Genetics and the Origin of Species is a 1937 book by the Ukrainian-American evolutionary biologist Theodosius Dobzhansky. It is regarded as one of the most important works of modern synthesis and was one of the earliest. The book popularized the work of population genetics to other biologists and influenced their appreciation for the genetic basis of evolution.
In his book Dobzhansky applied the theoretical work of Sewall Wright (1889–1988) to the study of natural populations. Dobzhansky uses theories of mutation, natural selection, and speciation to explain the habits of populations and the resulting effects on their genetic behavior. The book said evolution was a process that accounts for the diversity of all life on Earth. Dobzhansky said that evolution regarding the origin and nature of species, which at the time was deemed mysterious, had potential for progress.
Background In Darwin's theory of natural selection, more organisms are produced than can survive. Some have variations that give them a competitive advantage, and they have the best chance of surviving and procreating. The main element lacking in the theory was any mechanism that would allow organisms to pass on these favorable variations. Lacking such a mechanism, the theory of evolution faced competition from theories such as neo-Lamarckism, in which the environment acted directly on organisms, changing their structures. Darwin did not know that the monk Gregor Mendel was already working on experiments that would explain inheritance in terms of units of heredity that we now call genes.
When Mendelian genetics was rediscovered by several scientists, it initially increased the confusion. The Dutch botanist Hugo de Vries developed a theory called mutationism in which most variations were inconsequential and could not lead to species change. Instead, new species were formed by large mutations. At first, geneticists tended to support mutationism; but in the 1920s and 1930s a group of theoretical geneticists – particularly Ronald Fisher, J. B. S. Haldane and Sewall Wright – showed that Mendel's laws could explain continuous variation in biological characteristics; and that natural selection could act cumulatively, giving rise to large changes. Their work provided a theoretical framework for incorporating genetics into the theory of evolution.
Many biologists fell into two camps: the geneticists, who mostly worked in the laboratory; and naturalists, who studied natural populations in the field and museums, and put much of their effort into taxonomy. Each contributed concepts that were essential to the understanding of evolution. Naturalists introduced the biological species concept, the definition of species as a community that is reproductively isolated and occupies a distinctive ecological niche. They also recognized that species are polytypic, having variations in time and space; and that behavior and change of function can give rise to evolutionary change.
The two groups used such different methods and terminology that it was difficult for them to communicate. They were often battling over the same scarce academic resources, and each was often scornful of the other. As the paleontologist George Gaylord Simpson put it, paleontologists believed that "a geneticist was a person who shut himself in a room, pulled down the shades, watched small flies disporting themselves in milk bottles, and thought that he was studying nature." Meanwhile, naturalists were "like a man who undertakes to study the principles of the internal combustion engine by standing on a street corner and watching the motor cars whiz by."
Publication
The book began as a series of lectures at Columbia University in October and November 1936. Dobzhansky decided to attend the summer 1936 meeting of the Genetics Society of America in Woods Hole, Massachusetts. When the geneticist Leslie Dunn learned of this, he invited Dobzhansky to give a series of lectures at Columbia. He sent the invitation in April, and within a month he had suggested that the lectures be a springboard for writing a general treatise on evolutionary genetics. Dobzhansky was enthusiastic, and in May he wrote back, proposing the title "Genetics and the Origin of Species". He envisioned two parts – "part I containing data on the sources of evolutionary change, and part II containing a discussion of their interactions leading to race- and species formation." After the lectures (for which he received $500), Dobzhansky returned to Pasadena, and in December Columbia University Press accepted his proposal for a book. The pace of writing was helped by a riding accident in February 1937 in which he crushed his knee, rendering him immobile; and by April he was able to send a manuscript to Dunn for his perusal.
While reading the manuscript, Dunn persuaded the university to revive the Jesup lectures, a series that had involved some notable lecturers in the past. It was back-dated so that Dobzhansky's lectures became the first in the series. The Columbia University Biological Series was also revived, and when Genetics and the Origin of Species was published in October 1937 it became the 11th volume in that series.
Three main editions were published, in 1937, 1941 and 1951, each with significant changes. Dobzhansky considered Genetics of the Evolutionary Process (1970) a fourth edition, but one so much changed that it needed a new title.
Contents of the first edition Genetics and the Origin of Species has two main points to make. The first is that speciation is a genuine problem that must be explained by the theory of evolution. In nature, there is not a single population of organisms separated from each other by small variations. Instead, the natural world is divided into species, each with its own limited range of variability. The second point is that all the variations can be explained by the principles of genetics.
The 1937 edition was divided into nine chapters, whose contents are described below.
Organic diversity The first chapter is a short outline of the main points of the book: Evolutionary theory must account for variations on the level of the individual and also that of populations and species. It must explain how reproductive isolation can occur. And the goal is to explain all this using genetic principles that can be verified in the laboratory.
Gene mutation The second chapter argues that mutations occur frequently and are random in direction. Most are small in effect, but they affect all features of organisms and vary from beneficial to lethal. They are sufficient to provide the raw material for natural selection.
Mutation as a basis for racial and specific differences In the third chapter, Dobzhansky demonstrates that the mutations that are observed in the laboratory also occur in nature, where they are acted on by natural selection. Thus, there is nothing intrinsically different about the process by which new species evolve.
Chromosomal changes Dobzhansky continues the theme of continuity between the laboratory and nature in chapter 4, showing that variations in the structure, arrangement and number of chromosomes occur in both the laboratory and nature. He shows that chromosomal translocation, a rearrangement of parts in chromosomes, accounts for racial differences in Datura stramonium (jimson-weed). Chromosomal inversion, a reversal of a segment, is the basis for differentiation in Drosophila. He also points out that these effects demonstrate that the chromosome parts are interdependent.
Variation in natural populations Having established that the sources of natural variation are mutations and chromosomal rearrangements, Dobzhansky considers what shapes and preserves this variation. In the first edition of this book, he considers genetic drift as important as natural selection. As an example, he argued that drift was the reason for racial variations that were observed in Partula, a land snail, so these variations had no adaptive advantage. He argued that most such variations, which he called microgeographic race, was nonadaptive. Because variation is not always controlled by selection, we need to know the size of a population before we can predict its evolutionary dynamics.
Selection The sixth chapter discusses the evidence for natural selection from experiments in the laboratory and observations of nature. He considered examples of camouflage such as industrial melanism, in which moths and other arthropods evolve darker pigmentation when exposed to an environment with a lot of soot in it. He refuted some Mendelian misconceptions about the effectiveness of natural selection, but he also rejected the strict selectionism of Fisher.
Polyploidy In the seventh chapter, Dobzhansky discusses polyploidy, a condition (common in plants) where an organism has more than two complete sets of chromosomes. (Humans are diploid, having one set each from the mother and father.) He discusses case histories such as that of Raphanobrassica, a hybrid between the radish and the cabbage. This is an example of "cataclysmic" speciation, an exception to his general rule that speciation is a slow process.
Isolating mechanisms
An isolating mechanism prevents members of two different species from successfully producing offspring. Dobzhansky invented the term, and in the eighth chapter he discusses their role in speciation. His views were sufficiently original for him to publish them in a journal article as well. Although previous authors had recognized the importance of isolation, their reasons were entirely different from his. George Romanes thought that isolation was a cause of change, whereas Dobzhansky sees two competing effects. Isolation reduces the rate of formation of harmful gene combinations, but it also restricts the scope of genetic variation. Thus, a species remains near a given peak in the fitness landscape and does not find new adaptive peaks.
Dobzhansky presents a classification of isolating mechanisms. The main division is between hybrid sterility, which he discusses in the next chapter, and mechanisms that prevent organisms from mating. These include geographic and ecological isolation. After two populations are changed sufficiently, "physiological" isolating mechanisms prevent them from mating, so they will remain distinct even if they are no longer physically isolated. This isolation is the end of a continuous process that includes division into races, and involves multiple mutations.
Hybrid sterility In the chapter on isolating mechanisms, Dobzhansky deplores the "appallingly insufficient attention" that geneticists had devoted to isolation. The only isolating mechanism that they had studied was sterility in hybrid organisms, so he devotes the ninth chapter to a particularly detailed analysis of the literature on this subject.
Experiments
Through his work on Drosophila pseudoobscura, a species of fruitfly, Dobzhansky was able to identify that some populations of this species did not have identical sets of genes. Dobzhansky used experimental breeding in laboratories and gardens, and also surveys related to species in nature to help support aspects of organic evolution. The data in his book show the different genetic mutations and chromosomal changes that were observed. These experiments are vital to this book because they illustrate a transition from the laboratory to the greater field of genetics. Drosophila enabled researchers to gain a deeper understanding of how scientific knowledge about heredity has expanded and contributed to other areas of biology. By focusing on the opportunities and constraints of the research organism, persuasive accounts of the chromosomal genetics of Drosophila eventually evolved into the genetics of natural populations in the 1930s. All of the results from his experiments support the theory of modern evolutionary synthesis.
Mutations