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.
Discontinuity
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:
| Mammals | | 3,500 |
| Birds | | 8,600 |
| Reptiles and Amphibians | | 5,500 |
| Fishes | | 18,000 |
| | |
| | |
| | |
| Total Vertebrates | | 35,600 |
Mayr's estimates for the phyla of the animal kingdom are:
| Vertebrates | | 35,600 |
| Tunicates and Prochordates | | 1,700 |
| Echinoderms | | 4,700 |
| Arthropods | | 815,000 |
| Mollusks | | 88,000 |
| Worms and related groups | | 25,000 |
| Coelenterates and Ctenophores | | 10,000 |
| Sponges | | 5,000 |
| Protozoans | | 15,000 |
| | |
| | |
| | |
| Total | | 1,000,000 |
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:
| Angiosperms | | 150,000 |
| Fungi | | 70,000 |
| Mosses | | 15,000 |
| Algae | | 14,000 |
| Pteridophytes | | 10,000 |
| Liverworts | | 6,000 |
| Gymnosperms | | 500 |
| | |
| | |
| | |
| Total | | 265,500 |
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.
Evolution
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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