Unofficial SJG Archive

The Unofficial Stephen Jay Gould Archive

Unofficial SJG Archive

Genetics and the Origin of Species (1951)

by Theodosius Dobzhansky

Chapter I.  Organic Diversity

an has always been fascinated by the great diversity of organisms which live in the world around him. Many attempts have been made to understand the meaning of this diversity and the causes that bring it about. To many minds this problem possesses an irresistible aesthetic appeal. Inasmuch as scientific inquiry is a form of aesthetic endeavor, biology owes its existence in part to this appeal.

Organisms are amazingly varied in the gross and in the microscopic structure of their bodies. They are equally varied in their ways of life. Several generations of morphologists and anatomists have worked to describe the structures of recent organisms, and the end of this work is not yet in sight. Paleontologists keep discovering a tremendous variety of fossils. Ecologists have only begun to explore the multiform relationships between organisms and their environments. The extent of the diversity of physiological and biochemical traits in living beings is still quite imperfectly known.

All this diversity is at first sight staggering and bewildering. The greatest achievement of biological science to date is the demonstration that the diversity is not fortuitous. It has not arisen from a whim or caprice of some deity. It is a product of evolution, an outcome of a long historical process of development, the duration of which is surmised to be of the order of two billion years (Simpson 1949). Biology can not fathom whether life may be a part of some Cosmic Design. But biology does show that the evolution of life on earth is governed by causes that can be understood by human reason. Darwin was the first to infer that organic diversity is a response of the living matter to the diversity of environments on our planet.

The adaptedness of organisms to their environments is striking. The structures, functions, and modes of life of every species are at least tolerably consonant with the demands of its environment. Every organism is adjusted to occupy and to exploit certain habitats. But habitats vary in space. Evolution has, accordingly, brought about the diversity of allopatric organisms, which inhabit different territories. Diverse habitats occur also within territories which are accessible to an individual organism in its wanderings during its lifetime, or in which the sex cells or seeds of an individual are dispersed. Adaptation to such local diversities of habitats brings about the diversity of sympatric organisms. Finally, the habitats change with time, and the inhabitants often change hand in hand with the environmental changes. The evolutionary changes not only enable life to endure the shocks emanating from the environment; they permit life to conquer ever new habitats, and to establish progressively firmer control of the older ones.


Organic diversity is an observational fact more or less familiar to everyone. It is perceived by us as something apart from ourselves, independent of the working of our mind. Experience shows that every person whom one meets differs from all met before. Every human individual is unique, different from all others who live or lived. This is probably true also of individuals of organisms other than man.

The uniqueness and unrepeatability of individuals are aspects falling primarily within the province of philosophers and artists. Although individuals, limited in existence to only a short interval of time, are the prime reality with which a biologist is confronted, a more intimate acquaintance with the living world discloses a fact almost as striking as the diversity itself. This is the discontinuity of the variation among organisms. If we assemble as many individuals living at a given time as we can, we notice at once that the observed variation does not form any kind of continuous distribution. Instead, a multitude of separate, discrete, distributions are found. The living world is not a single array in which any two variants are connected by unbroken series of intergrades, but an array of more or less distinctly separate arrays, intermediates between which are absent or at least rare. Each array is a cluster of individuals which possess some common characteristics. Small clusters are grouped together into larger secondary ones, these into still larger ones, and so on in an hierarchical order.

Biologists have exploited the discontinuity of variation to devise a scientific classification of organisms. The hierarchical nature of the observed discontinuity evidently lends itself admirably to this purpose. For the sake of convenience the discrete clusters are designated races, species, genera, families, and so forth. The classification thus arrived at is to some extent an artificial one, because it is a matter of convenience and convention which cluster is to be designated a genus, family, or order. But the clusters themselves, and the discontinuities observed between them, are not, as sometimes contended, abstractions or inventions of the classifier. Classification is natural and not artificial, in so far as it reflects the objectively ascertainable discontinuity of variation, and in so far as the dividing lines between species, genera, and other categories are made to correspond to the gaps between the discrete clusters of living forms. Biological classification is simultaneously a man-made system of pigeonholes, devised for the pragmatic purpose of recording observations in a convenient manner, and an acknowledgment of the fact of organic discontinuity. A single example will suffice to illustrate the point.

Any two cats are individually distinguishable, and this is probably equally true of any two lions. And yet no individual has ever been seen about which there could be a doubt as to whether it belongs to the species of cats (Felis domestica) or to the species of lions (Felis leo). The two species are discrete because of the absence of intermediates. Therefore, one may safely affirm that any cat is different from any lion. Any difficulty which may arise in defining the species Felis domestica and Felis leo, respectively, is due not to the artificiality of these species themselves, but to the fact that in common as well as in scientific parlance the words "cat" and "lion" frequently refer neither to individual animals nor to all existing individuals of these species, but to certain modal, or average, cats and lions. These modes and averages are statistical abstractions which have no existence apart from the mind of the observer. The species Felis domestica and Felis leo are evidently independent of any abstract modal points which we may contrive to make. No matter how great may be the difficulties encountered in finding the modal "cats" and "lions," the discreteness of these species is not thereby impaired.

In organisms which reproduce sexually and by cross-fertilization, the reality of species as biological units can also be demonstrated by a quite different method. If mating and procreation are observed, it will soon be found that organisms form usually quite discrete reproductive communities. These communities consist of individual united by the bonds of sexual unions, as well as of common deseent, and common parenthood. It will doubtless be discovered that one of these reproductive communities consists of animals which, on the basis of previous morphological study, were called cats, while another community will consist of lions. No lion cub is ever born to a pair of cats, nor is the converse ever observed. A species is, consequently, not merely a group and a category of classification. It is also a supraindividual biological entity, which, in principle, can be arrived at regardless of the possession of common morphological characteristics.

What has been said above with respect to the species Felis domestica and Felis leo holds for innumerable other pairs of species. Discrete groups are encountered among animals as well as plants, among structurally simple as well as among very complex ones. Formation of discrete groups is so nearly universal that it must be regarded as a fundamental characteristic of organic diversity. An adequate solution of the problem of organic diversity must consequently include, first, a description of the extent, nature, and origin of the differences between living beings, and, second, an analysis of the nature and the origin of the discrete groups into which the living world is differentiated.

The true extent of organic diversity can only be surmised at present. In 1758 Linnaeus knew 4,235 species of animals. How many species are known at present, and how many remain to be discovered, can be estimated only very roughly. According to Mayr (1946a), 8,616 species of birds have been described, and it seems doubtful that even as many as 100 remain to be discovered. The systematics of birds is, however, known better than that of any other group of comparable size, not only because collections have been made in most parts of the world but also because the evaluation of the taxonomic status of the described forms as species or as races and subspecies has acquired a fair degree of reliability and internal consistency. In other groups—notably among insects—many new species are described every year, large additions may be expected in the future, and some forms now regarded as species will be eventually reduced to subspecific status and vice versa. The estimates of numbers of species known have, therefore, quite different margins of error in different groups. They are relatively more reliable for the vertebrates, for which Mayr gives the following figures:

Reptiles and Amphibians5,500
Total Vertebrates35,600

Mayr's estimates for the phyla of the animal kingdom are:

Tunicates and Prochordates1,700
Worms and related groups25,000
Coelenterates and Ctenophores10,000

Among the estimated 815,000 known arthropod species, some 750,000 are insects. These numbers are growing rapidly, and may eventually be more than doubled. The number of plant species is smaller than that of animals. The following estimates have been kindly furnished by Professor Carl Epling:


A million and a half species of animals and plants combined is, therefore, a minimal estimate of the number now living on earth.

Adaptive Peaks

Organic diversity and discontinuity of organic variation are perceived by direct observation. Similarly, we recognize, through observervation and experiment, that living beings with different body structures occur in different habitats, and that they possess organs, traits, and forms of behavior which permit them to secure food, shelter, protection from enemies, and to care for the offspring in countless different ways. It is a natural surmise as well as a profitable working hypothesis, that the diversity and discontinuity on one hand, and the adaptation to the environment on the other, are causally related. The present book is devoted to an inquiry into the nature of this relationship. It may, however, be useful at the outset, as an aid in arriving at clear-cut statements of the problems involved, to consider a symbolic picture of the relations between the organism and the environment devised by Wright (1932).

Every organism may be conceived as possessing a certain combination of organs or traits, and of genes which condition the development of these traits. Different organisms possess some genes in common with others and some genes which are different. The number of conceivable combinations of genes present in different organisms is, of course, immense. The actually existing combinations amount to only an infinitesimal fraction of the potentially possible, or at least conceivable, ones. All these combinations may be thought of as forming a multi-dimensional space within which every existing or possiblt organism may be said to have its place. (A more precise and realistic version of Wright's symbolic picture will be given in chapter 10 of this book.)

The existing and the possible combinations may now be graded with respect to their fitness to survive in the environments that exist in the world. Some of the conceivable combinations, indeed a vast majority of them, are discordant and unfit for survival in any environment. Others are suitable for occupation of certain habitats and ecological niches. Related gene combinations are, on the whole, similar in adaptive value. The field of gene combinations may, then, be, visualized most simply in a form of a topographic map, in which "contours" symbolize the adaptive values of various combinations (Fig. 1). Groups of related combinations of genes, which make the organisms that possess them able to occupy certain ecological niches, are then, represented by the "adaptive peaks" situated in different parts of the field (plus signs in Fig. 1). The unfavorable combinations of genes which make their carriers unfit to live in any existing environment are represented by the "adaptive valleys" which lie between the peaks (minus signs in Fig. 1).

Fig. 1. The "adaptive peaks" and "adaptive valleys" in the field of gene combinations. The contour lines symbolize the adaptive value (Darwinian fitness) of the genotypes. (After Wright.)

The enormous diversity of organisms may be envisaged as correlated with the immense variety of environments and of ecological niches which exist on earth. But the variety of ecological niches is not only immense, it is also discontinuous. One species of insect may feed on, for example, oak leaves, and another species on pine needles; an insect that would require food intermediate between oak and pine would probably starve to death. Hence, the living world is not a formless mass of randomly combining genes and traits, but a great array of families of related gene combinations, which are clustered on a large but finite number of adaptive peaks. Each living species may be thought of as occupying one of the available peaks in the field, of gene combinations. The adaptive valleys are deserted and empty. Furthermore, the adaptive peaks and valleys are not interspersed at random. "Adjacent" adaptive peaks are arranged in groups, which may be likened to mountain ranges in which the separate pinnacles are divided by relatively shallow notches. Thus, the ecological niche occupied by the species "lion" is relatively much closer to those occupied by tiger, puma, and leopard than to those occupied by wolf, coyote, and jackal. The feline adaptive peaks form a group different from the group of the canine "peaks." But the feline, canine, ursine, musteline, and certain other groups of peaks form together the adaptive "range" of carnivores, which is separated by deep adaptive valleys from the "ranges" of rodents, bats, ungulates, primates, and others. In turn, these "ranges" are again members of the adaptive system of mammals, which are ecologically and biologically segregated, as a group, from the adaptive systems of birds, reptiles, etc. The hierarchic nature of the biological classification reflects the objectively ascertainable discontinuity of adaptive niches, in other words the discontinuity of ways and means by which organisms that inhabit the world derive their livelihood from the environment.


Scientific study of the organic diversity and adaptation begins of necessity with description and classification. At the beginning of its existence as a science, biology was forced to reduce to a rational system the seemingly boundless variety of living things. In the eighteenth and nineteenth centuries systematics and morphology, two predominantly descriptive disciplines, took precedence among biological sciences. But description is only the first step in scientific inquiry. However great may be the satisfaction which an investigator derives from observation and accurate recording of facts, sooner or later he feels a desire to inquire into the causal connections between the phenomena observed. The theory of evolution arose in the nineteenth century through generalization and inference from a body of predominantly systematic and morphological data. It has furnished a rational framework for biological thought.

The theory of evolution asserts that (1) the beings now living have descended from different beings which lived in the past; (2) the evolutionary changes were more or less gradual, so that if we could assemble all the individuals which have ever inhabited the earth, a fairly continuous array of forms would emerge; (3) the changes were predominantly divergent, so that the ancestors of the now living forms were on the whole less different from each other than these forms themselves are; (4) all these changes have arisen from causes which now continue to be in operation, and which therefore can be studied experimentally.

Evolutionists of the nineteenth century were interested primarily in demonstrating that evolution has actually taken place. They succeeded eminently well. Evolution as a historical process is established as thoroughly and completely as science can establish facts of the past witnessed by no human eyes. At present, an informed and reasonable person can hardly doubt the validity of the evolution theory, in the sense that evolution has occurred. The very rare exceptions (such as Marsh 1947) prove only that some people have emotional biases and preconception strong enough to make them reject even completely established scientific findings. However that may be, the mass of evidence which can be adduced to show that evolution has indeed taken place in the history of the earth does not concern us in this book; we take it for granted.

Two distinct approaches to evolutionary problems became crystallized rather early in the development of evolution theory. The first concentrated on unraveling and describing the actual course which the evolutionary process took in the history of the earth, and which has led to the status of the organic world which we find at our time level. The historical process, phylogeny, is the central theme for the exponents of this approach, while their methods are mainly those of systematics, comparative morphology, comparative embryology, and paleontology. The second approach emphasizes studies on the mechanisms that bring about evolution, causal rather than historical problems, phenomena that can be studied experimentally rather than events which happened in the past. In general, the phylogenetic approach to evolutionary problems was predominant during the second half of the nineteenth century, while in the twentieth century the attention shifted toward the causal aspects, which were taken up by genetics and related biological disciplines. In fact, Darwin was one of the very few nineteenth-century evolutionists whose major interests lay in studies on the mechanisms of evolution, in the causal rather than the historical problems. In this sense, genetics and not evolutionary morphology is heir to the Darwinian traditions. Finally, the most recent developments indicate a trend toward synthesis of what were often divergent historical and causal approaches, and toward emergence of a unified evolutionary biology.

Genetics and Evolution

Genetics as a discipline is not synonymous with the evolution theory, nor is the evolution theory synonymous with any subdivision of genetics. Nevertheless, genetics has so profound a bearing on the problem of the mechanisms of evolution that any evolution theory which disregards the established genetic principles is faulty at its source. Every individual resembles its parents in some respects but differs from them in others. Every succeeding generation of a species resembles but is never a replica of the preceding generation. Evolution is the development of dissimilarities between the ancestral and the descendant populations. The mechanisms which determine the similarities and dissimilarities between parents and offspring constitute the subject matter of genetics. Genetics is the physiology of inheritance and variation.

The signal successes of genetics to date have been in studies on the mechanisms of the transmission of hereditary characteristics from parents to offspring, that is, on the architectonics of the germ plasm of the sex cells. The germ plasm has been shown to be composed of discrete particles known as genes. Chromosomes as carriers of genes have been studied in detail. The transmission of hereditary characters has been brought under human control, in the sense that in organisms which have been well studied genetically the characteristics of the offspring are frequently predictable, with a rather high degree of accuracy, from a knowledge of the characteristics of the parents.

The elegance and precision of methods devised by genetics to control the results of experiments which involve crosses of individuals differing in many hereditary characteristics have led to claims that the problem of heredity has been solved. Athough a large amount of work still remains to be done in this field, it is indeed fair to say that the laws of the transmission of hereditary characters are, by and large, understood now. But the problem of heredity is much wider. Between the genes of a fertilized egg and the characters of the adult organism which arises from it there lies the whole of individual development during which the genes exert their determining action. The mechanisms of gene action in development constitute the central problem of the second major subdivision of genetics; this has been variously labeled genetics of realization of hereditary characters, phenogenetics, or developmental genetics.

The problem of gene action is as yet unsolved, although much important work has been done in this field by geneticists and biochemists in recent years (see Beadle 1945, 1946, 1947, 1949, Wright 1941b, 1945, and Horowitz 1950 for reviews). The only possible way in which genes could influence the development of an organism is through physiological, and ultimately chemical, processes in the living body. Indeed, it is known in several instances that the formation of adult characteristics involves chains of chemical reactions, at least some of which are controlled by genes. The remarkable work of Beadle's school on the biochemistry of metabolism in the fungus Neurospora has disclosed that mutation of some genes blocks certain reactions at specific points in the reaction chains, and causes accumulation in the cells of substances which normally exist only as intermediate products.

Despite the great interest of these findings for chemical physiology, rather little insight has so far been gained into the gene action proper. Beadle and his colleagues have supposed that the "normal" alleles of the genes studied by them in Neurospora produce enzymes which mediate specific reactions, and that mutation (or destruction) of these genes causes nonproduction of the enzymes and blockage of the reactions. They further implied that every gene produces one and only one enzyme which catalyzes a specific reaction. This supposition is temptingly simple, and it has so far vindicated itself as a stimulating working hypothesis. Yet, there is no compelling evidence in favor of the one gene-one enzyme assumption, and the postulated enzymes have rarely been indentified. A brilliantly conceived attempt in this direction by Caspari (1946) in the moth, Ephestia kühniella and by Wagner and Guirard (1948) and Wagner (1949) in Neurospora, have led to ambiguous results.

A gene is a particle of molecular dimensions. It is located in a chromosome in the cell nucleus (or in the cytoplasm, in the case of plasmagenes). How can such particles bring about the often very striking macroscopic changes in living bodies? Gene action must of necessity start with intracellular processes, which may subsequently be translated into chains of reactions that culminate in the appearance of visible characters. Little is known regarding these intracellular processes—interactions between the constituent parts of the chromosomes and their nuclear and cytoplasmic surrounding, although Muller (1947), Rapoport (1947), Mather (1948), and Spiegelman (1948) have advanced interesting hypotheses concerning the possible mechanisms of the gene action in development. Are all genes continually active, or does each gene exert its determine function at a certain period of development and remain quiescent at other periods? Is the gene action merely a by-product of the self-reproduction of the genes in the course of cell division? What are the relations between gene specificity and the specificity of chemical substances, particularl proteins, composing the organism and manifested especially in serological reactions?

Biophysical and biochemical work has revealed a great complexity of cellular organization on the ultramicroscopic level of molecular aggregates. To a geneticist, it seems that genes should be determining agents of this "molecular morphology," and Delbrück (1941) and Emerson (1945) have advanced interesting suggestions which will be helpful in further thinking and experimentation in this important field. According to Emerson, gene specificity resides chiefly in the molecular surface configuration of the gene. This might permit the gene specificity to be transmitted to molecules of different chemical make-up, and some of the latter may in turn serve as "templates" for synthesis of new genes and enzymes (and, hence, for gene reproduction).

Genetics of Populations

Genetics of the transmission of hereditary materials, and genetics of development, are concerned with individuals as units. The former seeks to find out the rules which govern the formation of gene constellations in individual zygotes, so that the probable distribution of genes in the offspring may be predicted from a knowledge of the genotypes of the parents. The latter studies the mechanisms of gene action in ontogeny. A third subdivision of genetics has as its province the processes which take place in groups of individuals, in populations, and is therefore called genetics of populations.

In a broad sense, a population may be defined as "any single or mixed species association in the laboratory or in nature that presents a closely interacting system which can be studied and expressed with some quantitative rigor" (Park 1942, Allee et al. 1949). Allee et al. argue that a population is not a group concept but a spatiotemporal entity which possesses the following five organismic attributes. (1) A definite structure and composition. (2) "The population is ontogenetic. It exhibits (as does the organism) growth, differentiation and division of labor, maintenance, senescence, and death. (3) The population has a heredity. (4) The population is integrated by both genetic and ecologic factors that operate as interdependent mechanisms. (5) Like the organism, the population is a unit that meets the impact of its environment. This is a reciprocal phenomenon, since the population is altered as a consequence of this impact, and, in time, it alters its effective environment."

Among the different kinds of populations that exist in nature, the organism-like integration is most evident in the breeding associations which are formed in all sexual and cross-fertilizing organisms. The integrating agent in such Mendelian populations is the process of reproduction itself, which establishes mating, parenthood, and progeny bonds between the component individuals. A Mendelian population is, then, a reproductive community of individuals which share in a common gene pool (Dobzhansky 1950d).

A Mendelian population may be said to possess a corporate genotype. The population genotype is evidently a function of the genetic constitution of the component individuals, just as the health of an individual body is a function of the soundness of its parts. The rules governing the genetic structure of a population are, nevertheless, distinct from those which govern the genetics of individuals, just as rules of sociology are distinct from physiological ones, although the former are in the Iast analysis integrated systems of the latter (Novikoff 1945). Suppose for example, that some factors have arisen in the environment which discriminate against too tall or too short individuals of a species. From the standpoint of an individual, some growth genes would have acquired lethal properties, and the effects of these genes might be described adequately by stating the precise nature of the physiological reactions leading to death. From the viewpoint of population genetics, death of this category of individuals initiates a complex chain of consequences: the relative frequencies of homozygotes and heterozygotes for certain growth genes and for genes located in the same chromosomes would be altered; some genetic factors which previously were being eliminated because of their harmfulness might become neutral or even favorable; after some generations the genetic constitution of the whole species may be changed.

Evolution is a change in the genetic composition of populations. The study of mechanisms of evolution falls within the province of population genetics. Of course, changes observed in populations may be of different orders of magnitude ranging from those induced in a herd of domestic animals by the introduction of a new sire to phylogenetic changes leading to the origin of new classes of organisms. The former are obviously trifling in scale compared with the latter. Experience shows, however, that there is no way toward understanding of the mechanisms of macroevolutionary changes, which require time on geological scales, other than through understanding of microevolutionary processes observable within the span of a human lifetime, often controlled by man's will, and sometimes reproducible in laboratory experiments.

Many authors believe that microevolutionary changes are different in principle from macroevolutionary ones, and that while the former can be understood in terms of the known genetic agents (mutation, selection, genetic drift), the latter involve forces that are experimentally unknown or only dimly discerned. Views of this kind have been entertained by few geneticists (among whom there is, however, so eminent a man as Goldschmidt, 1940), but they have been popular among those who approach evolutionary problems on the basis of data of paleontology and comparative anatomy. Well-known writers have supposed macroevolutionary changes to be engendered by some directing forces either inherent in the organism itself or acting on it by some inscrutable means from the outside. These guiding forces received a variety of names, including orthogenesis, nomogenesis, aristogenesis, hologenesis, and finalism, but they escaped precise definition which would make them subject to experimental test or to any kind of rigorous proof or disproof (see Simpson 1949).

Methods of experimental genetics apply directly only to forms which can be crossed and which produce hybrids. Genetic analysis is, accordingly, limited to differences on the individual, racial, specific, and at most the generic levels, which are usually regarded the province of microevolution. A geneticist can approach macroevolutionary phenomena only by inference from the known microevolutionary ones. It is obviously impossible to reproduce in the laboratory the evolution of, for example, the horse tribe, or for that matter of the genus Drosophila. All that is possible is to examine the evidence bearing on macroevolution which has been accumulated by paleontologists and morphologists, and to attempt to decide whether it agrees with the hypothesis that all evolutionary changes are compounded of microevolutionary ones. This difficult but important task has been brilliantly accomplished in recent years by Simpson (1940) for paleontological and by Schmalhausen (1949) and Rensch (1947) for comparative anatomical and embryological evidence. The three authors find nothing in the known macroevolutionary phenomena that would require other than the known genetic principles for causal explanation. The words "microevolution" and "macroevolution" are relative terms, and have only descriptive meaning; they imply no difference in the underlying causal agencies.

Evolutionary Statics and Evolutionary Dynamics

Evolution is a process of change or movement. Description of any movement may be divided into two parts: statics, which treats of the forces producing a motion and the equilibrium of these forces, and dynamics, which deals with the motion itself and the action of forces producing it. Following this scheme, we shall discuss, first, the factors which bring about changes in the genetic composition of populations (evolutionary statics), and second, the interactions of these forces in race and species formation and disintegration (evolutionary dynamics).

In bare outline, the mechanisms of evolution as seen by a geneticist appear as follows. Gene changes, mutations, are the primary source of evolutionary changes and of diversity in general. Next come changes of a grosser mechanical kind, rearrangements of the genic materials within the chromosomes. Such rearrangements at least occasionally entail changes in the functioning of the genes themselves (position effects), since the effects of a gene on the development are determined not only by the structure of that gene but also by its neighbors. Combining chromosome complements of different species to produce a single new one (allopolyploidy) is an important evoltutionary method among plants. Finally, there is an insufficiently studied field of changes in extranuclear structures, usually associated with the cytoplasm.

Mutations and chromosomal changes arise in every species and supply the raw materials for evolution. But the origin of mutations and chromosomal changes is only the first stage or level of the evolutionary process. Once produced, mutations are injected into the gene pool of the population, where their further fate is determined by the dynamic regularities of the physiology of populations. The influences of selection, migration, and geographical isolation then mold the genetic structure of populations into new shapes, in conformity with the secular environment and the ecology, especially the breeding habits, of the species. This is the second level of the evolutionary process, on which the impact of the environment produces the historical changes in the living populations.

Finally, the third level is a realm of fixation of the diversity attained on the preceding two levels. Races and species are populations or groups of populations which remain distinct only so long as some cause limits their interbreeding. Unlimited interbreeding of two or more initially different populations results in exchange of genes between them and a consequent fusion of the once distinct groups into a single variable array. A number of reproductive isolating mechanisms encountered in nature (ecological isolation, sexual isolation, hybrid sterility, and others) guard against such a fusion of the discrete arrays and the consequent decay of discontinuous variability. The origin and functioning of the reproductive isolating mechanisms constitute one of the most important problems of the genetics of populations.

[ Theodosius Dobzhansky, Genetics and the Origin of Species, 3rd edition, New York: Columbia University Press, 1951, pp. 3-18. ]

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