EUGENE M. MCCARTHY, PHD GENETICS
(Continued from the previous page)
Though neo-Darwinism eventually gained wide acceptance, not everyone was satisfied that all aspects of evolution could be explained by models based on point mutation and the long-term effects of meiotic recombination. For example, E. B. Babcock (1918: 120-121) pointed out that many closely related forms treated as distinct species differ with respect to karyotype and that the mere reassortment of alleles occurring in intrachromoset meiotic recombination would not be expected to produce a new karyotype:
By "unique or irregular behavior of one or more members of a chromosome group," Babcock seems to have meant "processes producing chromosomal mutations."
Others insisted the fossil record was inconsistent with the gradualistic paradigms of the modern synthesis. Otto Schindewolf (1896-1971) was an adamant saltationist and, from the 1930s on, maintained that neo-Darwinism was an inadequate explanation of the abrupt changes seen in the fossil record. He summarized his position in his magnum opus Grundfragen der Paläontologie (1950). Schindewolf was perhaps the most prominent German paleontologist of the twentieth century. Reif (2004) comments that all German paleontology coalesced in Schindewolf's evolutionary theory, known as "typostrophism," and that this
But Schindewolf's ideas have had little impact in the United States and England since Grundfragen was only relatively recently (1993) translated into English. Moreover, even a sympathetic reading of that translation (by this writer) could discover no trace of a genetic explanation for the saltational phenomena Schindewolf describes. He simply argued that gradual genetic processes posited by neo-Darwinism failed to explain the saltational pattern observed in the fossil record and that some other mechanism must therefore be at work. Noting that Darwin had explained the discontinuities between extant forms by supposing that intermediate types had died out, Schindewolf (1993: 333) wrote,
According to Schindewolf,
If, instead of leaving the job to geneticists, he had made some attempt himself to specify the mechanisms involved, he might have made a more convincing case. But he really seems to have had no idea what mechanisms might be involved:
He put his faith in an unknown force.
Harvard paleontologist George Gaylord Simpson (1902-1984) proposed the idea of "quantum evolution" to account for the saltational fossil data. For example, in one publication (Simpson 1944: 206), he comments that
Later he claimed that gradual transitions between fossil forms "are not recorded because they did not exist," and
The botanist J. C. Willis (1868-1958), a fellow of the Royal Society, was also a saltationist. He noted, as others had before him (e.g., Robson and Richards 1936), that the features used by taxonomists to classify organisms are typically nonadaptive, and that these diagnostic traits of particular forms are nevertheless found in all individual specimens of a form. On this basis, he argued gradual natural selection could play no significant role in shaping such forms (Willis 1940: 52–54). Such a process could not explain, he said, why all the members of each form each have all the useless traits characteristic of that form. Willis' view of evolution relied largely on the occurrence of major mutations. Thus, he asserted "a single mutation, usually very divergent from the parent form, may give rise, at one step (not gradually as under Darwinism) to a new form, of family, generic, specific, or varietal rank." (Willis 1949: 14). In his opinion, "chromosome alterations"1 were the causes of these mutations (Willis 1940). Sewell Wright (1941: 345) commented that the view to which Willis was "most systematically opposed is that of evolution by gradual statistical transformation of populations." Wright himself was perhaps the greatest exponent of such statistical explanations of evolution.
Goldschmidt. But it is Richard Goldschmidt (1878-1958) who remains the best remembered of the twentieth century saltationists, at least in the United States. When Goldschmidt fled Hitler's Germany to become a professor at Berkeley, he was a geneticist of international standing. But his saltationist claims soon brought him lasting censure. Goldschmidt dismissed the evolutionary significance of point mutations and instead proposed that the "decisive change in the genetic material" actually causing abrupt, evolutionary changes is a "change in chromosomal pattern." The saltational shift producing new forms treated as distinct species occurs, he said, when the structure of the chromosomes is reshuffled and scrambled.2
In his book, The Material Basis of Evolution (1940), Goldschmidt explores these ideas. There he also points out, as Babcock and other biologists had before him, that organisms treated as distinct species very often have distinct karyotypes, a fact that has been firmly established by subsequent studies (see citations in Table 3.1). Across a broad range of organisms, closely related forms very often differ in chromosome number and/or with respect to the structure of individual chromosomes (this fact was emphasized in Section 3), so that a chromosomal mutation would be required to convert one's karyotype into the other's. Even members of the same genus with identical chromosome counts commonly differ with respect to the structure of one or more individual chromosomes. Such is the case for example with the chimpanzee (Pan troglodytes) and the pygmy chimpanzee, or bonobo (P. paniscus).3 Goldschmidt was convinced the process producing new sets of chromosomes — whatever it might be — was the same process producing the sort of morphological and physiological differences that prompt biologists to treat forms as distinct species.4 "We have long been seeking a different type of evolutionary process," he wrote in the best saltationist spirit, "and have now found one; namely, the change within the pattern of chromosomes [viz., changes in karyotype]."5
Goldschmidt also emphasized the discontinuous nature of the paleontological data. Thus, he says, Schindewolf (1936) "shows by examples from fossil material that the major evolutionary advances must have taken place in single large steps, which affected early embryonic stages with the automatic consequence of reconstruction of all the later phases of development. He shows that the many missing links in the paleontological record are sought for in vain because they never existed."6
But Goldschmidt gave no clear account of what the process might be that produced changes in karyotype. Nor did he specifically explain mechanisms whereby such changes could affect the development of an organism once they had occurred. He simply suggested that a "systemic mutation" can suddenly arise and produce a new organism with a new set of chromosomes. Such a mutation, he said, would rearrange many, or even all, of the chromosomes. Goldschmidt called gradual evolution (statistical changes in the frequency of allelic variants) "microevolution," and point mutations affecting individual genes, "micromutations." He dismissed the evolutionary significance of both. "Microevolution within the species," he said,
As Goldschmidt conceived them, systemic mutations were chance events that suddenly produced "a huge effect upon a series of developmental processes leading at once to a new and stable form, widely diverging from the former."8 He emphasized his belief that (1) no intermediates fill what he called "the bridgeless gaps" between forms treated as species; (2) systemic mutations allowed evolution to leap these gaps and create discretely distinct new forms.
He was one of the most prominent geneticists of his era. His claim was correct that many somatypes treated as distinct species are distinct chromotypes. And yet he was ridiculed when he suggested that a systemic mutation could both arise de novo in a single individual and nevertheless get established as a new type. He had failed to explain how a solitary, radically altered organism, created by a single, random, massive mutation, would find a mate of its own kind. Nor did he explain how a set of chromosomes could be rearranged so abruptly. How did the chromosomes get shortened and lengthened? How did the structure of the chromosomes get rearranged? Goldschmidt said only that the observed existence of such chromosomal rearrangements distinguishing closely related forms implied the existence of some unknown mechanism of rearrangement — an unknown force. The observation was correct. Therefore, his conclusion that such a mechanism exists was also correct. The only known mechanisms with such an effect are stabilization processes producing chromosomal mutations.
Goldschmidt's radical and largely unelucidated claims came at a time when nearly all of his colleagues had already embraced the new models of population genetics. This field seemed to offer an unprecedented numerical exactitude comparable to that of mathematics or physics. Adherents of this new discipline believed Mendel's Laws sufficiently described the genetic events underlying evolution and that evolution occurred gradually, not abruptly by way of the mysterious systemic mutations Goldschmidt was proposing. In particular, a major proponent of the new movement, Fisher (1930), had argued that large mutations would almost surely be maladaptive: "for greater changes the chance of improvement diminishes progressively, becoming zero, or at least negligible, for changes of a sufficiently pronounced character." This view was widely accepted. To most biologists,9 Goldschmidt's ideas seemed to hark back to the abandoned theories of Hugo de Vries. Not surprisingly, Goldschmidt was roundly rejected by most of his colleagues.
Nevertheless, it is now well known that large-scale changes in the genetic material do occur and that they can indeed be beneficial. Take polyploidy. Otto and Whitton (2000) note that while it is easy to speak of hypothetical adverse effects,
Actually, some of Goldschmidt's notions were not that radical. In a paper read at a general meeting of the American Association for the Advancement of Science, Goldschmidt (1933: 547) emphasized
Later workers such as Stebbins (1959), Gilliard (1959), and Rieseberg (1997) expressed the opinion that a markedly new type would be more likely to get established if an empty niche were available. Others later emphasized the idea that mutations in genes affecting early developmental processes could have a major effect. This idea is essentially the same as that set forward by Alan Wilson and his co-workers fifty years later (King and Wilson 1975; Prager et al. 1976; Wilson 1975).
But, in general, Goldschmidt's speculations were not well received. As Milner (1993: 222) notes, "There is a grotesque humor about the unfortunate phrase 'hopeful monster' that lent itself to caricatures of Goldschmidt's ideas and obscured the theoretical issues." The mechanisms he offered were vague, undocumented, and, as a result, unconvincing. Moreover, one can detect in Goldschmidt's tone a sweeping unapologetic condescension that probably rubbed many of his colleagues the wrong way. He expressed himself in absolutes as if he could somehow know that a systemic mutation is involved in the production of every form treated as a species. He insisted a mystery process no one had observed was the key to evolution. In the end, largely due to Goldschmidt, saltationist became anathema to an entire generation of evolutionary biologists. For many it remains a heresy even today.
Some later evolutionists have embraced a watered-down formulation of Goldschmidt's theories. For example, Lewis and Raven (1958) supposed that chromosomal rearrangements that were less massive would not have such a severe adverse effect on fertility and viability. Such changes, they suggested, might become established through some unknown mechanism, perhaps "mutator genes" (Lewis 1962; Lewis and Raven 1958) or possibly extreme inbreeding (Lewis 1966). Grant (1981: 173-175) notes there has been a school of thought that argues the establishment of new structural rearrangements can be driven by the ability of such arrangements to lock up favorable gene combinations from recombination. However, this ability would not be expected to offset the extremely deleterious effect of structural heterozygosity on fertility (Grant 1981: 172; Key 1968; Templeton 1981: 33–35). All the offspring of an individual such as Goldschmidt had described would, perforce, be structurally heterozygous (since the new individual would have to mate with an individual of the preexisting type). But the fact remains — forms treated as distinct species very often do have distinct karyotypes. NEXT PAGE >>
1. Quoted in Wright (1941: 345).
2. Goldschmidt (1982: 243).
3. Khudr et al. (1973).
4. Goldschmidt (1982: 249).
5. Goldschmidt (1982: 199).
6. Goldschmidt (1982: 395).
7. Goldschmidt (1982: 396).
8. Goldschmidt (1982: 96).
9. For example, Dobzhansky (1940a: 357); Mayr (1942: 155).
Human Origins: Are we hybrids?
On the Origins of New Forms of Life
Cat-rabbit Hybrids: Fact or fiction?
Georges Cuvier: A Biography
Prothero: A Rebuttal
Branches of Biology