflies face different environments in succeed- ing generations. It may operate to produce a highly canalized de- velopment leading to a "winning" phenotype in situations where the Changes in Populations [ 157 environmental stresses are highly similar, generation after genera- tion. Developmental aspects of genetic assimilation are discussed in Chap. 4. ADJUSTMENT TO THE ENVIRONMENT The diverse and ingenious ways in which organisms meet the prob- lems of survival and reproduction are inferential evidence for the great efficacy of natural selection. This adjustment to the environ- ment is usually called adaptation, but for reasons discussed in the final chapter this ambiguous term is avoided whenever possible. One need not go into the details of the evolution of the bird's wing, the giraffe's neck, the vertebrate eye, the nest building of some fish, etc., as the selective origins of these and other structures and of behavioral patterns may be assumed to be basically the same in out- line as those, such as industrial melanism, which have alreadv been discussed. Even a slight advantage or disadvantage in a particular genetic change provides a sufficient differential for the operation of natural selection. Thus the property of light sensitivity of unicellular organisms provides a starting point for the development, through selection, of the highly complex eyes found in vertebrates, insects, and certain mollusks. The old antievolutionist argument that the vertebrate eye would be useless unless present in its modern complexity is nonsense. Many organisms with less complex and less specialized photore- ceptors put them to good use, and it is easily seen that even a human being would be better off with a non-image-forming photo- receptor (one which gave information only on the amount of light present) than without any photoreceptor at all. Similarly, any non- detrimental variation in a highly edible butterfly, tending to make it look more like a sympatric distasteful species, puts this deviant at a selective advantage. The presence today of all degrees of refinement in the phenomena of mimicry and protective coloration argues strongly against the hypothesis that such resemblance must be virtually perfect before it is effective. Experimental evidence at hand indicates that less per- fect copies of certain distasteful model butterflies also enjoy a degree of protection, though perhaps not as great as that of the more nearly perfect mimics. It is interesting to note that mimetic forms of various butterflies generally do not occur in areas where the models are absent, indicating that a selection pressure favoring mimicry is required to prevent regression to the wild type. Even 158 I The Process of Evolution such difRcult-to-explain phenomena as the evolution of social be- havior in bees are now yielding to investigation (e.g., Michener, 1958). This problem is complicated because the unit of evolution is the colony, not the individual. ( Most members of a hive are non- reproductive. ) Selection in honeybees consists largely of differential reproduction of colonies rather than of individual genotypes. Loss of features when they no longer confer selective advantage is one of the most widely observed evolutionary phenomena. Se- lectively neutral characters presumably are rare. The eye, very useful to most animals, may become an easily injured, infection- prone liability to a cave fish. Body hair, which at one time protected human beings against cold and injury, became a happy hunting ground for lice with the invention of clothing. SUMMARY In this chapter has been given a series of examples of studies of evolutionary changes within populations, chosen for the diversity of approach and material. In addition, inferential evidence bearing upon the efficacy of the selective process is discussed. It becomes apparent that, although it is relatively simple to demonstrate changes, it is much more difficult to partition the responsibility for the changes among the various evolutionary forces. The problem is especially complicated because of complex interactions within genetic mechanisms and developmental systems. Most difficult of all to document is the role of genetic drift. It is nearly impossible to "prove" the efficacy of drift in natural populations, as one can always hypothesize the existence of some as-yet-to-be-discovered selection pressure that could account for the observed phenomena. Indeed, there have been several cases, especially in snails, where differences at one time attributed to the action of drift have been demonstrated to be caused by selection. However, the inevitability of drift, to- gether with results of studies of gene frequencies in small popula- tions of Drosophilo, both in nature and in the laboratory, leads one to believe that drift, interacting with the other pressures, can be an important evolutionary force. Extensive studies still are needed on a wide variety of organisms before broad generalities on the rela- tive contribution of the various forces can be made with real con- fidence. REFERENCES Allison, A. C. 1959. Metabolic polymorphisms in mammals and their bearing on problems of biochemical genetics. Am. Naturalist 93: Changes in Populations 159 5-16. Summary paper with extensive bibliography. (Allison, 1956, is the central reference on sickle-cell anemia.) Camin, J. H., and P. R. Ehrlich. 1958. Natural selection in water snakes {Natrix sipedon L.) on islands in Lake Erie. Evolution 12: 504-511. This is the principal paper on selection in N. sipedon. Clarke, B. 1960. Divergent effects of natural selection on two closely related polymorphic snails. Heredity 14: 423-443. The bibliography lists most of the important papers on Cepaea, including Sedlmair (1956). Dobzhansky, T. 1951. Genetics and the Origin of Species. Columbia Univ. Press, New York. The classic work on genetics and evolution in animals. A "must" for every serious student. The early work on chromo- somal polymorphism in Drosophila is summarized here. . 1959. Variation and evolution. Proc. Am. Phil. Soc. 103: 252- 263. The bibhography of the paper will introduce the reader to more recent evolutionary literature on Drosophila. The number of the Pro- ceedings containing this paper has many articles of interest to the evolutionist. . 1960. Evolution and environment. In Sol Tax [ed.], Evolution After Darwin, Univ. Chicago Press, Chicago, vol. 1, pp. 403—428. Con- tains a good discussion of drift in Drosophila. Genetics and 20th Century Darwinism. 1959. Cold Spring Harbor Svm- posia on Quantitative Biology, vol. 24. See especially Lamotte's paper on Cepaea for his side of the drift-selection controversy, and the in- teresting papers by Carson, Dobzhansky, and Mayr. Lerner, I. M. 1954. Genetic Homeostasis. Wiley, New York. Highly theoretical and well worth reading (partly out-of-date) . . 1958. The Genetic Basis of Selection. Wiley, New York. This is a comprehensive source on artificial selection. Michener, C. D. 1958. Distinctive type of primitive social behavior among bees. Science 127: 1046-1047. Evidence of the origin of worker-queen differentiation. Sheppard, P. M. 1959. Natural Selection and Heredity. Hutchinson, Lon- don. An excellent httle book covering much of the material in this chapter. . 1961. Some contributions to population genetics resulting from the study of the Lepidoptera. Advances in Genet. 10: 165-216. See this for further information and references on the Lepidoptera exam- ples and a guide to the literature on disruptive selection. Stebbins, G. L. 1950. Variation and Evolution in Plants. Columbia Univ. Press, New York. The plant equivalent of Dobzhansky's Genetics and the Origin of Species. Summary and references for all plant examples in this chapter— and much more. Waddington, C. H. 1961. Genetic assimilation. Advances in Genet. 10: 257-293. Summary and discussion of the work and literature. 3 genetic systems I The cytogenetic mechanisms discussed in the preceding chapters provide for the production of offspring sufficiently similar to the parental types that, barring unduly rapid environmental change, the group survives. They provide also for sufficient variability that, when the environment changes gradually, the organisms change also and thus survive. The persistence of an organism depends on the proper balance between these two phenomena: fitness for the im- mediate environment and fitness, in the long-range view, for what- ever changes take place in the environment in the course of time. It has already been pointed out that the genetic mechanism of most organisms provides for storing variability in unexpressed form, as well as for regulating the release of this variability. Diversity of means of storage and release of variability, which, like other traits, must be under genetic control, implies that these means change in the course of time; in other words, they evolve. The collective ways in which the amount and type of new gene combinations are con- trolled may be referred to as the genetic system. Thus one may speak of the evolution of genetic systems, which is the evolution of those mechanisms effecting and affecting variability. In this chapter the various sorts of genetic systems found in plants and animals will be described. In the following chapter sev- eral specialized genetic systems which have become important in certain organisms will be discussed in greater detail. Elucidating the interrelationships and ultimate significance of these systems and integrating them into evolutionary theory are among the greatest challenges facing the modern evolutionist. Although the actual course of evolution of genetic systems is not known, there has been considerable speculation as to the main lines that it may have taken. Here genetic systems will be discussed, starting with the simplest and concluding with the most complex. This does not indicate an evolutionary sequence. The simplest bio- logical phenomena often can be interpreted as reduced to this state from something more complex. GENETIC SYSTEMS IN MICROORGANISMS Transformation Transformation is a phenomenon involving genetic change in some bacteria. For example, studies of strains of pneumococcus bacteria (Diplococcus) have shown that the DNA extracted from a strain | 161 162 I The Process of Evolution with a particular trait can transform a strain lacking this trait into one that possesses it. Virulence in pneumococcus depends on the polysaccharide envelope of the bacterial cell. If a nonencapsu- lated, nonvirulent strain is grown with purified DNA extract from a virulent strain, virulent cells with a capsule will develop in the culture. Thus the DNA determines the polysaccharide coat. Trans- formation has been achieved for a large number of traits, and there is an equal probability of transformation in either direction. Linkage of traits in the DNA material also has been found, since in experi- ments involving differing traits double transformations occur more frequently than would be expected for independent events. Recombination in Viruses A rather complex relationship exists between certain bacteria and bacterial viruses known as bacteriophages. A single phage consists of two major structural elements, a tail and an enlarged ( often hexag- onal) head. A protein coat surrounds a DNA core. When a bac- terial cell is infected, the phage attaches itself by the tail to a specific receptor site on the cell wall. During the course of the next few minutes the DNA leaves the phage and enters the bacterium. Within the bacterium, the phage multiplies and the cell eventually breaks open (undergoes lysis), releasing the new phages. During the period of multiplication, new phage DNA is synthesized and new phage protein formed, the result being a hundred or so new phages of the same type as the infector. The original infector has, with its DNA, managed to preempt the synthetic processes of the bacterium and turn them to its own use, that of duplicating the phage. Since the beginnings of their scientific study, phages with many different traits have been found. It is possible to infect a culture of bacteria with a mixture of phages with different traits. When this is done, recombination of traits occurs in the phage progeny. With some characteristics, recombinants appear with equal frequency; with others there is a reduction in the number of recombinants, in- dicating that linkage exists between the genetic factors affecting the traits. What has happened then is recombination of the genetic material of the phage in the host cell of the bacterium. Radioactive- tracer studies have shown this genetic material to be the DNA. Such evidence, taken with that from experiments with bacterial transformation, in which the transforming agent is DNA, shows that in these forms the genetic material is DNA. It also shows that, in the Genetic Systems I ' 163 absence of sexual reproduction, recombination of the genetic ma- terial can occur during the synthetic processes producing more DNA. Transduction The bacteriophages discussed previously are virulent, and the host bacterium is killed as the cell undergoes lysis. However, there are also temperate phages capable of establishing a sort of symbiotic relationship which need not result in lysis of the host bacteria. These are called lysogenic bacteria (for example. Salmonella). The noninfectious stage of the temperate phage is called prophage. Re- production of the prophage and bacterium is so regulated and inte- grated that there is no detrimental effect. Occasionally, however, at a rate of 10 "- to lO"-" per generation, lysis of a bacterial cell occurs and the phages released are able to adsorb on other bacterial cells. In the course of DNA synthesis, the prophage may incorporate some of the genetic material of the host bacterium. If the prophage should convert to the phage stage, the host will lyse. When the phage is released and infects a new bacterium, it carries with it genetic material from the former host. This in turn has a chance of being incorporated into the genotype of the recipient bacterium, a phenomenon known as transduction. The genetic material trans- ferred usually is not a single genetic factor but groups of factors. Transduction thus is a special sort of genetic recombination, again involving not the sexual process but an infectious process. The phage acts as a vector for infectious transfer of bacterial genes. Viruses, including phages, with RNA are also known, but their study is just beginning. Sexual Recombination in Bacteria Sexual recombination also occurs in bacteria. Tatum, Lederberg, and others have shown that in Escherichia coli conjugation of cells may take place. Genetic material is transferred from one cell to an- other, one of the conjugating cells being a recipient cell, the other a donor. Recombination of traits from diflFerent strains has been studied in some detail. The process of conjugation can be inter- rupted by separating the cells by agitation, and it can be shown that the amount of genetic material transferred is proportional to time. The genetic material of the bacterium behaves as if it were on a single chromosome. Linkage maps of this chromosome have been 164 I The Process of Evolution made. Spontaneous breakage of the transferring chromosome also occurs. Hybrids of E. coli and Shigella dysenteriae, as well as of E. coli and Salmonella typhimurium, have also been studied. Microbial Genetics and Evolution The occurrence and significance of such phenomena as transforma- tion, transduction, and sexual recombination in many microorganisms in nature are unknown. The extent to which these processes are distributed among microorganisms also is largely unknown. It is obvious that population genetics of these forms is apt to be rather different from that known in larger organisms. The possibility of some sort of "genetic" relationship between very small replicating systems and larger organisms is only beginning to be studied. Most of what has been observed in this category has been termed infection. At this level, of course, the distinction between genetics, infection, and development breaks down. Many viruses, particularly the RNA viruses of plants (which may be transmitted by sucking insects), produce morphological effects similar to those controlled by chro- mosomal genetic material. Some strains of Parameciiwi are known as killer strains, since they secrete into the medium in which they grow a substance poison- ous to sensitive strains of Paramecium. It has been found that killer- strain individuals contain particles of DNA called kappa particles. In order for kappa particles to be maintained in the cell, the geno- type of the strain must contain a dominant nuclear factor K. In the course of asexual fission the kappa particles reproduce in such a way that all members of the resulting clone contain the particles. Should the rate of cell reproduction exceed that of particle reproduction, KK individuals lacking kappa may appear. Such individuals are not killers, but sensitives, and can regain kappa particles only by con- tamination from a cellfree suspension of killer animals. They are not able to initiate the formation of particles despite their chromo- somal genotype. Sexual reproduction in Paramecium involves conjugation of cells and cytoplasmic exchange, during which kappa particles may or may not be transferred. In the cross between KK and kk individuals, the resulting Kk paramecia will be killers if conjugation has lasted long enough that kappa particles are transferred. Paramecia that are homozygous for the recessive factor k may inherit kappa par- ticles in the cytoplasm, but these are lost during subsequent genera- tions of fission. It is obvious that there is a resemblance between this situation in Genetic Systems I 165 Paramecium and the more variable ones in bacteria and their asso- ciated viruses. From the kappa-particle inheritance to the inheri- tance of differences in chloroplasts in plants is but a short step. Chloroplasts are bodies with genetic continuity in the cytoplasm which may mutate to a different form or color. The variant may then continue to reproduce the altered form. Portions of a plant may be affected, producing a mosaic of yellowish or white areas inter- spersed with the usual green. Chloroplasts are inherited in the cytoplasm of the egg but not through the pollen. Some mutants are initiated and maintained by chromosomal gene changes; some are initiated by nuclear change but continue in the absence of the chromosomal factor; finally, there are plastid mutants which ap- pear to be independent of nuclear control. If other cellular com- ponents, such as mitochondria, could be studied as easily as plastids, perhaps they would be found to exhibit the same behavior. It should be noted that virus infection of plants may simulate the same sort of mutations of plastids that occur spontaneously or can be induced by irradiation with X rays or ultraviolet light. This digression into what is usually termed cytoplasmic inheri- tance will serve to illustrate the difficulty of drawing distinctions between nuclear and cytoplasmic inheritance and between genetic recombination and infection at the level of cell and microorganism. It should be reiterated that the evolutionary significance of such phenomena as cytoplasmic heredity and infection is largely un- known, but their importance can scarcely be doubted. There are indications from work with both plants and animals that what have been regarded as genetic traits are the effects of viruses or that genetically determined characters are mediated by the presence of viruses. When these processes are considered in relation to the role of microorganisms in the ecosystem or community, very important relationships may emerge. Bacteria and other microorganisms may be involved in the transmission of genetic information in the eco- system in diverse ways, in addition to their role as reducers or decomposers. GENETIC SYSTEMS OF OTHER ORGANISMS The basic features of the genetic systems in the vast majority of living organisms are the same. There is an alternation of haploid and diploid phases, the result of alternating haplosis ( meiosis ) and diplosis ( syngamy ) . In the few viruses and bacteria that have been studied, the genetic system is simpler and more variable. Only in 166 I The Process of Evolution the blue-green algae ( Cyanophycophyta ) is recombination un- known. In their rather specialized habitats the blue-green algae seem to be a successful and widespread group. One is led to suspect that they must have some mode of genetic recombination at present not detected. Perhaps it is related to that in bacteria, and possibly it involves viruses as well. Sexuality and Diploidy The phenomenon of sexual reproduction is so widespread and its evolutionary significance is so immediately apparent that most bi- ologists place its origin very early in the evolution of life. In many groups of plants and animals, every individual produces both female and male gametes. Even so, cross-fertilization is frequently the rule. Where the organisms are not hermaphroditic, the nature of sexual development may be affected by environmental factors such as light intensity, photoperiod, temperature, chemical composition of the medium, etc. It would seem that, in the course of evolution, this rather variable sex determination is replaced with more precise con- trol systems or, at least, have such imposed upon them. The first question to be considered is the origin of diploidy. It is usually assumed that diploidy has high selective value because of the opportunity it provides for the storage of recessive genes and thus of variability. There seems to be little doubt that diploidy also pro- vides necessary buffering in development and thus greater freedom from environmental effects. One would expect that this would be increasingly important as the complexity of organisms increases. Fusion of free cells occurs spontaneously in tissue cultures and cul- tures of unicellular organisms with such frequency that it is not difficult to imagine the origin of syngamy. Perhaps the resulting buffering effect in the diploid cell would have immediate selective value. In many instances, fusion of cells leads to instability which is re- solved by division. It was suggested in Chap. 1 that in early proto- organisms a sort of protorecombination might have taken place. Since a great many of the simpler algae and fungi are haploid for most of their lives (the only diploid cell being the zygote which immediately divides), one might conclude that recombination oc- curring with division immediately after syngamy had high selective value. Sexual reproduction is a complicated process having many com- ponents that must be integrated in function. The stages in its evo- lution are not known. With the completely unstable genetic Genetic Systems I | 167 mechanism of protoorganisms, each "individual" would presumably have been difiFerent from all others. Fusion would occasionally com- bine complementary "genotypes," and this may have been the foun- dation of a selective advantage of fusion. On the basis of the speculations presented in Chap. 1, it may be suggested that the stabilization of the genetic mechanism of early protoorganisms in- volved loss of superfluous genetic material or its assumption of new functions; association of the DNA nucleotides with protein mole- cules to form chromosomes; and the restriction of gene function so that expressivity became less variable and control more precise. The variety of genetic systems in plants and animals includes manv bi- zarre and unusual phenomena. Their common features suggest that, in nearly all instances, the advantages of diploidy and of recombina- tion have been combined. Meiosis of the nuclear genetic material and fusion of cells are combined in a life cycle of varying degrees of complexity. This combination appears to have arisen inde- pendently in a number of diff^erent ways in plants and animals. Stebbins has concluded that the wide occurrence of haploidv in the flagellates and filamentous algae is a result of their short and simple development and their rapid rate of reproduction, which makes the establishment of complex gene-developmental systems less important. The selective value of buffering and long-term storage of recessives would also be less important. With increasing complex- ity have come increased length of the developmental period and concomitant lengthening of the life cycle. The build-up of inte- grated gene complexes with ontogenetic buffering and genetic home- ostasis is thus favored, and the diploid state has high selective value. Diploid Life Cycles and Alternation of Generations Some groups of Protozoa are predominantly diploid, with complex mating behavior effecting recombination. Certain groups of algae, notably some of the brown algae (Phaeophycophyta), diatoms (Bacillariophyceae), and some green algae (Siphonales), also are diploid during most of their life cycle. Meiosis results in the produc- tion of haploid gametes. The Metazoa, of course, ha\c tlic same sort of genetic system. Presumably this diploid life cycle arose independently several times from organisms with a predominantly haploid cycle. In addi- tion, there also arose in plants and in fungi life cycles that involve alternation of generations. In these forms there is a regular cycle of haploid individuals that produce gametes and diploid individuals that produce asexual spores. Gametogenesis takes place by mitosis. 168 1 The Process of Evolution Sporogenesis involves meiosis and resultant recombination. In some algae and fungi, the alternating generations are isomorphic (indis- tinguishable morphologically ) . With increasing complexity, there seems to be a tendency for reduction of the haploid gametophyte generation in proportion to the diploid sporophyte. Thus in most ferns the gametophyte is a small thallus, usually a centimeter or less in diameter, whereas the sporophyte may be quite massive. In gymnosperms and flowering plants the gametophytic generation is reduced to relatively few cells (the pollen grain is a male gametophyte) and the sporophyte is the conspicuous stage. In mosses, on the other hand, the gameto- phyte is the conspicuous stage. It is perhaps better to regard the bryophytes (mosses and liverworts) as a specialized offshoot of ancient terrestrial plants and not on the main phyletic line of the vascular plants. No process comparable to alternation of generations is known to occur in animals, with the possible exception of some Sporozoa. In other animals the haploid phase is represented by the gametes only. The Coelenterata have the so-called alternating generations of medusae and polyps, but these are morphologically, not cytologi- cally, different. The fungi, as a group, have a number of distinctive genetic sys- tems of interest here in so far as they shed light on the selective forces involved in the evolution of genetic systems. These highly specialized organisms are poorly understood cytogenetically. The oc- currence of somatic crossing-over and systems of multiple-mating types attests to the selective advantage of recombination. The water molds, Phycomycetes, which are filamentous and without cross cell walls, build up numerous haploid nuclei in the common cytoplasm of the filaments or hyphae. Within the mycelium of these fungi genetically different populations of nuclei may arise, producing the so-called heterokaryotic state. Mushrooms and toadstools (Basidiomycetes), the most complex of the fungi, and some of the sac fungi ( Ascomycetes ) show an inter- esting parallel with the evolution of diploidy in plants and animals. Haploid mycelia, often with specific mating types, develop in the soil. The cellular hyphae of different mycelia, coming into contact, may fuse. Eventually a mycelium results in which each cell has two haploid nuclei— one from each haploid mycelium— which do not fuse until reproductive structures are formed. This special sort of diploidy is known as dikaryosis. Just as diploidy is associated with developmental and structural complexity in plants and animals. Genetic Systems I | 169 dikaryosis apears to be requisite for great morphological complexity in the fungi. Recombination and Genetic Systems To recapitulate, when the many different genetic systems that have evolved are compared, certain common features stand out. Except where simple unicellular structure and rapid reproduction are found, it seems that diploidy has had selective value. Simple, rapidly re- producing organisms have great flexibility for these reasons alone, and mutation rate is the chief source of variability. Such organisms as yeast and Faramccium are exceptions. With increased complex- ity, integrated combinations of genes controlling the developmental pathways are built up. The diploid state (or dikaryosis) provides the necessary buffering. It also permits the storage of variability in the form of recessive genes and of polygenes in balanced systems. Meiosis and syngamy produce the release of variability as new gene combinations. It is usually assumed that, without environmental change, most new gene arrangements will have lower selective value than exist- ing ones. Thus a certain wastage of zygotes occurs in addition to the wastage of gametes in sexual reproduction. Nevertheless, the wastage in diploid outcrossing organisms is surely much less than that in haploid organisms. The wastage of recombinants in the bac- teria-bacteriophage systems is probably very much higher still. This suggests that there has been selection, also, for genetic systems that not only provide for buffering and the build-up of gene complexes and for storage and gradual release of variation but also reduce the wastage of biological materials and thus energy in the course of evolution. (It must not always be assumed, however, that what appears to be "wastage" is disadvantageous. ) Reduction of Recombination In nearly all genetic systems, modifications that reduce the amount of recombination have occurred. The result is reduction of wastage, as immediate fitness is increased. It has been generally assumed that the most primitive organisms were asexual, and these modifications are usually referred to as reversions to asexualitij. Since recombina- tion mechanisms have been found in nearly all organisms and since the ability to effect recombination appears to be a fundamental 170 I The Process of Evolution property of DNA, this is perhaps an inappropriate designation. Sexuality and diploidy probably evolved relatively early in time. Reduction of recombination takes many forms. In unicels there may be absence of sexual reproduction. In the more complex multicels, reduction of recombination may occur as a result of reduction or elimination of crossing-over, assumption of specialized mating sys- tems, inbreeding, self-fertilization, or loss of sexuality. As Stebbins has pointed out, such modifications are found most often in pioneer forms whose populations experience pronounced fluctuations in size. Under conditions of environmental change, such forms can ex- ploit newly opened habitats through rapid duplication of closely similar genotypes. Often there is reduction in body size and in developmental complexity, as well as increased reproduction rate. Cytogenetic mechanisms affecting recombination are discussed in Chap. 9; systems of mating will be considered here. Since the amount of recombination is affected by selection, the genetic system of nearly any organism is always in a state of flux. The simultaneous existence of variable mechanisms producing in- crease and decrease in recombination provides a system buffered against short-term environmental change but able to respond to long-term change. Most of the mating systems are bivalent in this sense or are combined with other mechanisms to produce this bi- valence. Mating Systems and Recombination The basic type of mating against which the others may be compared is random mating. If like individuals are more apt to mate than would be expected by chance, the system is said to be positively assortative. If unlike individuals mate more frequently than expected by chance, the system is negatively assortative. The nature of repro- ductive mechanisms in plants and animals makes it unlikely that truly random mating ever occurs. It is difficult to specify precisely the extent of deviations from randomness. Mating systems do not, of course, fall into discrete classes, and any population in nature may show several of the arbitrarily delimited types. Furthermore, mating-system type is affected by such things as selective advantage of particular characters and population size. As pointed out in Chap. 6, random mating results in constant gene frequency with no change in variability. With selection, gene frequencies change, but the variance of the population and genetic correlation between relatives are little affected. As the population size reaches lower limits, the effects of sampling error lead to genetic Genetic Systems II 171 drift. Small population size also leads to inbreeding and a deviation from randomness of mating when the species is considered as a whole. Inbreeding Systems Inbreeding may be referred to as positive genetic assortative mating for it increases the chance of mating by organisms with like geno- types. Mating systems leading to inbreeding result in the break-up of a population into smaller groups that only rarely exchange genetic information. The heterozygosity of the population is reduced as fixation occurs in the subgroups. Genetic variance is increased in the population as a whole unless there is strong selection for par- ticular homozygotes. Morphological and physiological mechanisms leading to inbreeding are common in plants. Their degree of re- striction ranges from facultative self-pollination to obligate self- fertilizing types with cleistogamous flowers (see Chap. 9). Detailed studies by Stebbins, Grant, and others have shown that the degree of restriction of recombination in plants is closely corre- lated with their growth form and habitat. As is discussed in Chap. 9, populations often combine genetic systems that have opposite effects on recombination. Herbaceous plants, which have short generations and thus more recombination, tend to have low chromosome num- bers. Perennial plants with longer generation time have higher chromosome numbers. Plants that occur in ecological communities usually thought of as "closed," i.e., those in which most offspring do not survive to maturity, tend to have genetic systems that promote genetic recombination. Oaks are an example. On the other hand, pioneer organisms, wherever they may occur, are members of "open" communities. In order for zygotes to survive, they must have the proper genotvpes; there is no time for the organism to experiment with recombinants. It is not surprising, therefore, to find that plants of desert regions, grasslands, and cleared areas in the tropics generally have genetic systems that result in reduced recombina- tion. When inbreeding is imposed on populations that are usually outbreeding, a loss of fitness referred to as inbreeding depression occurs. The relationship between fitness, heterozygosity, and out- breeding is not well understood as yet. Many groups of plants have successfully employed inbreeding as a mating system for long pe- riods of time with only occasional outbreeding. It cannot be assumed that all organisms necessarily maintain developmental and genetic homeostasis through outbreeding and heterozygosity. 172 I The Process of Evolution Outbreeding Systems Negative assortative mating is the mating of unlike individuals with a frequency greater than that expected under random mating. The differentiation of sexes in animals generally assures that self- fertilization cannot occur. In addition, most animals have developed systems of varying degrees of complexity which influence the degree of outbreeding. These are briefly discussed in Chap. 5. There is evidence that genetically controlled components of dispersal affect outbreeding and gene flow in insects. Behavioral mechanisms in both invertebrates and vertebrates often operate to reduce the frequency of nearest-neighbor matin gs. In Homo sapiens such ethological mechanisms reach their extreme. A diversity of mating types affects recombination in microor- ganisms. Dispersal mechanisms in plants, as in animals, make near- est-neighbor matings less frequent than would be expected if chance alone determined the pairings. Flowering plants have floral pollina- tion mechanisms which also function to determine the amount and type of recombination. It is commonly assumed that there has been a general evolutionary trend from open flowers, composed of numer- ous parts, which are pollinated more or less indiscriminately, to flowers with the few stamens and stigmas positioned in such a way that pollen is precisely applied to and withdrawn from the body of the pollinator. Other plants have physiological incompatibility systems that en- sure outbreeding. Since reproduction in the higher plants requires pollination by male gametophytes, as well as fertilization by male gametes, the process can be interrupted at many steps. A common system has a multiple allelic basis, the gene for incompatibility, S, existing in many states. The various possible genotypes affect polli- nation so that pollen tube growth is very slow in a style with the same allele of S as is found in the male gametophyte. Other more complex incompatibility systems have been studied in the flowering plants. SUMIVIARY Recombination is a basic property of nucleic acids. Viruses and bac- teria show simple, variable genetic systems producing recombination with great wastage of recombinants. In flagellates and filamentous algae, the life cycle may be predominantly haploid with a diploid Genetic Systems I | 173 zygote that divides by meiosis immediately after formation. From this situation, diploid life cycles and cycles with alternation of gen- erations seem to have evolved independently a number of times in plants and animals. In the fungi, specialized mechanisms producing recombination and diploidy have developed. The selective advan- tage of diploidy and sexuality seems to lie in their provision for de- velopmental buffering and the storage and release of variability, as well as in the reduction of the amount of gamete and zygote wast- age. Recombination may be modified in multicellular organisms by mating systems leading to predominantly inbreeding or predomi- nantly outbreeding individuals. REFERENCES The references are given at the end of Chap. 9. 9 genetic systems II As has been seen in the consideration of evolutionary change in the quantitatively measured characteristics of organisms, recombination is a factor more significant than mutation as a source of variability. Since the genotype of an organism evolves under conditions de- termined, among other things, by the constitution and organization of its chromosomes, the investigation of the chromosomal mechanism is particularly important in evolutionary studies. The basis of \aria- tion lies in the genetic code, but changes in the structure and num- ber of the chromosomes bearing the code may directly or indirectly affect the amount of recombination of the code. Genes are defined here as those regions of the chromosome be- tween the closest points of crossing-over. They are therefore the smallest units of recombination in higher organisms. Larger units of recombination may involve particular portions of the chromosome in which crossing-over or its effects are restricted; these may be called supergenes. There are whole chromosomes, e.g., the sex chromosomes, which behave as recombinational units. The entire nucleus is the unit of transmission and recombination in organisms that reproduce asexually (some kinds of apomictic organisms). The cytological mechanisms that determine, in part, the amount of recombination in a population are an important aspect of the genetic system of the organism. Often they are visible in the pheno- tvpic appearance of the chromosomes, the karyotype, flowever, structural and genie changes may occur without any obvious changes in the karyotype. The cytological mechanisms about to be discussed usually are referred to as aberrations or imitations. Since they ap- pear to occur with measurable frequency in most organisms and since they become characteristic of entire populations or taxonomic groupings of animals and plants, they are best thought of as aspects of the cytogenetic repertoire of organisms. As with any other char- acteristic, they arise spontaneously and are maintained by selection as long as they confer an advantage. MEIOTIC DRIVE In a variety of organisms, all alleles of a locus do not have an equal chance of inclusion in the gametes. This phenomenon has been called meiotic drive, since one allele is favored or driven. It is part of the genetic system, as it reduces the amount of recombination. Cytogenetic mechanisms that affect the distribution of alleles have been studied in some detail in Drosophila melanogaster. It is obvi- | 175 176 I The Process of Evolution ous that, with meiotic drive, an allele might increase in a population even if it had a deleterious effect. In such a case, selection presum- ably would result in the accumulation of factors that reduce the effect of the gene responsible for meiotic drive. It is possible that meiotic drive might result in the spread of a beneficial gene. The extent to which this has occurred in nature is unknown. CHANGES IN CHROMOSOME STRUCTURE The usual cytological mechanisms of haploid and diploid organisms already have been described. The student should review the ma- terial on chromosome behavior in Chap. 3 before reading the fol- lowing discussion of derivative genetic systems, those in which cytological changes operate to increase or ( more often ) to decrease what usually is thought of as the standard amount of recombination. Inversions Chromosome inversions are structural changes that can lead to supergene formation. Here a section of the chromosome is in re- verse sequence as compared with a standard. Upon synapsis in meiosis, one of the pairing chromosomes must become twisted, and a characteristic loop is formed in an organism heterozygous for an inversion. As has been seen, crossing-over and recombination are profoundly affected. Single crossing-over within the loop of a para- centric inversion (one that does not include the centromere) leads to the formation of a dicentric chromatid and a chromosome seg- ment without a centromere. At anaphase I, these appear as a bridge connecting the daughter nuclei and a fragment which does not move to the poles (Fig. 3.2). Depending upon the number of the cross- overs and the chromatids involved, all or a portion of the gametes are unbalanced genically and therefore nonviable. Thus the genes within the inversion remain together as a supergene that is the length of the inversion. The gametes containing recombinations of the genetic code within this supergene may not survive, or the resultant zygote may be inviable. One way or the other, recombi- nants are not produced. In a long inversion, more than one crossover may occur; the results depend upon whether the same or different chromatids are involved in the second as were concerned in the first. If the same chromatids are affected, the second crossover will compensate for the first and there will be no detectable cytological effect. If a new pair cross over, a double bridge and two fragments are found at anaphase I. Genetic Systems II 1 177 Various other configurations may occur, depending upon the number and position of crossovers. Should an inversion be pericentric (i.e., include the centromere), a supergene will be formed, but the inver- sion is not detectable by a bridge and fragment as with paracentric in\ersions. The evolutionary effect of inversions depends upon whether they are relatively long or short. Short or moderately long inversions fre- quently are found "floating" in populations of plants and animals where they appear spontaneously. They reach high frequency when they become associated by chance with a favorable combination of genes. The genus Drosophila provides numerous examples of this (see Chap. 7). It has been found that, concomitant with inversions regularly associated with certain chromosomes, there may be marked increase of crossover frequency in other chromosomes. Thus the effect of the inversion must be to form supergenes rather than simply to reduce the amount of recombination. For if selection were operating to reduce recombination generally, the increase in cross- over frequency in other chromosomes probably would not be ob- served. When individuals from geographically isolated populations or from different species are crossed in the laboratory, they are often found to differ by very long inversions (perhaps of an entire arm). Several such inversions prevent the formation of fertile offspring; thus they may be thought of as isolating mechanisms. Their impor- tance in nature is not as clear-cut as that of shorter inversions. It seems clear that spontaneously occurring inversions might rather rapidly differentiate karyotypes of populations that become sepa- rated. When they occur with regularity, as in Drosophila and Sciaro, they seem to be playing a special role. Reciprocal Translocations Reciprocal translocations likewise may function in setting up super- genes in populations heterozygous for the change. If the inter- changed segments are short, usually only two competent combina- tions can be formed in a heterozygous organism. Should crossing- over occur between the centromere and the point of interchange, only two of the four chromatids associated at any one place may be separated as competent combinations (see Chap. 3). With more than one interchange between the same two chromosome pairs, rings of more than four chromosomes are formed at meiosis. In some organ- isms, such as the plant Rhoeo and several species of Oenothera (eve- ning primrose), all the chromosomes exchange segments and all are 178 I The Process of Evolution united, forming one great ring at meiosis (Fig. 9.1). Directed alter- nate disjunction may occur in these specialized types. Since synapsis is very precise, homology determines that each chromosome has its own place in the ring which can be changed only by further struc- tural changes. When more than one interchange has occurred, a new type of chromosome segment is set apart. The homologous chromosome arms are called pairing segments, and the portion of the chromosome be- tween the point of interchange and the centromere is the interstitial segment. The new area, which is associated with the central part of a chromosome with which it is not paired terminally, is the diflFer- ential segment which may or may not include the centromere. In this area, crossing-over occurs rarely; when it does, the interchanges are modified, and the arrangement of the chromosomes or the num- ber of rings may change. The study of reciprocal translocations requires careful and painstaking analysis of crosses between many different individuals so that the homology of the pairing segments can be determined. Floating translocations occur in a large proportion of diploid plants (e.g.. Campanula, Paeonia), as well as in scorpions and cock- roaches. In many invertebrate animals translocations are involved in the sex chromosome mechanism. When an XX female and XO male chromosome mechanism exists, one of the autosomal chromo- somes may become translocated to an X. This leaves one of the autosomes with its homologue as part of a modified sex chromo- some (which may be called a neo-X). This autosome subsequently behaves as do Y chromosomes or heterochromatic regions of chro- mosomes in general. It is said to have become heterochromatized and is called a neo-Y. Apparently this process has occurred in several grasshoppers and mantids, as well as in other invertebrates. Pre- sumably some readjustment of gene function takes place. The genes in the autosome translocated to the X are now present in half their previous number in the males. This is because heterochromatization presumably results in their loss in the homologous autosome (the neo-Y). Multiple sex-chromosome mechanisms (with several X's and Fig. 9.1 I ( see opposite page ) Chromosome behavior in two species of Oenothera, a, seven bivalents in O. hookeri; h, ring of 14 in meiotic prophase of O. biennis; c, ring of 14 in alternate disjunction at anaphase I in O. biennis. {After Cleland, 1936, Bot. Rev. 2, and after Abrams, 1951, Illustrated Flora of the Pacific States, Stanford University Press.) Genetic Systems II 179 (a) p\m Oenothera hookeri (b) (c) Oenothera biennis 180 I The Process of Evolution Y's) may evolve by this process, as has been the case apparently in Drosophila miranda. Karyotype evolution in the Drosophila virilis group also involves translocations between autosomes and sex chro- mosomes. An extreme case of multiple sex-chromosome mechanisms is found in the Palestinian beetle Blaps polychresta. In the males there are 12 X chromosomes and 6 Y chromosomes. The females have 24 X chromosomes. Genetic systems based upon regularly occurring structural hybrid- ity for reciprocal translocations are characteristic of a few groups of plants. The behavior of translocation systems in nature is not well understood for most plants, although rings are reported for a num- ber of genera, for example, Rhoeo, Paeonia, Datura, Hypericum, and many Onagraceae. Progress toward an understanding of such mech- anisms in natural populations of Clarkia and Oenothera is being made by Lewis, Raven, and their associates. Clarkia has been studied in some detail cytologically. Species invariably differ by chromo- some rearrangements, and interspecific hybrids are often highly sterile. It has been estimated that in many species of Clarkia more than 20 percent of the plants in nature have rings of chromosomes, indicating the presence of translocations. Such rearrangements form a part of the genetic system and may characterize whole populations. An interesting example of evolution involving chromosome re- arrangements is Clarkia franciscana, studied by Lewis and Raven. This largely self-pollinated plant is restricted to an area of serpentine rock in San Francisco which is within the geographic range of the closely related C rubicunda. Apparently C franciscana is also re- lated to C amoena, a northern species. Studies of meiosis in hybrids show that C. franciscana differs from C. amoena by at least two translocations and two paracentric inversions. It difiFers from C. rubicunda by at least three translocations and four inversions (see Fig, 9.2). (Clarkia amoena and C. rubicunda differ by at least three Fig. 9.2 I (see opposite page) Evolution involving chromosomal re- aiTangements in Clarkia. a, seven bivalents in C. franciscana; h, ana- phase I showing two bridges and two fragments in cross between C. rubicunda and C. franciscana; c, anaphase I showing chain of five chromosomes, a chain of three chromosomes, and three bivalents in cross between C rubicunda and C franciscana; d, anaphase I showing two bridges, two fragments, and a lagging chromosome in a cross be- tween C. amoena and rubicunda. Map shows distribution of three spe- cies in San Francisco Bay area. ( After Lewis and Raven, 1958, Evolution 12, Brittonia 10; and Lewis and Lewis, 1955, Univ. Calif. Publ. Bot. 20. ) Genetic Systems II | 181 182 I The Process of Evolution translocations and two inversions. ) Lewis and Raven have concluded that C. franciscana has originated relatively recently from C. rtibi- ciinda and that it may have been the result of a rapid repatterning of the chromosome set, producing many differences in a relatively short time. It also seems likely that C rubicunda may have arisen from C. amoena at an earlier time. Some plants, such as Oenothera, subgenus Euoenothera, have evolved translocation systems of amazing complexity. The develop- ment of these systems seems to have involved selection for hybrid- ity. This group is well known through the work of Cleland and others. Indeed, the "mutation hypothesis" of De Vries was based upon studies of Oenothera erythrosepala (O. lamarckiana) . Some species of the subgenus, for example, O. hookeri, have seven pairs of chromosomes (Fig. 9.1) or small rings (floating translocations). In others, such as O. biennis, all their chromosomes are in a ring of 14 at meiosis (Fig. 9.1 ). In addition to the reciprocal translocations that involve all the chromosomes, there are balanced sets of lethal genes, the Renner complexes, which prevent the survival of homozygous offspring by acting to produce nonviable gametes or zygotes. (As mentioned above, there is directed alternate disjunction.) The ring-forming species have small flowers and are largely self-pol- linated (in contrast to O. hookeri, for example). They consequently form a very large number of highly heterozygous, mostly true-breed- ing, and partially isolated races. Occasional outcrossing between the races leads to the origin of new ring systems and new racial types. The great number of such races, which are the primary evolutionary units in the ring-forming Oenotheras, makes the taxonomy of the group very difficult. As Cleland has pointed out, these ring-forming Oenotheras illus- trate that various types of "mutations," which individually might be considered harmful, may, in combination, produce a workable genetic system. They form a widespread and even weedy group. Reciprocal translocations (which usually lead to sterility), lethal genes, and self-pollination combine to form a system in which hetero- zygosity of all the chromosomes is preserved and the plants are highly fertile. There are what appear to be disadvantages of such a genetic system. Recombination is reduced, and there may be wastage of gametes and zygotes. The failure of any part of the system de- stroys the whole. Polyploidy apparently is excluded since it is in- compatible with a complete ring system and lethal gene complexes. Changes in Chromosome Size and Shape By their nature, inversions and translocations also change the size and appearance of the chromosomes. Pericentric inversions may Genetic Systems II | 183 change the position of the centromere and thus alter the relative length of the arms. Unequal reciprocal translocations also may change the chromosome length. Since the number of crossovers is proportional to the length of a chromosome arm, there may be a change in the amount of recombination simply as a result of size changes. As is discussed below, if a chromosome is very small, for example, because of translocation, and does not pair properly, it may be lost and the basic chromosome number changed. Changes in Chromosome Number One way of expressing the amount of recombination afforded by the genetic system is to use the recombination index of Darlington. This simple measure is equal to the haploid number of chromosomes plus the average number of chiasmata. The larger the recombination index (R.I.), the greater the number of new gene combinations formed by recombination and segregation. The index does not take into account the existence of supergenes formed as a result of structural changes or apomixis, but its use leads to interesting comparisons. An increase in the basic number will increase the R.I.; for this reason, chromosome-number change may be regarded as an aspect of the genetic system related to the balance between immediate fit- ness and long-range flexibility. Stebbins has made an analysis of the distribution of R.I. as indicated by chromosome number in flowering plants. In woody plants (trees and shrubs) the basic gametic number is significantly higher than that of herbaceous genera. This suggests that among long-lived plants a high recom- bination index has been favored by selection. In short-lived rapidly reproducing organisms, the genetic system usually seems to favor immediate fitness at the expense of flexibility. There is, however, a certain amount of flexibility inherent in a short life cycle with its rapid turnover of genes. Within strictly cross-pollinated herbaceous flowering plants, Stebbins finds that annual species tend to have a lower recombination index than perennials. Most of these annual species are either cross-pollinated with a low R.I. or pre- dominantly self-pollinated, with a high R.I. They appear to be specialized for rapid occupation of uniform habitats. POLYPLOIDY The basic chromosome number clearly appears to have had selective value. How are changes in the basic number accomplished? The minimum number of chromosomes, though all different, that func- tion as a harmonious and integrated unit is a genome. For purposes of discussion, the genomic number may be symbolized as x. The 184 I The Process of Evolution total number of chromosomes in any nucleus is its chromosome com- plement. It is easy to see that the complement may include one or more genomes or parts of genomes. In discussing the regular alterna- tion of gametic and zygotic chromosome numbers, it is convenient to use a diflferent symbol n. The haploid and diploid numbers of all organisms are n and 2n. Organisms that have experienced an increase in chromosome number are called polyploid. Here n in- cludes more than one x or portions of x. In polyploid organisms n and 2n usually are called haploid and diploid, even though in a tetra- ploid, for example, n = 2x and 2n = 4x. The commonest type of change is eupolyploidy, which is irreversible increase by whole genomes. The oldest members of a eupolyploid series are those with the lowest numbers. Thus if a plant is found to have 2n = 22 (x=ll) chromosomes, its closely related tetraploid derivative would have 2n = 44 (x = 11). The number of chromosomes in one genome may also change, leading to aneupolyploidy. Strictly speak- ing, an aneupolyploid series is a series of numbers, such as 11, 12, 13, 15, not a series of organisms; aneuploidy is reversible and may arise in several ways. Aneupolyploidy There is cytogenetic evidence that centromeres do not arise anew in populations in nature. Nor can genetically active chromosomes be added or subtracted from the genome. Nevertheless, conditions favoring the loss or gain of chromosomes can be brought about by unequal reciprocal translocations. In many organisms the region of a chromosome adjacent to the centromere is genetically inert ( hetero- chromatic). Darlington has suggested that, if the active euchromatic arms are translocated to other chromosomes, the remaining hetero- chromatic centric fragment usually will not pair properly and may be lost. Thus a chromosome is removed from the genome, but the same amount of genetic material remains (Fig. 9.3). The resulting off- spring is isolated cytogenetically from its parent; if it forms a new population, further difiFerentiation may be expected to occur. On the other hand, a second translocation might apportion some active material back to the centric fragment, dividing the genetic material of another chromosome. Again pairing will be upset and a variety of gametes will be produced. A zygote that has an extra chromosome added to its genome may be formed. The original amount of genetic material must be present or the combination will not be viable. Aneuploid change in chromosome number has been studied exten- sively in plants. Generally a decrease in basic chromosome number Genetic Systems II | 185 is involved. In Crepis (false dandelion, Compositae) the chromo- somes may be recognized individually. Species considered to have retained primitive characteristics in other features of the phenotype have X = 7, 6, or 5. The most specialized species have x = 4 and X = 3. The correctness of the above-described model of Darlington has been shown by Tobgy, who demonstrated that Crepis fuliginosa (n = 3) was derived from C. neglecta or its ancestor (n = 4). One arm of the C chromosome of C. neglecta apparently is inert, and the other arm was translocated to the A chromosome. This is shown by pairing behavior in the Fi hybrid between the two species. Fig. 9.3 I Diagram showing how basic chromosome number can be in- creased or decreased by reciprocal translocation of unequal chromosome segments. Nonhomologous chromosomes are white and hatched; black- indicates supposedly inert segments. {From Stebbins, 1950, Variation and Evolution in Plants, Columbia University Press, and after Darling- ton, 1937, Recent Advances in Cytology, Blakiston.) Origina n + 2 pairs offspring from similar gametes n+3 pairs 186 I The Process of Evolution Similarly, C. kotschijana (u = 4) was derived from an n = 5 form close to C. joetida. There are numerous examples suggestive of progressive increase in chromosome number but unfortunately little experimental evi- dence. The genus Clarkia apparently is one in which chromosomes have been added to the genome. This increase may be associated with the formation of supernumerary chromosomes (see below). Aneuploidy that simulates progressive increase may result when loss or gain of one chromosome is followed by amphidiploidy (doubling of the chromosome number following hybridization of two diploids ) . Part of such a series has been produced in Brassica ( mustard ) where X = 8, 9, and 10 may represent a phylogenetically ascending series. The numbers known in nature or experimentally produced are n = 17, 18, 19, 27, and 29. The genus Carex (sedges) has the most extensive aneuploid series known. Haploid numbers ranging from n = 6 to n = 56 have been reported, and every number from 12 to 43 is represented by one or more species. Presumably structural changes and polyploidy have produced some of the numbers in this series. Structural rearrangements in which two acrocentric chromosomes give rise to a large metacentric chromosome and a minute fragment, which subsequently disappears, are common in Drosophila, grass- hoppers, and reptiles. The process is known as centric fusion and represents a special sort of reciprocal translocation. In many families or genera, the number of long arms remains constant while the rela- tive number of acrocentrics and metacentrics fluctuates. Many ex- amples could be given. An interesting one is the cricket genus Nemobius. Netnohiiis fasciatus has a metacentric X chromosome and seven acrocentric autosomes. Other species have additional meta- centrics and fewer acrocentrics, presumably the result of structural rearrangements. In addition to the basic number of chromosomes in the genome, both plants and animals may have extra chromosomes called super- numeraries. Ordinarily extra chromosomes are not tolerated, for they cause genetic unbalance and upsets in meiosis, as in experi- mentally produced trisomies, tetrasomics, etc. This is why, in gen- eral, only reciprocal translocations can change the basic number. The genetic material remains the same; only its distribution among the centromeres is changed. When supernumerary chromosomes are found, it is evident that they must be neutral in some sense or have a special function. Often they are variable in number from cell to cell, or individual to individual. Nevertheless, it seems unlikely that they are completely inert, since they may in some cases remain in the population. Genetic Systems II 187 These supernumerary chromosomes in plants commonly are called B chromosomes, and they are surprisingly frequent. In general, the B chromosomes are smaller than the others, and they pair only among themselves. In most instances they are heterochromatic. They vary in number among individuals, they may be in odd or even numbers or absent, and their presence usually cannot be detected in the phenotype of the plant. B chromosomes may have physio- logical eflFects, however, the evolutionary importance of which is unknown. In Clarkia, supernumeraries probably have arisen as the result of structural changes in the chromosomes. These plants often have ring or chain arrangements of chromosomes in translocation heterozygotes, and unequal separation may lead to the formation of trisomies. For some reason, in this genus extra chromosomes do not disturb the genetic balance or reproduction. Supernumerary chromosomes are found among many inverte- brates. They appear to be largely heterochromatic. Some are mitot- ically stable; others show nondisjunction. Perhaps related to these supernumeraries are the so-called "limited" chromosomes of some Diptera. These chromosomes are limited to the germ line, v/here they often show bizarre cytological behavior. The fungus gnat Sciara copropliila, for example, has seven chromosomes in somatic cells of males and eight in somatic cells of females (Fig. 9.4). Cells in the germ line that will produce gametes contain 10 (sometimes 9 or 11 ) chromosomes; this group includes the four pairs of somatic cells plus a pair of large chromosomes never found in cells other than the germ line. These are the limited (L) chromosomes. In meiosis female flies produce eggs with a full haploid set including an L. Meiosis in males is unusual, in that synapsis does not occur and at the first division a unipolar spindle is formed. The L's and one of each of the other chromosomes proceed to the pole of the spindle, while the others (which genetic evidence shows are all paternal chro- mosomes) are eliminated as they go in the opposite direction and are cut off in a small bud. During the second division a bipolar spindle forms and the chromosomes behave normally except for the X, which divides but sends both daughters to the same pole. Only the spermatid that receives the X's matures; therefore only one sperm is formed at each meiosis. It contains 2 L's, 2 X's, and one of each autosome; except for the L's, these chromosomes come origi- nally from the maternal parent. After fertilization, which results in a zygote with three pairs of autosomes, 3 X's, and (usually) 3 L's, a series of cleavage divisions takes place in which the L's and X's are selectively eliminated from cells. Eventually the chromosome numbers and distribution t E 53 .E CD ~ \5 cu CD '^-i •"^P H g H Q. CO / CuO >:^ c/) >^ \5 03 « E ^ CO O-t- I I I I > e > ss: >^ t O ll O CO \d ^> CO a; E J o c/o 0-+ ns Q. C/5 (/) T- b- c^J- o-r /I 6' f f c f a- - a' EgJ b- -b' b' Single , polar body MEIOTIC Type la MEIOTIC Type 16 a' b' AMEIOTIC 204 I The Process of Evolution Agamic complexes in plants other than in Crepis are less well known. They may be quite small or exceedingly large and complex. The chromosome numbers may become very high and unbalanced. Study of other genera has proved to be quite difficult where the ancestral sexual diploids have become extinct (Riibiis) or where there may be several agamic complexes in one genus (Poa). Large agamic complexes are less common in animals. The case of Artemia salina ( brine shrimp ) with sexual diploids and thelytokous triploids, tetraploids, pentaploids, octaploids, and decaploids is well known. In the Curculionidae (weevils) and Lumbricidae (earth- worms) larger complexes have been found; these may include diploids, triploids, tetraploids, and pentaploids, or even hexaploids and decaploids. In the weevils there is reason to believe that occa- sional fertilization of parthenogenetically developing eggs by sperm from bisexual races or species takes place. Thirteen species of thelytokous earthworms have been studied cytologically; all are polyploids ranging from triploids to a possible decaploid. Some sexual species are polyploid as well. Since the Lum- bricidae generally are hermaphroditic, thelytoky involves modifica- tion or loss of the male organs. Oogenesis is complex, there being chiasma formation and bivalents even in odd-numbered polyploids. The apparent success of these forms, if their wide geographic dis- tribution is to be a criterion, may be accounted for by postulating heterosis as a result of allopolyploidy. Apomixis has been reported in the vertebrates, but its extent and evolutionary importance are virtually unknown. Some subspecies of the European lizard Lacerta saxicola are parthenogenetic. Several species of the American lizard Cnemidophorus may consist only of females, since no males have ever been collected; this suggests that parthenogenesis may occur. An interesting situation has been studied in some detail in the fish genus Mollienesia. Several populations of M. formosa have been sampled in southern Texas, where they occur in streams and drainage ditches. The fishes have also been raised in the laboratory and their genetic similarity studied by means of tissue transplants. Grafts of donor tissue are rejected by the host fish (because of the presence of tissue antigens produced by the host genes) in a period of time roughly proportional to the degree of genotypic similarity between the host and donor. Mollienesia formosa is parthenogenetic, but eggs do not develop without the stimulation of sperm. Since males of M. formosa are exceedingly rare in nature, sperm from related species (in this in- stance M. latipinna ) is necessary to initiate development. This mode of reproduction, in which the genetic information of the sperm is Genetic Systems II | 205 not incorporated into the zygote, is known as gynogenesis. As in other instances of apomixis, the genetically identical progeny of a female form a clone. Tissue-transplant studies have shown that two clones of M. formosa make up about 80 percent of the population in one drainage ditch near Olmito in the valley of the Rio Grande. The remainder belong to a third clone or cannot be identified. Clones sampled in 1961 were the same as those found in 1960. In 1954, several dozen Mollienesia formosa were taken from the Olmito ditch and released in the San Marcos River some 250 miles to the north. The species has become established, as has M. htipinna which was introduced into the area many years earlier. The San Marcos population thus was available for comparison with the Olmito fishes; it was also sampled in 1960 and 1961. Only two clones were found; these were the common clones at Olmito. Thus the clones of these Mollienesias probably have remained relatively un- changed (within the limits of tissue-transplant discrimination) for at least a decade and possibly for much longer. There remains to discuss only cyclical parthenogenesis, a peculiar genetic system found in aphids, gall wasps, Cladocera, and rotifers. The cytological mechanisms differ from group to group, but we may single out a species of aphid as an example. In Tetraneura idmi, which produces galls on elm leaves, there is a sequence of genera- tions which have been given names descriptive of their behavior: fundatrices, emigrantes, exules, sexuparae, and sexuales. In the spring, females of the fundafrix generation become adults within the elm leaf galls. There each produces, parthenogenetically, female offspring which later develop wings and fly away to feed on the roots of grasses. They are the emigrantes, which produce, also par- thenogenetically, several generations ( females ) of exules. Eventually the exules give rise to the sexuparae, winged females which fly back to the elm and there parthenogenetically produce both males and females called sexuales. The latter pair, and from fertilized eggs appear once more the female fundatrices, the gall-making genera- tion. Cytological investigation shows that female sexuales have 2n = 14, while males have 2n = 13; there is evidently an XX:XO sex-chro- mosome system. The fundatrices, emigrantes, and exules types of thelytokous females have a diploid set of 14 chromosomes. There is a single maturation division in oogenesis. The eggs produced are diploid because the division is not reductional and they develop into females that are identical genetically except for mutation. The sexuparae produce eggs of two kinds. In those eggs that will be female-determining, all the chromosomes split, as in mitosis. In those 206 I The Process of Evolution that will give rise to males, the chromosomes behave similarly, ex- cept for the X chromosomes, which pair and are reduced as in meiosis. One X remains in the egg; the other goes into the polar body. In spermatogenesis of the male sexuales, the X chromosome is apportioned to one of the secondary spermatocytes in normal fashion. However, those cells without the X chromosome eventually degenerate and only X-containing cells produce sperm. Thus the sexuales males can have only daughters, which complete the cycle as fundatrices. Other aphids have similar cycles in which the number of genera- tions may differ, in which there are two kinds of sexuparae: male- producing and female-producing, etc. In such complexes as those described for plants and animals, the usual concept of species is very difficult to apply. The sexually re- producing diploids may be comparable to species in other organisms. But the autopolyploids and allopolyploids that combine the char- acteristics of two or more diploids in asexually reproductive and therefore very fertile organisms break down the utility of criteria based upon morphological intergradation, gene exchange, and geo- graphic distribution. Combining the classic techniques of taxonomy with the methods of cytogenetics, however, the biologist may be able to identify the major evolutionary units within the complex. To these he customarily gives the rank of species, while the multitude of apomictic forms may be described, with or without formal tax- onomic recognition, as appears most useful. Aside from greatly complicating the work of the biologist, what are the effects of apomixis as a genetic system? It is obvious that apomixis makes possible the survival of many genotypes that are vigorous and well-adapted but sexually sterile for one reason or another, e.g., in unbalanced polyploids. Apomixis also permits the building up of large numbers of genetically similar individuals for the rapid colonization of newly available habitats. One finds apo- mixis often to be the genetic system of weedy or pioneer organisms and of those in habitats subject to frequent or regular catastrophe, such as sand bars, lawns, etc. It is also true that apomixis limits the genetic variability of the organisms that have adopted it as their sole mode of reproduction. For this reason, it generally is found to be an alternative or second- ary genetic system. Apomixis usually is not combined with other systems that reduce the long-range flexibility of the organism for the sake of immediate fitness (e.g., self-fertilization). It is interesting that, even in those groups, such as Poa, where apomixis and high Genetic Systems II [ 207 polyploidy are carried to what appear to be extremes, the situation is not, as usually described, "dead end." The pollen of obligate apomicts may be functional, and pollination of an apomict may occa- sionally result in the segregation that leads to an escape from asexuality. SUMMARY Populations of plants and animals often exhibit cytogenetic mech- anisms controlling the amount and nature of genetic recombination. These mechanisms, along with others previously mentioned, make up the genetic system of the population that determines how many new gene combinations are produced in a unit of time. They range from inversions and translocations, which produce relatively small groups of linked genes, through polyploidy with its diverse and vari- able eflfects, to apomixis, in which recombination is eliminated. Such mechanisms are often considered disadvantageous in the very long- range view. However, they are extremely common in both plants and animals and must result in a selective advantage. The bizarre and complicated genetic systems of some organisms discussed are poorly understood and have not been satisfactorily integrated into evolutionary theory. REFERENCES Darlington, C. D. 1958. The Evolution of Genetic Systems. 2d ed. Basic Books, Inc., New York. A remarkable attempt to unify cytology and genetics in evolutionary terms. Sager, R., and F. J. Ryan. 1961. Cell Heredity. Wiley, New York. This is a recent source for material on the genetic systems of microorganisms. Stebbins, G. L. 1960. The comparative evolution of genetic systems. In Sol Tax [ed.], Evolution After Darwin, Univ. Chicago Press, Chicago, vol. I. The Evolution of Life, pp. 197-226. A thorough recent account of the problem, together with speculations concerning answers. White, M. J. D. 1954. Animal Cytology and Evolution. 2d ed. Cambridge Univ. Press, New York. The standard reference for genetic systems in animals, though now somewhat out-of-date. Other references to animals will be found in Evolution After Darwin, vol. 1, cited above. 3 populations: differentiation The process of evolution is sometimes divided into microevolution (changes within populations) and macroevoltition (the origin of major variation patterns). Where to draw the distinction is an arbitrary decision, which we prefer not to make. In the preceding section we have considered prijnarih/ changes witJiin populations. In this section the ivays in which evolving populations change and interact to produce the diverse life forms on the earth are presented. Chapter 10 deals with the basic splitting process of evolution: the ways in which a single evolving entity becomes two or more entities. This subject is discussed first by comparing different patterns of diversification which have been observed and then attempting to explain how thcij jnight hove come about. Chapter 11 is concerned with the patterns produced over long periods of time by populations evolving and dividing and also becoming extinct. No special factors are postulated to account for the evidence derived from a study of the fossil record, which is accepted as fragmentary and biased in various ways. The same processes that produce elaboration of different populations across a diversified habitat arc viewed as being responsible for the elabora- tion of populations through time. The apparent problem of how "higher" taxonomic categories arise is considered an artifact created by the taxonomic method applied to situations where much extinction and loss of data have occurred. 10 ^^^ differentiation of populations It is obvious to anyone observing the variation of living things in nature that organisms do not vary continuously. Variants of one type of organism may be arranged in a continuum, but there are gaps in the variation from continuum to continuum. Plants and animals, viewed by our usual techniques of studying organisms, seem to be aggregated into discrete or nearly discrete clusters usually called species. Certainly the living world may be structured by the scientist in many ways diflFerent from this customary taxo- nomic one; some of these may be of considerable interest to the evolutionist. In the last chapter of this book some of the problems involved in perceiving and describing structure and pattern in nature are discussed. Nevertheless, it is possible to recognize taxo- nomic units and to classify them; this has led biologists to attempt to understand the origin of such units in nature. This generally has been studied from the point of view of how a single sup- posedly interbreeding population can differentiate into discrete clusters. The processes presumed to be involved make up what is frequently referred to as speciafion. • Elucidating the mechanisms of speciation often has been regarded as the central problem of evolution. Darwin's classic work was en- titled The Origin of Species . . . , and many monographs in both botany and zoologv in recent years have emphasized the so-called species level of recognizable biological difference. This emphasis may have had the effect of obscuring some exceedingly important and interesting problems usually thought of as falling within the province of ecology (for example, the nature and evolution of com- munities of plants and animals). However, in discussing here the question of how recognizable aggregates of similar organisms arise in nature, we for the moment shall accept the commonly used analyses and designations. One usually gains the impression from even a casual study of living things that there is a spectrum of degree of similarity among organisms. Some forms appear to be very distinct from all others; some appear to intergrade almost imperceptibly with others that are closely similar. In approaching the problem of how populations become differentiated, it will be useful to consider the nature and size of the gaps in variation between clusters of similar organisms. In this chapter, examples from the spectrum of variation will be discussed, examples in which the degree of differentiation is rela- tively small. To put it another way, we shall examine situations that seem to be close to branch points in the evolutionary tree— organ- | 211 212 I The Process of Evolution isms that seem to be on the verge of fragmenting into multiple en- tities, and multiple entities that appear to be of rather recent origin. A series of examples is presented first, to give the reader some "feel" for the types of patterns that occur. The probable causes of these patterns are then discussed, illuminated with further brief examples. The very distinct forms will be dealt with in Chap. 11. In what follows, the term character will denote any trait that varies in the overall group under discussion. Thus the presence or absence of plastids is a character when one considers all organisms. For any given organism one can determine whether or not it possesses plastids. Their presence or absence is not a character in roses for they are uniformly present. Femur length is a character in man because it varies within the group and can be measured for any individual. Femur length is also a character when adult mice and adult men are compared, but the ranges of observed variation in this character are not overlapping. Such discontinuities in variation ( in single characters or in constellations of characters ) are here re- ferred to as gaps. EXAMPLES OF tD I F F E R E N T I AT I N Continuous Geographic Variation In many instances, variation is sufficiently continuous so that no dividing lines between segregates are obvious. Variation in some characters may occur in gradients. These gradients in single char- acters are called dines, and the variation is then called clinal. Color, Pattern, and Size Variation in Animals. Geographic vari- ation in color, pattern, and size is one of the most widely studied of all biological phenomena. This variation is often of the sort already described in the previous chapter ( Biston, Cepaea, Natrix ) in which populations diflFer primarily in the frequency of diflFerent types of individuals present. Another example is the North American tiger swallowtail butterfly, Papilio gluucus, some populations of which are composed of yellow and black striped (tiger) males and females, and other populations of tiger males, tiger females, and uniformly dark- brown females. In southern Canada and the extreme northern United States the populations of P. glaucus are composed only of tiger individuals. In south central Florida the proportion of dark- brown females is very low (6 to 8 percent), and in southern Florida dark females may be completely absent. In most of the southern United States, however, populations show high frequencies (up to 50 percent) of dark-brown females. The Differentiation of Populations j 213 In many cases, variation is not in frequency of types (partially intrapopulational ) . Individuals within a given population may all be closely similar (little intrapopulational variation), but color or pattern may change from population to population in broad geo- graphic trends. Among mammals and birds the tendency for popu- lations in colder, drier parts of the range to be hghter than those in the warmer, more moist parts is so common as to have been dignified as Gloger's rule. Other so-called "ecological rules" deal with varia- tion (not necessarily continuous) in size and shape. One (Berg- mann's ) states that homoiothermal vertebrates in warm areas tend to be smaller than those from cool areas. Another (Allen's) states that all projecting parts (wings, legs, noses, etc.) tend to be shorter in cooler areas than in warm ones. Ecotypic Variation in Plants. Botanists have attached more im- portance than have zoologists to the local population as a basic unit, perhaps because of the greater ease with which the less motile plants may be studied. The work of Clausen, Keck, and Hiesey over many years has been directed to an analysis of the variation within and between populations of plants widely distributed in California. Mak- ing use of field growing stations at Stanford (sea level), Mather (4,600 feet), and Timberline (10,000 feet), they have been able to separate, to a large extent, environmental and genetic components of variation. Perennial plants that can be propagated vegetatively may be grown at all these locations and their physiological responses to environmental factors thus investigated. In effect, the same genetic individual mav be studied simultaneously in three different eco- logical situations. Studies such as these have led to recognition of the ecological race or ecotype of plants. The genus Achillea (yarrow) in the sunflower family has already been mentioned. By means of transplant studies, Clausen, Keck, and Hiesey have analyzed the A. millefolium complex in some detail. The plants are found throughout the Northern Hemisphere, where they grow from sea level to timberline. There is continuous morpho- logical variation, from plants some 6 feet high in the San Joaquin Valley to alpine plants only a few inches in height. Other morpho- logical traits also intergrade from population to population, so that taxonomic distinctions are difficult to determine. Adjustment of Achillea plants to their environment depends on the proper integration of many physiological processes, such as rates of photosynthesis and respiration, resistance to cold, and time of dor- mancy and other periodic phenomena. Each local population is composed of many different genotypes. Depending upon the level of study, these can be viewed as aggregated into groups of varying 214 I The Process of Evolution size. Clausen, Keck, and Hiesey concluded from transplant studies along a 200-mile transect of California that the genotypes and local populations are arranged into at least 11 physiological races. Two taxonomic species are represented along this transect, where they occur in different habitats. Achillea lanulosa, of the higher eleva- tions, is primarily a species of continental habitats, whereas A. borealis occurs at lower altitudes and is a coastal species, in the main. It is interesting to note that in the northern portion of its range, where A. lanulosa comes to the coast, it has developed coastal ecotypes that mimic those of A. borealis. It may well be that plants, being rooted, become adjusted to the local conditions with a precision that would not be of selective value in animals. Clinal Variation in Plants. The butterfly weed (Asclepias tuhe- rosa) also shows geographic variation, but it has been studied in a very different manner. The subspecies occurring in the eastern two- thirds of the United States have been studied in great detail by Woodson. The distribution of these subspecies is shown in Fig. 10.1. Only A. tuberosa tuberosa and A. t. interior will be discussed here. In most parts of their range these subspecies can be dis- tinguished by the shape of the leaf. Fortunately, two important components of leaf shape can be quantified and the change in shape studied geographically. These components are angle A, a measure of the taper of the apex of the leaf, and angle B, which measures the shape of the base of the leaf. The two subspecies meet along a broad front in the eastern United States, and there is a zone of intergradation, as can be seen in Fig. 10.1. Woodson has studied geographic variation by dividing a map of the country into equal- area quadrats and measuring the herbarium specimens collected in these areas. He has also studied local-population samples and has measures of variation within and between individuals and colonies. By comparing the measurements of specimens collected in 1946 along a 1,200-mile transect from Kansas to Virginia with the available herbarium specimens from the quadrats in which the transect falls, Woodson was able to study the effect of time. The herbarium speci- mens, collected over a period of many years, represent a sample which is, on an average, older than the 1946 transect. It was clear that characteristics of A. t. interior were moving eastward, while those of A. t. tuberosa were moving westward but at a much slower rate. When, in 1960, samples were once again collected along the transect, the changes that had occurred in the 14-year interval could be accurately measured. Apparently reciprocal diffusion of the The Differentiation of Populations | 215 eastern and western genotypes of both Z A and Z B has occurred. Woodson interprets the eastern subspecies to be in the process of genetic submergence by the western one, since its western move- ment is proportionately less. Nevertheless, its effects on the western leaf shape can be clearly seen. Clinal Variation in Animals. In some cases, although the geo- graphic variation is continuous, experimental evidence indicates that a considerable amount of differentiation has occurred. Variation in the leopard frog, Rana pipietis, is extensive and discordant. The Fig. 10.1 I Map showing distribution of AscZeptas fufoero^fl. Each symbol represents a county record. Large dots, A. t. interior; small dots, A. t. tuherosa; hollow circles, putative hybrids between subsp. tuberosa and subsp. interior; half-circles, A. t. rolfsii. {From Woodson, 1947, Ann. Missouri Bot. Qard. 34. ) 216 I The Process of Evolution variation in 12 characters is summarized in Table 10.1. No overall pattern of variation is evident; indeed, many of the characters seem to vary completely independently. Moore's detailed studies of vari- ation in developmental processes have yielded abundant provoca- tive data. For instance, northern and southern populations of R. pipiens show different temperature-tolerance ranges for normal embryological development (Fig. 10.2). These differences parallel those found between northern and southern frogs belonging to clearly distinct clusters. (For example, the northern R. sylvatica, which ranges from the subarctic to the central United States, can develop normally between 2.5 and 24°C, whereas R. catesbiana, Table 10.1 | Population Formula: i fo r Meadow Frogs of Easte rn 1 ^orth America Quebec A B C D E F G H ] [ J K L Maine A B C D E F g H ] [ T k L Vermont A B C D E F G H ] [ "j K L N. New York A B C D e F G H ] [ J K L Massachusetts A b C D E F G H ] [ J K L Rhode Island A B C D E F G H ] [ J K L S. New York A b c d E f g h 1 k 1 New Jersey A b c d e f g h ] k 1 Maryland a B c D e F g h ] k 1 North Carolina A b c D e F G h J k 1 South Carolins I — b — D e f g h — — Georgia a b c d e f g h k 1 Florida a b c d e f g h 1 K 1 Ontario B c D E F G H I 1 K Michigan — B C D e F G H [ J — — Wisconsin A B C D E F g H ] K L Minnesota B C D e F g H 1 K L South Dakota A B C D e F g H K L Nebraska — B C D e F g h ] k L Indiana A B C D e F g H I k 1 Kentucky a B c D e F g H — — Illinois a B C D e F g H ] k 1 Missouri A B c D e F g H k L Kansas A B c D e F g H k L Arkansas A B c D e F g H 1 k 1 Oklahoma a B c D e F g H 1 k 1 Mississippi a b c D e F G h k 1 Louisiana a B c d e F G h k 1 Texas a B c D e F g H ] K L After Moore, Bull. Am. Mus. Nat. Hist., 82, 1944. The Differentiation of Populations I 217 Definition of Symbols A, Head width/head length 0.92 or greater B, 50'^'r or more with tibia bars C, Average number of tibia bars (when present) 1.4 or greater D, 50''/r or more without femur bar E, 50% or more without tympanic spot F, 50% or more with Hght reticulum G, Number of dorsal spots less than 13 H, Number of lateral spots 12 or more I, More lateral than dorsal spots |, 50% or more without lateral reticulum K, 50% or more of males with ovi- ducts L, 50% or more of males with no, or poorly developed, external vocal sacs a, Head width/head length less than 0.92 b, Less than 50% with tibia bars c, Average number of tibia bars ( when present) less than 1.4 d, Less than 50% without femur bar e, Less than 50% without tympanic spot f, Less than 50% with light reticulum g, Number of dorsal spots 13 or more h. Number of lateral spots less than 12 i, Lateral spot number equal to, or less than, dorsal spot number ], Less than 50% without lateral reticulum k, Less than 50% of males with ovi- ducts 1, Less than 50% of males with no, or poorly developed, external vocal sacs living from southern Canada to Mexico, has a range of 15 to 32 °C; where these two overhip, R. sylvatica breeds in the early spring, R. catesbiana in midsummer. ) Laboratory crosses of individuals from difiFerent populations of R. pipiens produce normal offspring when the parents are drawn from populations that are geographically adjacent (e.g., central and southern Florida) or lie at roughly the same latitude (e.g., Texas and central Florida). However, the greater the north-south gap separating the home populations of the parents, the greater also is the proportion of defective (inviable) oflFspring. Eggs from Ver- mont females fertilized by New Jersey or Wisconsin males do not differ in development from those fertilized by Vermont males. If the spermatozoa that fertilize the eggs come from a Louisiana male, some abnormal development occurs, but there is no significant in- crease in mortality. Texas-Vermont hybrids have many develop- mental difficulties, and mortality may reach 100 percent. In a single cross between a male from an eastern Mexican population and a Vermont female most of the hvbrid embrvos died in the gastrula or neurula stage. Thus, the R. pipiens situation might be considered analogous to that of Ensatina discussed below. In the 218 I The Process of Evolution Latitude Locality 46°N Quebec 45 Vermont 44 Wisconsin 40 New Jersey 30 Louisiana 29 Ocala, Fla. 27 EngL, Fla. 32 Texas 22 Mexico I I I I I I I I I 5 10 15 20 25 30 35 40°C Embryonic temperature range Fig. 10.2 I Temperature tolerance ranges for normal embryological development of Rana pipiens from difPerent localities. The lower limit for Quebec and Wisconsin has not been determined but is believed to be identical with Vermont. {From Moore, 1949, Evolution 3. ) Rana case, however, the distribution does not form a ring, and the terminal populations of the series do not occur together in nature. The British satyrine butterfly Coenonympha tullia shows a pattern of differentiation reminiscent of Rana. Crosses between individuals from widely separated populations resulted in some broods in which a number of "females" were intersexual, indicating some genetic incompatibility. (This result follows Haldane's rule that inviability or sterility in hybrids will most likely appear in the heterogametic sex, in this case the females. ) In crosses between less distant popu- lations no abnormalities were found. However, this butterfly has not been as intensively studied as the gypsy moth, Lijmantria dispar, for which Goldschmidt has described many degrees of intersexuality in crosses between populations of various levels of differentiation. For those interested in details, this work is well summarized by Dobzhansky. The First Stages of Genetic Isolation. Populations of some ani- mals that are connected by long series of intermediate forms may occur together and remain distinct. For instance, an interesting pattern of variation has been described in the plethodontid sala- The Differentiation of Populations | 219 manders of the genus Ensatina. These animals hve along the west- ern coast of North America from southwestern British Columbia to southern California. In California they are confined to coastal areas, the Sierra Nevada, and southern interior mountain ranges. There is considerable geographic variation in color pattern and, to a lesser extent, in size (Fig. 10.3). The coastal populations are brown or reddish-brown above, while the Sierra and interior populations become progressively more spotted with yellow, cream, or orange as one travels southward. In the Sierra Nevada, at the latitude of San Francisco Bay, there is an enclave of populations similar to those of the coast, and individuals intermediate between the Sierra Nevada and coastal types are also found. In the characters studied (and with the exception just mentioned) there seems to be rather continuous north-south variation, although taxonomists have broken the continuum into a series of "subspecies" and "zones of intergrada- tion." However, where the southern coastal and inland types meet south of the Central Valley, there is a rather sharp discontinuity in the variation. Strikingly different uniformly colored and blotched forms have been found within 0.2 mile of each other in habitats on the southeast side of Mount Palomar. In Mill Canyon, above Banning, California (about 50 airline miles north of the Palomar locality), in 1962 R. C. Stebbins ^ and C. W. Brown discovered both forms living together, as well as one apparent hybrid and several possible backcross individuals. Whether or not hybrids will also be found where the two forms meet on the slopes of Mount Palomar remains to be seen. A similar situation has recently been reported for neotropical fruit flies, Drosophila paulisiorum. In this case, the pattern is more complex than that described for Ensatina, there being three areas where two groups occur together without interbreeding. In these areas not only is hybridization not detected, but in laboratory tests where the forms were denied the opportunity of mating with their own kind, not even cross-insemination (let alone the production of viable hybrids) was found. However, in laboratory tests it was possible to exchange genetic information between these forms by using a series of intermediate "bridging" cage populations sampled from other geographic areas. How much actual exchange takes place in nature through such bridging populations is an open question. Such complex situations are found in more and more organisms as detailed studies are made. ' We are deeply indebted to Dr. Stebbins for keeping us informed of the progress of this most interesting work. 220 I The Process of Evolution Closely Related Isolates Species Swarms in Fishes. The east African lakes, Victoria, Tanganyika, and Nyassa, support a large number of closely related fish species of the family Cichlidae. For example, in Lake Victoria are found some 70 endemic and 6 nonendemic species of the genus Haplochromis living in three different ecological zones. One group consists of deep-bodied forms with short snouts, horizontal mouths, equal jaws, and bicuspid outer teeth (Fig. 10.4). These fish are found inshore and are bottom feeders. The cichlids of a second group have more slender bodies and longer snouts, their mouths are slightly oblique, their lower jaws prognathous, and their outer teeth conical and caniniform (Fig. 10.4). The members of this group are fish-eating predators, hunting the middle depths of open and inshore waters. A third group of Haplochromis are slender and long- snouted. They have very oblique mouths (in two forms almost vertical), extreme prognathism of the lower jaw, and caniniform outer teeth (Fig. 10.4). These are predaceous surface feeders, eat- ing principally other fish and insects. There is only a moderate amount of diversity in this large com- plex of closely related distinct clusters. Although some of the forms are virtually indistinguishable morphologically, they have been found to be ecologically differentiated and to have distinctive breed- ing coloration. The greatest morphological variation is in the teeth and structures of the head, which is hardly surprising in view of the diverse feeding habits within the group. Sibling Species of Alpine Butterflies. In some cases, superficial similarity may disguise a rather large amount of diversity. Lorkovic has shown that the holarctic butterflies of the Erebia tyndarus group, although very much alike in outward appearance, have wide di- vergence in chromosome number (n = 8, 10, 11, 15, 24, 25, 51, and perhaps 52) and (to a lesser extent) in the morphology of the male and female genital structures. Such outwardly similar forms are often called sibling species. In the western Alps (Fig. 10.5) two forms, Erebia cassioides (n = 10) and E. nivalis (n = 11), occupy two barely overlapping ecological zones, the former in the subalpine (1,400 to 2,400 meters) and the latter in the alpine (2,300 to 2,700+ meters). Although E. cassioides and E. nivalis share a narrow border strip, there is little evidence of gene flow between them. Only 2 of 400 specimens examined were not unequivocally assignable to one species or the other. The two forms have quite distinct life cycles, E. cassioides completing its development in one year, E. nivalis The Differentiation of Populations \ 221 Fig. 1 0.3 [ Ensatina in western North America. Discussion in text. (After Stcbbiti.^, 1949, Universitt/ of California Publications in Zoology 48.) 222 I The Process of Evolution Fig. 10.4 I Cichlid fishes (Haplochromis) from Lake Victoria, Africa. Top, H. macrops; center, H. haijoni; bottom, H. cavifrons. These fishes are approximately 100, 180, and 145 mm long, respectively. [After Boulenger, 1915, Catalogue of the Fresh-water Fishes of Africa in the British Museum {Natural History) III.] The Differentiation of Populations | 223 requiring two. Laboratory crosses indicate strong behavioral isola- tion (females not responsive to males of the wTong form; copulation, when induced, abnormally brief), and hybrids, when produced arti- ficially, are wholly sterile and unlike any individuals found in na- ture. In short, the two forms seem to be completely isolated from each other genetically. Erebia cassioides, however, has a wider geographic distribution than E. nivalis, and in regions where E. nivalis is absent it extends to altitudes as high as those occupied by E. nivalis where both are present. Conversely, in some areas where the mountains do not reach great heights, E. nivalis lives at lower elevations, with E. cassioides correspondingly lower or absent. Erebia tyndarus (n = 10) occurs in essentially the same life zone as E. cassioides, but as one can see from Fig. 10.5 the two are not sympatric. Erebia tyndarus occurs in the central Alps, with E. cas- sioides on the east and west. Experimental crosses (£. tyndarus females X £• cassioides males) showed little behavioral isolation, and about 15 percent of the eggs from these crosses hatched with 25 percent survivorship among the young larvae. Although the ranges of E. tyndarus and E. cassioides adjoin very closely, both in Fig. 1 0.5 1 Map showing distribution of butterflies of the Erebia tyndarus group in central Europe. {From Lorkovic, 1957, Biolo'ski Glasnik 10.) 224 I The Process of Evolution the east and west, there seems to be no significant overlap. Indeed the three species, E. tyndarus, E. cassioides, and E. nivalis, show a striking aversion to coexistence. A fourth alpine species, E. calcarius (n = 8), is found in the Julian Alps, but its exact spatial relationship with E. cassioides is not known at this time. The Galapagos Finches. A large cluster of distinct closely re- lated groups is found in the subfamily Geospizinae of the finch family ( Fringillidae ) . These birds, known collectively as Darwin's finches, are restricted, with a single exception, to the Galapagos Is- lands. One member of this group is found on Cocos Island. First studied by Darwin, they have been the subject of brilliant mono- graphs by Lack and Bowman. There are some 14 distinct kinds of finches, considered by ornithologists to represent six genera. Table 10.2 lists these species. The 14 species are distributed in various patterns over the islands, individual islands within the Galapagos having between 3 and 10 species each (Fig. 10.6). The birds differ primarily in size and in the form of the beak and in other structures related to their feeding habits (Fig. 10.7). There is almost a complete continuum in the amount of differentiation. The cluster known as Platyspiza crassi- rostris is distributed over eight of the islands but shows almost no inter-island differences in the characters studied. The warbler finch, Certhidea olivacea, is found on all the Galapagos and shows con- siderable variation in color from island to island. For instance, the upper parts of both sexes vary from gray -brown (James Island) to very pale gray (Barrington Island). Superimposed on this is varia- tion in the amount of olive tinge. The under parts range from pale Table 10.2 { Species of Darwin's Finches (Geospizinae) Number of Islands on Which Species Is Name Description and Habits Permanent Resident Geospiza magnirostris Large; forages on ground and in bushes and trees. Feeds on small variety of very hard, generally large seeds. 14 Geospiza fortis Medium; habits as above. Feeds on large variety of moderately hard, small to large seeds. 12 Geospiza fuliginosa Small; forages on ground more than G. fortis or magnirostris. The Differentiation of Populations | 225 Geospiza difficilis Geospiza scandens Geospiza conirostris Platyspiza crassirostris Camarhynchus psittacula Camarhynchus pauper Camarhynchus parvulus Cactospiza pallida Cactospiza heliobates Certhidea olivacea Pinaroloxias inornata Feeds on large variety of soft, generally small seeds. 14 Medium; forages on ground. Pre- sumably takes soft seeds. 7 Medium; rests mostly on Opuntia cactus. Feeds on a small variety of moderately hard seeds, also soft plant tissues and nectar. 11 Large; habits poorly known but similar to G. scandens. 3 Large; mostly in dense brush and high trees. Feeds mainly on fleshy fruits, soft to moderately hard seeds, young leaves, and flowers. 10 Medium; forages in trees, brush, and occasionally on ground. Primarily insectivorous, ex- cavating fairly deeply into woody tissues, usually on larger branches. 11 Same as C. psittacula, which it replaces on Charles Island. 1 Small; forages in trees, brush, on cactus, and on ground. Primarily insectivorous, excavat- ing less deeply than C. psittacula, usually on smaller twigs and in lichens. 12 Medium; tanager-like habits. Probes with stick or cactus spine; primarily insectivorous. 7 Medium; habits poorly known, primarily insectivorous. Re- stricted to coastal mangroves. 2 Small; warbler-like. Forages at all levels in trees, occasionally at ground level. Takes only animal food, especially small larvae. 16 Medium; reported feeding on ground and in trees. Presumably takes insects, nectar, and some fruits. 1 Source: Modified from Bowman ( 1961 ', 226 I The Process of Evolution olive-buflF (James Island) and wash-brown (Culpepper, Wenman, Charles Islands ) to white ( Barrington Island ) . The peripherally distributed G. conirostris is, except for size, quite similar to the central G. scandens, the two forms being completely allopatric. Three ground finches (G. magnirostris, G. fortis, and G. fuliginosa ) are widely sympatric and differ superficially in overall size and relative beak size. Where the three forms occur together Fig. 1 0.6 I Map of Galapagos Archipelago showing main islands. Numbers give the total of different kinds of Geospizine finches recorded from each island. {After Bowman, 1961, University of California Publications in Zoology 58. ) 92° 91° 90° « Culpepper © ftWenman © t Cocos © N t 1 1 1 1 10 10 20 i Albemarle © ^Abingdon o® Bindloe © Tower r Narborough >. ( Villi (lo; I'-^^ames ,o ® Uerviso • h Seymc vDuncanv, J *\ y^ — X ^w - (Academy ? (9) ^-^ -^c ^ • ^-^ Barrin| t -T )ur ^ fatigable W /r;^ Chatham (10) ^^ Wreck 1 / iton ^"^CS/ @ Charles Black r'^ Beach / /• , © ) CiN^Hood © 92° 91° 90° The Differentiation of Populations I 227 Geospiza magnirostris Certhidea olivacea Fig. 10.7 I Schematic representation of the relationship between bill structure and feeding habits in 10 species of Geospizinae from Indefati- gable Island. (From Bowman, 1961, University of California Publications in Zoology 58. ) there is almost no overlap in measurements, and observations indi- cate that individuals recognize and mate only with members of the proper form. The three forms sometimes take the same food; their feeding habits are overlapping but not congruent, as shown in Table 10.2. The two members of the genus Cactospizo, C. pallida (the tana- ger-like finch) and C. heliobates (the mangrove finch), are very similar. Both forms are primarily insectivorous, C pallida having the unique habit of excavating for beetles and other insects with its 228 I The Process of Evolution beak and then probing the excavations with a cactus spine or twig. This remarkable behavior compensates for the lack of a long tongue to use as a probe and is one of the few instances of birds using a "tool." On the only island occupied by both C. pallida and C. helio- bates (Albemarle Island) their ecological differences isolate them (C. heliobates, in the coastal mangrove belt; C. pallida, inland). Fig. 1 0.8 I Extremes of ditierentiation in skulls of Galapagos finches. Upper, Geospiza magnirostris ; lower, Certhidea olivacea. {After Bowman, 1961, University of California Publications in Zoology 58.) The Differentiation of Populations 229 Therefore they are allopatric, although in one case they both Hve on the same island. The extremes of differentiation within the genus can be seen (Fig. 10.8) by comparing the skulls of the broadly svmpatric Gcospiza magnirostris (the large ground finch) with Ceiihidca oUvacca ( the warbler finch ) . Darwin's finches show a pattern of differentiation opposite to that found in most groups of birds. The most closely related forms of these finches differ primarily in the size and shape of the beak, whereas closely related forms of other birds are usually differenti- ated most strongly by plumage color. The very closely related Asi- atic nuthatches, Sitfa tephronota and S. neummjer, are clearly differentiated by plumage pattern as well as bill length but only where they are sympatric (Fig. 10.9). Where their ranges are sep- arate they are almost indistinguishable. This exaggeration of differ- ences in an area of sympatrv is an example of character displacement, a phenomenon common in birds. Other instances have been de- scribed in ants, beetles, crabs, fishes, and frogs. Two kinds of termites have been reported to swarm at the same time of day where they are allopatric and at a different time of day where their ranges overlap. Possible causes of character displacement will be discussed in the second half of this chapter on pages 242-244. Host Preference in Parasitic Organisms Differentiation in host preference is widespread among parasitic organisms. This phenomenon has been studied in such diverse or- ganisms as cuckoos, human lice, and nematodes. Cuckoos have developed an unusual form of differentiation. They show what has been called "brood parasitism"; that is, they lay their eggs in the nests of other kinds of birds. In most cases the cuckoo egg is incu- bated by the foster parents, and the voracious cuckoo hatchling crowds its pseudosiblings out of the nest, eventually monopolizing the food brought by its adopted parents. Usually the egg laid by the cuckoo bears a remarkable resemblance to that of the host bird (Fig. 10.10). Cuckoo species seem to be subdivided into groups, each of which tends to lay its eggs in the nests of only one kind of host bird. These subdivisions, called gentes, are not geographically isolated from each other, individuals of one being found in close proximity to individuals of one or more of the others. There are two distinct forms of human Hce (Pediculus humanis), head lice and body lice, which differ strongly in their "ecology" but are nearly identical morphologically. Head lice, as the name implies, are found primarily in the relatively fine hair of the head. Their eggs are glued to hair. Body lice, on the other hand, hve in the clothes, 230 I The Process of Evolution sucking blood where the clothes are in contact with the body. Body lice attach their eggs to the clothing. The common human nematode parasite, Ascaris lumbricoides, is morphologically and serologically indistinguishable from the pig parasite, A. suum, but in most cases eggs from one will not develop properly in the host of the other. DISCUSSION OF OBSERVED PATTERNS The basic reason for the diverse patterns of differentiation described in the preceding section is that the physical environment is, and always has been, heterogeneous. This heterogeneity has meant that evolutionary forces, especially selection, have operated differentially. In turn, this has produced a heterogeneous biotic environment and further differentiation in the forces operating in any portion of that environment. It is all too easy to fall into the mistake of assuming that differ- entiation of populations into genetically isolated forms is somehow the "goal" of evolution. Differentiation often takes place, but usually those instances where incipient species become submerged again are unrecorded. When morphological divergence of two populations has progressed to a certain point, often we no longer say that they interbreed, we say that they hybridize. This implies that they have somehow or other made a "mistake." The fact that occasional genetic interchange enriches the variability of both populations is forgotten. Differentiation is not necessarily a step toward the formation of isolated populations. It is merely one of the many things that hap- pen to populations in nature. Geographic Variation in Selection Pressures Geographic variation, in most cases, may be attributed primarily to the different selection pressures prevailing in different areas. Thus Achillea growing at high altitude is under selection pressure that favors dwarfed forms physiologically adjusted to rigorous mountain- top conditions. Lowland Achillea are not subjected to the same pressures but to others. Nafrix sipedon populations have adjusted both to "normal" swamp habitats and to the special conditions pres- ent on the Lake Erie islands. Cepaea populations face selection pres- sures partially determined by local vegetation types, and Bistort populations to pressures varying with the relative presence or ab- sence of industrial pollution. The frequency of the so-called "sickle- cell" gene in human populations varies geographically; it is high in regions where malignant tertiary malaria is present, low elsewhere. The Differentiation of Populations j 231 Fig. 1 0.9 I Character displacement in Asiatic nuthatches. Bill length and facial stripe in the two species are very different in areas where they occur together but are quite similar where they occur alone. Populations west of the zone of overlap ( Siita neumatjer) : A, Dalmatia and Greece; B, Asia Minor. In the zone of overlap: C, Azerbaijan and Northern Iran; D, Kermanshah; E, Luristan and Bakhtiari; F, Fars; G, Kirman. East of the zone of overlap (S. tephronota) : H, Persian Baluchistan; /, southern Afghanistan; /, Khorasan; K, north-central Afghanistan north of the Hindu Kush; L, northeastern Afghanistan (Pamirs); M, Ferghana and western Tian Shan. (After Vaurie, 1950, Am. Mus. Novitates 1472.) Bill length 32 mm 31 30 29 28 27 26 25 24 23 22 21 r Sitta tephronota (broken lines) I I I I + I I I I X T 1 1 1 T + 1 1 1 + J- 1 C D E F G Sitta neumayer (solid lines) I I I + I I I HI J K L M 232 I The Process of Evolution Biologically sophisticated readers will be familiar with myriad examples in which differences due to different selective pressures ( due to different environments ) have been inferred. For the student a very few more are added. Arctic foxes (following Allen's rule) have short ears and snouts, whereas tropical foxes have long ears and snouts, presumably because the low surface-volume ratio in the former helps conserve body heat, whereas the high ratio in the latter permits the heat to dissipate more rapidly. Indeed, both Allen's and Bergmann's rules seem to be simply functions of the surface-vol- ume ratio problems concerned with heat retention and dissipation. A great many homoiotherms show clinal variation in conformance with these rules, but physiological work to support their validity is mostly lacking. Populations of Papilio glaucus have high frequencies of dark fe- males in certain areas, supposedly because these dark-brown females resemble the Aristolochia swallowtail (Battiis philenor), which is also found in these areas. Battus philenor has been shown to be distasteful to birds, and birds have been observed feeding on adult P. glaucus in the field. Selection apparently favored the develop- ment and maintenance of the mimetic form of P. glaucus in these areas. In the high Sierra Nevada of California two forms of the butterfly Oeneis chryxus are found, a light form in areas of granite rock and a dark form in areas of basaltic rock. The selection pressure involved has not been discovered, but the correlation suggests the work of an as-yet-undetected visual predator. Similar examples of geographic variation in color and in the habitat of many groups of animals have been reported and could be multiplied indefinitely— geographic vari- ation is ubiquitous, and selection has been shown to play a major role in differentiating most populations that have been studied thoroughly. It is a truism to state that populations of organisms in different places will, under most circumstances, be genetically dif- ferent. The variation described in Asclepias seems attributable to a com- bination of differing selection pressures and genetic drift. Asclepias tuberosa is a long-lived perennial growing in colonies of several to many plants. The effective population size is estimated to be less than the median census size of 11 plants. One would expect that, in populations of this small size, genetic drift would become an im- portant factor in evolution, and the data suggest that it is. Neverthe- less, there appears to be a strong selective component in the changes that have occurred along the transect during the 14 years. It is not possible to discuss these in detail, but it is clear that the western genotypes for Z A and Z B are selectively better off than the com- The Differentiation of Populations | 233 parable eastern genotypes. In A. tuberosa interior the genotype for Z B has a selective value about three times that for Z A. During the 14 years there has been a decrease of about 30 percent in vari- ability for both Z A and Z B. In both years, about midway along the transect there appeared to be a zone of heterozygosity expressed as greater variability as well as greater size and apparent vigor. It is probable that hybridization between these subspecies, which were separated from one another during the Pleistocene glaciation and which have subsequently come together, has resulted in an increase in vigor and an extension of range. This study is one of the few in which populations have been investigated over a period of years. Unfortunately, however, Asclepias tuberosa is not a good subject for genetic studies. The cytogenetic bases for the phenomena that have been detected bio- Fig. 10.10 I Egg mimicry. Upper row, eggs of Asiatic crows, Coruus coronoides, C. splendens insolens, and C. s. protegatus; lower row, eggs of the cuckoo Eudynamys scolopacea laid in the same nests as the three crow eggs. {After Baker, 1923, Proa. Zool. Soc. Lond.) I ■■■ 234 I The Process of Evolution metrically are not known. Many other interesting aspects of varia- tion in this species of Asclepias also are poorly understood but are being studied by Woodson. These include the apparently centrif- ugal variation of A. tuberosa interior, which has resulted in a concentrically distributed peripheral subspecies in the western and northern parts of the range, and an interesting variation pattern with respect to color of the flower (which can be studied biochem- ically ) . Exchange of Genetic Information In organisms with relatively continuous distributions, exchange of genetic material among populations will limit the amount of diflFer- entiation that can take place. The "mixing" effect of recombination is most pronounced in adjacent populations, which, because of the relative similarity of their environments, are less subject to diflFer- entiation through selection. The situation is reversed for widely separated populations in a series. A certain amount of genetic in- formation is passed back and forth through the intervening popula- tions, but the ability of this weakened gene flow to swamp out genetic differences is greatly reduced. In most instances these more distant populations have been subjected to quite different selection pressures, enhancing the trend toward differentiation. This is well illustrated by the Rana pipiens and Coenonijmpha tullia cases, in which distant populations are strongly differentiated but pairs of adjacent populations show little differentiation.^ A classic example of isolation by distance is found in populations of a single species of a California desert plant. Around the southern and western edges of the Mojave Desert a small annual, Linanthus parrijae (Polemonia- ceae), shows an interesting pattern of variation. With good rainfall, the plants form an essentially continuous carpet over large areas. In 1941 L. parryae was studied intensively between Palmdale and Lucerne Valley, where it had developed a practically continuous population over some 700 square miles. In most areas investigated, the plant samples consisted of white-flowered forms, but in three isolated sections samples with varying numbers of blue-flowered individuals were collected. The composition of the samples taken from the westernmost of these areas between Palmdale and Llano is shown in Fig. 10.11. The central variable area is separated from ' It is possible that gene flow between distant populations of some series is so slow that the migration effect might be practically indistinguishable from mutation as a source of variabihty. The Differentiation of Populations \ 235 the western one by a 25-mile gap and from the eastern one by an 8- mile gap. The frequency of blue-flowered individuals in samples from these "variable areas" is shown in Fig. 10.12. This frequency distribution of phenotypes is reminiscent of the theoretical gene-frequency dis- tributions in small populations subjected to some selection pressure (see Fig. 6.9). The curve in Fig. 10.12 cannot be fully interpreted because the genetic bases of the observed variation are unknown. However, the U shape is strongly suggestive of populations suffi- ciently small for considerable random fi.xation and loss of genes producing blue flowers to have occurred. It seems likely that selec- tion, drift, and isolation by distance all interact to produce the ob- served pattern of variation in Linanthiis. Some unknown selective factor probably gives a small advantage to blue-flowered plants in the "variable areas," while the small effective population size (cal- culated by Wright to be 14 to 27 plants ) permits considerable drift. In addition, the usual desert year (much drier than 1941) would Fig. 1 0.1 1 I Map showing composition, with respect to flower color, of samples of Linantlius parrijae from an area in southern California. Black sectors, blue flowers; white sectors, white flowers. ( From Epling and Dobzhansky, 1942, Genetics 27, and after Ahrams, 1951, Illustrated Flora of the Pacific States, Stanford University Press. ) 236 I The Process of Evolution produce only scattered Linanthus populations. This, combined with a possible low gene flow in the species ( caused by pollinators trans- ferring pollen mostly between nearby flowers), would help to maintain the established pattern. Cessation of Gene Exchange When the distribution of an organism becomes broadly discontinu- ous, exchange of genetic information between populations (or groups of populations ) may cease entirely. Such situations may de- velop in many ways. Emerging land may divide a marine habitat. The Isthmus of Panama has been repeatedly submerged through geologic time. With each new emergence some populations of Fig. 10.12 I Frequency ot samples of Linanthus parryae containing different proportions of plants with blue flowers. Ordinate, percentage of samples; abscissa, percentage frequency of blues. ( From Epling and Dobzhansky, 1942, Genetics 27.) 30 28 26- 24- 22- 20 18H 16 14 12 10 8- 6- 4- 2 10 20 30 40 50 60 70 80 90 100 30 28 26 24 22 20 hl8 16 14 12 10 -8 -6 4 2 The Differentiation of Populations j 237 marine organisms, previously more or less continuously distributed, become isolated. There is considerable evidence that glacial ad- vances during the Pleistocene repeatedly fragmented the ranges of many organisms. Repeated changes of continental seaways (Fig. 10.13) have isolated and reconnected portions of the continents, causing manifold changes in the continuity of the distributions of organisms. Climatic changes have profound effects on the distributions of plants and animals. Trends toward aridity produce desert or steppe barriers to the passage of organisms requiring high humidity or dense plant cover. Drought may cause large lakes to divide into numerous smaller lakes and rivers to be reduced to isolated series of pools. Increasing rainfall, on the other hand, tends to reunite iso- lated bodies of water, encouraging gene exchange in aquatic or- ganisms while forming barriers for terrestrial organisms. Belts of high humidity form barriers for desert and steppe organisms. It is interesting to note that discontinuities in the distribution of an animal are not necessarily the result of barriers which the indi- viduals are unable to cross. In many cases behavior patterns prevent dispersal across areas that could easily be traversed if the attempt were made. Thus rivers may serve to isolate bird populations on opposite banks, or a narrow strip of woods may effectively separate two meadow populations of butterflies. Isolation Whatever its cause, physical isolation permits the differentiation of populations of sexually reproducing organisms. Isolation is often referred to as if it were the cause of differentiation. It is not, of course; recombination between isolates is prevented, so that each isolate responds only to the selection pressures of its own environ- ment. If an organism is continuously distributed along a humidity gradient, the establishment of populations at the ends, which are adjusted to high and low humidity, will be hindered by the transfer of genetic information back and forth along the gradient. If the middle of the continuum is destroyed, differentiation can continue without such interference. By definition, the environments in which two newly isolated fragments of a population find themselves cannot be identical. Thus the two isolates are subjected to different selection pressures. Because of sampling error, the two new isolates will be of different sizes and will have gene pools that are not identical, so that evolutionary forces will be operating in unlike genetic environ- ments. Since mutation is a random process, it is not to be expected that identical mutations will show up in the two isolates. Thus, 238 I The Process of Evolution through physical isolation, a single evolving unit may become tw^o or more evolving units. As long as such evolutionary units remain isolated, they are free to respond independently to evolutionary forces. However, should circumstances permit such units to regain contact, numerous possibilities are presented. Through the course of time there have occurred innumerable environmental changes that might have brought previously isolated populations back together. The effects of ancient cataclysms have been modified or completely erased from the record. Two relatively recent overlapping events have undoubtedly produced effects which are conspicuous today. The first of these events is the Pleistocene glaciations, which changed climate on a vast scale, causing plants and animals to migrate or perish. The glaciers also scoured the earth, mingling soil types, creating lakes, and rearranging drainage systems. The advent of man has had similar effects. It is difficult to over- estimate the importance of man as a factor in evolution. First with fire and then with domesticated plants and animals, he has modi- fied the environment. Many of the instances of interbreeding of previously separated forms are the result of man's conscious or un- conscious intervention. By breaking into the vast stored-energy reserves of climax ecological communities, man has diverted energy for his own purposes and grossly modified what we even yet think of as the "natural" ( prehuman ) environment. Fusion of Populations. At one extreme, populations may have been isolated for such a short time or subjected to such similar con- ditions that divergence has been minimal. When the populations reunite, individuals mate at random and the offspring from matings between parents of different populations are as successful as those from members of the same isolate. The two previously isolated populations then fuse into a single population. This is seen when- ever Drosophila lines, isolated for a few generations in the labora- tory, are combined, or when guppies kept for a few years in one aquarium are dumped in with those that have been kept in another. The situation is certainly very common in nature also; it has not been given very much attention by zoologists. Meeting with No Gene Exchange. At the other end of the spec- trum, populations that have diverged a great deal may come to- gether and all attraction between individuals of the different populations may have been lost, resulting in no interpopulation matings. In most cases this means that gene flow between the evolu- tionary units has ceased, although, as in Drosophila panlistorum, genetic information may still be exchanged by such isolates via a "ring of races." The pattern in Ensotina is of great interest, as it is The Differentiation of Populations i 239 Fig. 10.13 I Distribution of seas in tlie northern Western Hemisphere during four geologic periods. Black, oceanic areas; dark gray, most persistent seaways on the continental platform; light gray, areas submerged during part of period only; dotted light gray, areas of temporary flooding; white, persistent land areas. ( From Moore, 1949, Introduction to Historical Geology, McGraw-Hill. ) 240 I The Process of Evolution not yet known whether the differentiated terminal populations will fuse or whether they will differentiate further to produce a typical ring-of-races pattern. Situations where two or more similar kinds of organisms live in close proximity without apparent hybridizing are numerous, but there seem to be no documented cases where rejoin- ing segregates have shown no interest in each other. This does not indicate that such situations never develop, merely that they have not been observed. Perhaps the most likely candidate for a situation in which two closely related kinds of animals have become sympatric and have not required selection against hybrids to reinforce isolating mech- anisms (as described below) is in the flycatcher genus Empidonax. N. K. Johnson has recently studied an area in eastern California in which two forms, Empidonax wrightii (gray flycatcher) and E. oberholseri (dusky flycatcher), are sympatric. It is possible that this sympatry is of very recent origin, the result of habitat changes brought on by logging operations in the middle of the last century. Empidonax wrightii breeds in sagebrush or small trees and tends to forage in open areas, whereas E. oberholseri is a forest-chaparral bird. Where they overlap in an area of mixed clearings and broken forest the birds retain their habitat separation but defend their territories interspecificalhj . The two kinds are so similar that they can be distinguished with assurance only by careful wing measure- ments and wing-tail ratios. Johnson has been able to detect distinct differences in vocalizations, presumably concerned with pairing and pair-bond reinforcement, but the challenge calls and appear- ance are so similar that E. oberholseri territories are defended against E. wrightii and vice versa. In spite of this similarity, several dozen mated pairs collected by Johnson all showed positive assort- ment (no oberholseri-wrightii pairings), indicating that the birds have no trouble in properly choosing mates. Perhaps the recogni- tion signs (whatever they may be) were originally weaker and were reinforced by selection, but if this was the case the process must have been completed very rapidly. Limited Gene Exchange. In a similar case of sympatry, pre- sumably permitted by human disturbance of the environment, two kinds of Mexican towhees, Pipilo erijthrophthalmus and P. ocai, have come into contact and are now hybridizing. In this case previ- ous ecological isolation (P. erijthrophthalmus mostly in oaks and brushy undergrowth, P. ocai mostly in coniferous forest and associ- ated undergrowth) seems to have broken down when lumbering and agriculture produced second-growth situations suitable to both forms. Indeed, hybridization at one level or another is widespread The Differentiation of Populations | 241 among birds (e.g., gulls, ducks, grackles, grosbeaks, honeyeaters, birds of paradise) and is usually interpreted as occurring in zones of secondary contact. Whatever its interpretation, it is evident that differentiation of bird populations may be a much more complicated process than one would assume from the neat arrays of "species" and "subspecies" found in bird checklists. Similarly, it has been shown rather clearly that in many groups of plants considerable genetic interchange is possible after the so-called specific level of differentiation has been reached. This may be en- tirely at the diploid level, or it may involve polyploidy and apomixis. Examples of the latter have been discussed in Chap. 9. Most in- stances of such hybridization fall into the category of what has been called introgressive hybridization. The chances of an Fi hybrid offspring crossing with another such hybrid in the early stages of hybridization are much less than the probability of crossing with one or the other of its parents. The hybrid derivatives are almost always intermediate with respect to their ecological requirements, just as they are intermediate with respect to morphological traits. Backcrosses to one parent or another will thus be more likely to find an appropriate ecological niche than the Fi or Fo individuals. The result is that genetic submergence of the two hybridizing enti- ties is unlikely to occur. Rather, portions of the germ plasm of one species will infiltrate the genotype of the other. Variability of the parental types will be increased in the direction of the hybridizing entity, and the species or subspecies may be able to increase its range and to move into habitats previously unoccupied. An interesting example of this in the sunflower genus (Helian- thus) in California has been studied by Heiser. Helianthus annuus is a common weedy species found in most of the United States; there is considerable evidence that it was introduced into California by Indians in relatively recent times. Helianthus holanderi occurs in California in two races. One is almost completely restricted to serpentine soils, an ecological situation in which relatively few and specialized plants are found. The other subspecies is a weedy form. Heiser has shown that the weedy race of H. bohnderi probably originated from the introgression of genetic material of the wide- spread weedy H. annuus into the serpentine race of H. holanderi. The details of how this came about need not concern us here, but by making the appropriate crosses the derived subspecies can be syn- thesized in the laboratory. It is interesting that the introgression has been reciprocal; in addition to creating a larger, weedier form of H. holanderi, it has resulted in the formation of a smaller form of H. annuus, apparently with extended ecological amplitude. 242 I The Process of Evolution Most species of plants seem to be strongly restricted ecologically. Where the ecological barriers are strong, genetic interchange does not often take place, and the hybrids with their intermediate eco- logical preferences do not survive. But when the ecological barriers break down, as in habitats subject to erosion or disturbance by man or glaciers, for example, the intermediate types may suddenly find suitable habitats and become common. When crossing between two species does occur, the result usually is not the swamping of the original species but the enrichment of the variation of the parental forms. Indeed, this appears to be very common in many genera of perennial plants studied in the United States. Selection against Hybrids. It seems likely that, when highly differentiated populations rejoin, selection operating against hybrid individuals usually reinforces factors tending to prevent hybridiza- tion. The exchange of genetic information between the isolates often becomes negligible. It is entirely possible that, when more is known about the processes of differentiation, it will be discovered that hybridization between individuals of rejoining segregates is almost universal, in other words, that mechanisms preventing exchange of genetic material between differentiated forms usually arise only through relatively unsuccessful hybridization after sympatry has been reestablished. An interesting experimental demonstration of this mechanism was obtained by Koopman, who synthesized artificial mixed populations of Drosophila pseiidoobscura and D. persimilis and held them at low temperatures (16°C) at which sexual isolation between the two is at a low ebb (i.e., hybrids are formed more readily at low than at high temperatures ) . Under the experimental conditions, the hybrids were extremely unsuccessful, but Koopman intervened to produce complete failure of hybridization by removing all hybrid individuals before they could reproduce (hybrids were identified by genetic markers ) . Over a period of several generations the proportion of hybrids formed showed a marked decrease, indicating a reinforcement of whatever factors were operating to prevent hybridization. Koopman was able to show that the isolating mechanism was at least in part sexual; i.e., males "preferred" to mate with females of their own kind. In nature, D. pseudoobscura and D. persimilis, although occurring in the same geographic areas, presumably do not hybridize for two reasons. First of all, there is considerable ecological isolation, D. persimilis usually occurring higher in the mountains and preferring cooler, shadier spots than D. pseudoobscura. Sexual isolation must also play a part, for except at low temperatures newly captured The Differentiation of Populations 243 flies show little tendency to hybridize. It is suspected that other un- detected factors also help to keep the two entities apart in nature. In the experiments just described, the two known factors were re- moved by crowding the flies together at low temperatures. In a very short time the action of natural selection established a barrier that was at least partly sexual where one had not existed previously. Unfortunately, we know very little about which combinations of gene flow and hybrid inviability lead to fusion of reuniting segre- gates and which lead to total differentiation. This is a wide open field for study. Patterns of Differentiation The Galapagos Finches and African Cichlids. The complexity of differentiation patterns must not be underestimated. A great many forces seem to have interacted to produce the complicated pat- tern observed today in the Galapagos finches. It is likelv that the Geospizinae are all descended from a small flock (perhaps a single pair) of fringillid ancestors which accidentally reached the islands from the South American mainland. It is highly unlikely that the immigrants would have represented a large and random sample of the parental population; indeed, they were most likely a small and biased sample, containing only a restricted segregate of the parental gene pool. This sampling error means that the selective forces on the islands, even if they were similar to those of the main- land, would operate differentially on the island birds, because they are operating in a genetic environment quite different from that on the mainland. Remember that, in discussing the operation of evolu- tionary forces, a single locus cannot be considered in isolation; the effects of pressure on one locus will depend in part on the composi- tion of the entire genotype. Sampling error and the resultant change in genetic environment that commonly occurs when new colonies are established have been described as the founder principle. It is, of course, a special case of genetic drift. Once the finches were established on one island, it was only a matter of time before migrants reached others in the group. Popu- lations on different islands, being subjected to different selection pressures and the effects of the founder principle, probably differ- entiated rapidly. When migrations and remigrations brought differ- entiated forms into contact, selection in many cases must have operated against the tendency to hybridize, as described above. Size and shape of the beak of Galapagos finches are related to the kinds of food eaten and also are used by the birds for identifying mates. Abundance of different food sources varies among the 244 I The Process of Evolution islands, and it is likely that selection caused some differentiation in the beaks and associated structures in isolation. When contact was reestablished, selection probably caused greater differentiation of these structures because they were important in recognition. Such differentiation also seems to have an additional advantage in reducing the types of individuals eating the same kind of food. Such selection for "reduced competition" obviously occurs, but its exact mode of operation is unclear. The history of the Haplochromis swarm must have been similar to that of the Galapagos finches. Migrants from river systems col- onized ancient Lake Victoria. Multiple colonizations (separated by appropriate time intervals) could alone account for the observed diversity. New arrivals found old immigrants already partially dif- ferentiated; selection against hybridization finished the job. In addition, repeated cycles of drying and inundation may have al- ternately fragmented and reunited segregates, permitting the mech- anisms of differentiation to act. It is also possible that areas of different types of bottom, shoreline, water depth, etc., acted (and act today) as intrinsic barriers to the dispersal of the various forms within the lake. Sibling Butterfly Species. The Erebia tijndarus group is another excellent example of the subtle interactions possible between differ- entiated forms. For instance, in spite of their different life-cycle adjustment to altitude, E. cassioides and E. nivalis seem to be strongly influenced by each other's presence or absence, the alti- tudinal restriction appearing where the two forms occur together. The present distributional picture seems to represent the results of differentiated populations interacting over a varied field. As with Darwin's finches, the exact nature of these interactions is difficult to specify. "Selection to avoid competition" is not an explanation in itself. It is necessary to know exactly how differential reproduc- tion of genotypes came about. Presumably there was some differentiation before meeting. For instance, when the populations ancestral to today's E. nivalis and E. cassioides populations came together, the E. nivalis may have, on an average, lived more toward the upper limit of the joint range and E. cassioides, on an average, nearer the lower limit. Then there must have been some advantage accruing to the E. nivalis genotypes that "preferred" the high altitude location and those E. cassioides that tended to remain low. Perhaps the relative scarcity of the other form reduced the possibility of wasting gametes through unsuccess- ful hybridization. Then again, maybe unlike larvae tended to can- nibalize each other. Perhaps waste products of the E. nivalis larvae tended to inhibit the growth of E. cassioides, and/or vice versa. The Differentiation of Populations 245 The food requirements of the various isolates may have been too similar, or the number of available niches too restricted to permit the sort of habitat specialization seen in Darwin's finches. The result is that each form now occupies the areas to which it is best suited. The historical details will probably never be known. Some forms, perhaps, lacking the genetic variability to differentiate further, became extinct. This could happen when severe conditions in an area greatly reduced the food supply, aijd larvae of another Erebia species proved to be much more efficient at utilizing the restricted food supply. Some forms (e.g., E. calcarius and western E. cassioides) may never have met. It is interesting to note that, in the E. tijndarus group, hybrid sterility exists in crosses between dis- tant relatives such as E. iranica (n = 51) and E. calcarius (n — 8) where there is no behavioral isolation. These forms have apparently never been in contact; thus the selective basis for the development of isolating mechanisms has never been present. Differentiation of Parasites. Patterns of differentiation that in- volve strains preferring different foods or hosts are poorly under- stood. It has frequently been observed that, in the laboratory, strains of parasitic organisms may be successfully switched from the usual host organism to another by transfer of large numbers. For instance, the human louse, Pediculus hiimanus, can be converted into a rabbit louse. Large numbers are transferred, and the relatively few able to feed survive and reproduce. This process of selection eventually results in a strain that is happy on rabbits but unenthusiastic about men. (Individuals will feed on men, but a colony will not thrive.) A similar selective process may well be responsible for the transforma- tion of head lice into body lice when the former are subjected to the normal environment of the latter. (It has been suggested that the genetic structure of louse populations encourages a plasticity that makes the transformation in either direction relatively simple.) Needless to say, this structure is itself undoubtedly a result of selec- tion. The Ascaris may differ from the Pediculus principally in that the Ascari are not so protean genetically. There can be little doubt about the selective advantage accruing to those cuckoos whose eggs closely match those of the host bird. Rates of desertion of eggs laid in the nests of the usual host are much lower than of those laid in the nests of unusual hosts. How- ever, the exact way in which gentes developed and are maintained is still somewhat of a mystery. It seems unlikely that gentes are genetically isolated from each other. There is no sign of differentia- tion among them except in the egg habitus; such differentiation would be expected if each gens was an isolated exolutionary unit. However, because only superficial characteristics are studied in many 246 I The Process of Evolution ornithological investigations, such differentiation may be present but undetected. There also seems to be no mechanism that could prevent interchange of genetic information in places where several gentes occur together, especially in view of the rather large ter- ritories occupied by the males. The resemblance between the cuckoo egg mimics and the series of mimetic polymorphs found in females of Papilio dardanus is largely superficial. The major difference lies in the necessity for the female cuckoos to lay their eggs in the nest of the proper host spe- cies. (The choosing of the proper nests by the female cuckoos may be explained by the phenomenon of imprinting. ) It is not sufficient for a population to be genetically structured so that a multiplicity of distinct forms is produced; the structure must be such that the proper egg type is laid by a bird which chooses a particular foster parent. Thus if the female cuckoo raised in the nest of a reed warbler were to be inseminated by a male of the gens parasitizing the white wagtail, her eggs might be better mimics of white wagtail eggs than of reed warbler eggs. She would continue to lay her eggs in the "wrong" nest, and in all probability her young would have a rela- tively low chance of survival. Unfortunately, nothing is known of the genetics of cuckoo egg color, and so the importance of the male con- tribution is conjectural. It seems likely that, in both cuckoo gentes and Papilio dardanus mimetic polymorphs, differentiation may have taken place in isola- tion. When two differentiated isolates of the butterflies came into contact in areas where both models were present, disruptive selec- tion established a polymorphic system (as described in Chap. 7). However, when two cuckoo gentes, each parasitizing a different host, came into contact in an area occupied by both hosts, another factor was probably added. In addition to disruptive selection (lower re- productive success of genotypes producing eggs intermediate be- tween the hosts), there was probably also selection favoring be- havioral patterns that encouraged positive assortative mating (pair- ing of individuals raised by the same foster parents ) . This seems to have been accomplished most successfully where the host forms have somewhat different ecological requirements, the male cuckoos remaining in the familiar habitat where they were reared and con- sorting with females of like background. In places where the habitats of the host forms tend to overlap, intermediate-type eggs are often laid and the mimicry breaks down. The gentes of cuckoos seem to be somewhat intermediate between polymorphic forms and what might be called host races. The degree of perfection of the mimicry apparently is dependent on the degree The Differentiation of Populations 247 of ecological isolation enjoyed by the gens. Where gentes occur together in the same general area, there is no sign that this degree of isolation is sufficient to produce enough genetic differentiation to result in reproductive incompatibility. There is every indication that such gentes are not "incipient species" and that divergence at the species level occurs in geographic isolation. Needless to say, the exact status of the cuckoo gentes is deserving of much additional study. Allopatric Speciation Differentiation of physically isolated populations, when this differ- entiation goes to the point that reunion of the populations does not occur if contact is reestablished, is known as allopatric speciation. An abundance of evidence suggests that allopatric speciation is the fundamental cause of organic diversity. It is the splitting mech- anism, v/hich, coupled with extinction, is necessary to explain the large numbers of relatively distinct kinds of plants, animals, and microorganisms that populate the earth. Little is known about the time required for populations to differ- entiate. In any given case, many variables would affect the required time span, including population sizes, magnitude of selection pres- sures, degree of isolation, and the genetic system of the organism. In most cases speciation seems to be a much more drawn-out process than could be conveniently observed by the evolutionist. However, much recent work (e.g., that on industrial melanic moths and on Cepaea and Nafrix) indicates that selection pressures in nature may be generally higher than once thought; if this is the case, then spe- ciation may also occur more rapidly than has been assumed in the past. At any rate, the evidence on which is based the view that allopatric speciation is the primary splitting mechanism in evolution is not direct observation but the presence of patterns of variation that seemingly represent every conceivable stage in the postulated process. Some of these have been discussed in this chapter; the bio- logical literature is replete with others. Sympatric Speciation Can distinct new kinds of organisms arise in the absence of physical isolation of populations? The answer is certainly "yes" in the case of allopolyploidy (discussed in Chap. 9) and for organisms in which sexual processes are absent. Each individual of completely asexual organisms (such as some rotifers) is a genetic isolate, and species are 248 I The Process of Evolution clusters of clones that owe their similarity to interclone selection. One of the enduring controversies among evolutionary theorists concerns the possibility of sympatric speciation by sexual organisms. Certainly, as stated above, the vast majority of evidence indicates allopatric speciation to be the rule, and many cases that have been presented as evidence for sympatric speciation are easily explained on other grounds (e.g., the cichlid-species swarms). However, one argument against sympatric speciation is that gene flow will swamp out any differences produced by a disruptive selection pressure. There has recently been some provocative work by Thoday and his coworkers and by Streams and Pimentel which indicates that this is not necessarily so. These workers have shown in laboratory experi- ments with Drosophila that disruptive selection can produce diver- gence in the absence of isolation. For instance, Thoday and Gibson subjected a wild-type population to disruptive selection for chaeta number, with both high and low selected individuals being placed in a common vial for mating. At the end of 12 generations the original population had split into two populations, which produced few hybrids. Therefore, sympatric speciation may not be theo- retically impossible, but its significance in nature is yet to be deter- mined. As Slobodkin has aptly stated: In one sense, the distinction between theoretician, laboratory worker, and field worker is that the theoretician deals with all conceivable worlds while the laboratory worker deals with all possible worlds and the field worker is confined to the real world. The laboratoiy ecologist must ask the theoretician if his possible world is an interesting one and must ask the field worker if it is at all related to the real one. In the sympatric-speciation controversy the ball seems to have been passed to the field worker. SUMMARY Available evidence indicates that differentiation in isolation is the primary source of organic diversity. Populations physically sepa- rated from each other (so that gene flow is minimized or absent) have different evolutionary "experiences" and thus differentiate genetically. If this process proceeds beyond a certain point, the populations will not reunite if contact is once again established. The investigation of this genetic "point of no reunion" and the develop- ment of generalizations concerning it are among the most difficult The Differentiation of Populations | 249 problems confronting evolutionists. Populations that have become so distinct as to obviate the possibility of future merger may still ex- change genetic information, reciprocally obtaining variation that may stimulate further evolution. It should not be assumed, how- ever, that the "purpose" of evolution is to create diversity nor that occasional genetic interchange between nearly isolated groups is in some sense bad or aberrant. REFERENCES Bowman, R. I. 1961. Morphological differentiation and adaptation in the Galapagos finches. Univ. Calif. Publ. Zool. vol. 58. The comprehensive source on the Geospizinae. Includes extensive references to the litera- ture, including Lack's classic 1947 monograph. Brown, W. L., and E. O. Wilson. 1956. Character displacement. Sys- tematic Zool. 5: 49-65. A good series of examples of the phe- nomenon. Clausen, J. 1951. Stages in the Evolution of Plant Species. Cornell Univ. Press, Ithaca, N.Y. A brief survey of the classic studies of Clausen, Keck, and Hiesey on ecotypic differentiation in plants. Dobzhansky, T. 1951. Genetics and the Origin of Species. Columbia Univ. Press, New York. This is the main source for the genetic aspects of population differentiation. Keck, David D. 1957. Trends in systematic botany. In Surveij of Bio- logical Progress, vol. 3. Academic, New York. Many examples of dif- ferentiation of populations of plants are discussed. Mayr, E. 1963. Animal Species and Evolution. Harvard Univ. Press, Cam- bridge. This scholarly and exhaustive treatise supersedes the author's earlier classic Systematics and the Origin of Species (Columbia Univ. Press, New York, 1942). It will long remain the source book on specia- tion in animals. [ed.j. 1957. The species problem. Am. Assoc. Advance. Sci. Symposium 50. A series of articles, many of which are pertinent to this chapter. Stebbins, G. L. 1950. Variation and Evolution in Plants. Columbia Univ. Press, New York. Contains many examples of differentiation in plants. Thoday, J. M., and J. B. Gibson. 1962. Isolation by disruptive selection. Nature 193: 1164-1166. This paper and Streams and Pimentel {Atner. Naturalist 95: 201-210) are the critical references on svmpatric divergence in the laboratory. Woodson, Robert E. 1962. Butterflyweed revisited. Evolution 16: 168- 185. The resampling of a transect made in 1947 across the United States is described and the results evaluated. Many papers on differ- entiation of populations are published in Evolution, and a survey of the back numbers will be of interest to any student of the subject. 11 major patterns of variation Can the processes that account for the differentiation of populations be the same ones that are responsible for the great diversity of life? The efficacy of mutation, selection, migration (in the genetic sense), and drift in producing different colors of Linantlms flowers, geographic variants among butterflies, or species of birds has been described in the preceding chapter. Now the question may be asked: Is the same constellation of factors also responsible for the differences among flowers, butterflies, and birds? Are these factors responsible for the existence of extremely distinct clusters as well as for those separated by relatively small gaps? Because of the pri- marily taxonomic orientation of early evolutionary studies, this is often considered to be the problem of the origin of higher taxonomic categories. Some paleontologists and geneticists have felt that higher categories, such as genera, families, and orders, may have resulted from evolutionary processes ( macroevolution ) different from those studied at the species level ( microevolution ) . It seems clear from the evidence from many fields of biology that, because of the immense amount of time during which evolution has been taking place, there is no need to postulate other processes in addi- tion to those previously discussed. EXTINCTION AND B I G E G R A P H I C PROVINCIALISM The existence of extremely distinct clusters can be accounted for by extinction or by inadequate geographic sampling. Of course, with possible very rare exceptions such as certain fish species whose entire populations seem to occur in single small springs, even the most distinct clusters are made up of smaller subclusters with some degree of variation among them. Extinction The sole surviving member of the reptilian order Rhynchocephalia, the tuatara {Sphenodon punctatus), is found only on about a dozen islets off the coast of New Zealand. The groups of individuals on different islands certainly belong to different mendehan populations, but the degree of genetic divergence among these populations ( and the amount of interchange among them ) is unknown. Other animal isolates of this sort are numerous. The strange Peruvian butterfly Styx inf emails is a member of the family Lycaenidae (related to | 251 252 I The Process of Evolution our common blues and hairstreaks ) , but its many structural pe- culiarities clearly set it apart from other lycaenids, and it is placed in a separate subfamily. Again, nothing is known of the degree of differentiation that may exist within the cluster. If one ignores his fossil record, man is a very distinct organism. There is complete reproductive isolation between Homo sapiens and his nearest living relatives (the anthropoid apes). As far as is known, differentiation within the human species has not progressed to the point where segregates within the species are infertile upon crossing. However, in contrast to Sphenodon and Styx, there is a great deal of geographic variation within Homo sapiens. Our con- cepts of "race" are based primarily on variation in a few conspicu- ous external characters (skin color, hair type, skull shape), but there is also variation in less obvious characters, notably blood type and hemoglobin type (Chap. 7). The Homo sapiens cluster therefore does not fragment easily into distinct subclusters. Virtually all sub- groups of man exchange genes to some extent, with the result that patterns of variation are exceedingly complex. Indeed, discordant variation in which patterns for the various characters studied are widely different is very common. A good example of this discordant variation can be seen in the comparison of distributions of blood types in human populations (Fig. 11.1). The distributions of blood group genes A and B show little resemblance. Examples of very distinct forms are also found commonly in the plant kingdom. Ginkgo biloba, the maidenhair tree, is, like Spheno- don, the only living member of its group, a very distinct order of gymnosperms. The phyletic line to which it belongs can be traced far back into the fossil record. During the Mesozoic there were many genera and species of Ginkgoales (Fig. 11.2). For some rea- son unknown to us, the line became extinct in the Tertiary, with the exception of Ginkgo biloba. The exact native habitat of the ginkgo is not known, but for centuries th