On the Origins of New Forms of Life

7.3: From de Vries to the Modern Synthesis



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But as the twentieth century unfolded, de Vries' theory gradually fell into disrepute. De Vries knew the sudden “single variations” he had witnessed were connected with hybridization, but genetics was still in its infancy. He had no clear notion of the nature of the actual genetic processes underlying his observations. He was unable to predict when his single variations would occur. In the years after the publication of his books, far more progress was made in understanding the genetic basis of trait inheritance than in elucidating the nature of single variations. As a result, the mechanisms of intrachromoset meiotic recombination (see Section 3) were worked out very early in the history of genetics, before 1920, primarily by T. H. Morgan's group at Columbia University. It was realized concurrently that Mendel's Laws accurately described the sort of variation arising from ordinary meiotic recombination, the sort occurring in intrachromoset matings. Thus, at the time, Babcock (1918: 117) commented that

During the decade following de Vries's announcement of his theory, biological interest shifted from the general problem of evolution to the more specific problem of heredity. The rediscovery of Mendel's law at once focused attention upon the inheritance of particular characters. Then began the era of experimental evolution in which, under the leadership of Morgan, most remarkable progress has already been made. The traditional problem of heredity, its mechanism, has been solved. We know, not only that the ultimate hereditary units are germinal, but also that they are located in that particular portion of the germ cell called the chromatin [the complex of DNA, RNA, and protein making up the chromosomes], and there is an ever-growing body of evidence proving that each hereditary unit occupies a particular locus in a particular chromosome. These hereditary units have been designated by various terms, but are most commonly referred to as genes, genetic factors, unit factors or simply factors.

Morgan's work, elucidating the effects of meiotic recombination, examined the inheritance of point mutations in Drosophila (Morgan 1911, 1915), but it soon became apparent that genetic factors following Mendelian inheritance exist in a wide array of sexual organisms (Babcock 1918). On the other hand, progress in understanding the stabilization processes producing chromosomal mutations went more slowly. Since meiotic recombination had been elucidated and chromosomal mutations had not, many felt that de Vries' theory was too vague, or even unscientific. Moreover, at that time it was still widely thought that hybridization, an important factor in de Vries' theory, was rare or nonexistent in a natural setting.

A new faction of biologists rose up, who placed great emphasis on the evolutionary potential of meiotic recombination and point mutation. Like Darwin, this new faction assumed the origin of new forms could be explained in terms of traits that (1) showed variation within a population, and (2) could be imagined as gradually becoming more or less common in a population under the influence of natural selection. Atop Darwin’s theory they erected a mathematical superstructure based on Mendel’s Laws. The only sort of change described was the sort that could occur in intrachromoset matings, because Mendel's Laws could describe nothing else. This new way of explaining evolution, combining Mendel and Darwin, became known as the “modern synthesis,” and the theory associated with this movement became known as “neo-Darwinism.”¹ This new theory had a tremendous impact on future biological research. The pioneers of this approach (e.g., Dobzhansky 1937a; Fisher 1930; Haldane 1932; and Wright 1931) took simple formulas—which Mendel had used to predict the outcome of a generation or two of peas—and extended them to describe long-term evolutionary processes. Their basic approach was to use these rules to quantify certain mating procedures employed by agricultural breeders — “assortative mating,” “inbreeding,” “mixed mating,” etc. (Provine 1971, 1986: 138–142).

This agricultural influence is especially apparent in the case of the American geneticist Sewall Wright, who contributed more models to population genetics than anyone else. Wright was trained in genetics at Harvard's Bussey Institution, which emphasized biological sciences related to agriculture and horticulture (Provine 1986: 44). He went on to spend the first decade of his post-graduate career (1915–1925) working for the U.S. Department of Agriculture. During this time, one of his major responsibilities was the analysis of data from a long-term experiment on inbreeding in guinea pigs, an experience that was greatly to influence his later work in evolutionary theory (Provine 1971, 1986: 138–142). Provine (1986: 142) notes that “from very early in his career, Wright saw evolution as deeply related to what he knew of evolution in domestic populations.”

However, it is now apparent that many genetic processes bringing about abrupt, major evolutionary change (such as the various types of stabilization processes discussed in Section 4) were left out of account by Wright and his contemporaries. This omission is understandable since at that time such processes were poorly understood and little known. Indeed, it must have seemed, and it actually was, a great scientific advance to understand intrachromoset meiotic recombination, which Mendel's Laws describe. It must also have been satisfying to use those laws to construct concise, seemingly valid, analytical models of evolutionary processes. But it now seems those early evolutionists rushed too soon to consensus. Simply identifying a process that can hypothetically produce change is not the same as showing it is the typical source of new forms. As Francis Bacon (Novum Organum, 1620) said

Even when men build any science and theory on experiment, yet they almost always turn with premature and hasty zeal to practice, not merely on account of the advantage and benefit to be derived from it, but in order to seize upon some security in a new undertaking of their not employing the remainder of their labor unprofitably and by making themselves conspicuous to acquire a greater name for their pursuit. Hence, like Atalanta, they leave the course to pick up the golden apple, interrupting their speed and giving up the victory.

Scientists of this new school, who called themselves “population geneticists,” thought of evolution in terms of an entire population gradually changing and taking on new traits. They equated the word species with “reproductively isolated population.” The production of new types of organisms via hybridization between such populations, then, did not enter their picture of evolution. They tended to focus on the gene and to ignore the chromosome. In short, they thought in terms of Mendelian models that described variation resulting from intrachromoset mating (see Section 3). This approach allowed them to construct tidy mathematical models predicting how evolution would occur, given their assumptions. Such models were attractive because they seemed to prove the feasibility of evolution under the influence of natural selection. Even more, they lent the field an air of scientific rigor, something previously lacking.

No one in this ascendant school thought in terms of chromosomal mutations. Or, if they did, they thought of them as aberrations. So the new models of evolutionary change left out this potent source of variation. By the 1930s, people who constructed such models were the leaders of evolutionary biology. More and more, de Vries' observations in Oenothera were dismissed as the result of “chromosomal irregularities” (Jennings 1935: 45). By the time de Vries died in 1935, he had seen his theories fall into neglect. Nevertheless, his direct observation — that new types of organisms can come suddenly and repeatedly into being and remain stable thereafter — is fundamentally at odds with the basic tenets of orthodox theory and should not be forgotten. NEXT PAGE >>



1. The name neo-Darwinism predates the modern synthesis by several decades. The term was already in use by the early 1890s (Ward 1891). But it has become the epithet usually used to refer to the theory of evolution developed during the modern synthesis, which describes evolution as a process of statistical change in isolated populations. Originally it merely referred to a faction of biologists that placed great emphasis on natural selection as opposed to the inheritance of acquired traits.

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