EUGENE M. MCCARTHY, PHD GENETICS
(Continued from the previous page)
Perhaps the most successful post-Darwinian saltationist was Hugo de Vries. De Vries dominated evolutionary thought during the first decade of the twentieth century. His theories, which “achieved an enormous popularity,”1 grew out of his own experimentation.
Hugo de Vries|
In 1886, beginning with nine evening primroses (Oenothera lamarckiana), he began a long-term study in which he sought to identify individuals with new traits (Hugo de Vries 1902: 726). Among 15,000 offspring of these nine plants he found two new types, five plants of each type. On the basis of their distinctive morphology, he later decided these two types should be treated as new species. He named them O. lata and O. nanella. In the next generation, among 10,000 plants produced from O. lamarckiana parents, he found three more of each of these types and one of a third type, which he named O. rubrinervis.
Having found these new types, he was prompted to very carefully examine the next generation of 14,000 plants (Hugo de Vries 1902: 726; 1901–1903). He found 63 O. lata, 60 O. nanella, and eight O. rubrinervis individuals. He also found three additional new forms, which he named O. albida (15 plants), O. oblonga (176 plants), and O. scintillans (a single plant). In the same generation he raised 41 plants by self-fertilizing a single individual that had arisen from O. lamarckiana in a previous generation. This type he later named O. gigas.
All these new types arose under culture, but he also found O. lata and O. nanella growing wild. Most of these types appeared repeatedly among the progeny of O. lamarckiana in successive generations. Most, too, were reproductively stable.2 De Vries thought he had discovered the real way that new types of organisms came into being and that it was not the way Darwin had described. As de Vries (1902: 724) put it,
He was impressed by the fact that these new types appeared repeatedly, in different generations, de novo among the offspring of O. lamarckiana (except O. gigas, which made its appearance only once as a single individual from which all subsequent O. gigas plants were derived).
“A species therefore is not born only a single time,” said de Vries (1901–1903, 1902: 726), “but repeatedly, in a large number of individuals and during a series of consecutive years.” “In fact,” he says elsewhere (Hugo de Vries 1902: 725), “one can study the birth of a species as readily as that of any individual, be it plant or animal.”
De Vries’ experience with these new forms, and others obtained in subsequent experimentation, led him to believe that the type of variation giving rise to forms treated as distinct species was different from ordinary, ongoing variation. He therefore assigned variation to two distinct categories. In English translations of his works, the first type is termed "fluctuating variability" or "individual variability." The second type is called “discontinuous variability.” To refer to individual changes falling under the heading of this latter type of variation, de Vries used one of the same terms, single variations, Darwin had used to refer to sudden changes producing new forms.³ Thus, says de Vries (1901–1903: vol. I, 32–33),
De Vries was right in asserting that his single variations had a different genetic basis. Later research by other workers showed his “fluctuating variability” corresponds to variation resulting from ordinary meiotic recombination). His single variations, we now know, resulted, for the most part, from stabilization processes such as those described in Section Four, in particular those associated with permanent translocation heterozygotes, of which O. lamarckiana is an example (Cleland 1962, 1972; Darlington 1937; Emerson 1935; Harte 1994: 102–109, 133). His O. gigas, for example, is a polyploid derived from O. lamarckiana (Emerson 1935; Gates 1909; Lutz 1907). His O. albida, O. lata, O. oblonga, and O. scintillans were trisomics produced by aneuploidization (Emerson 1935).
However, O. rubrinervis and O. nanella were products of ordinary crossing-over, not chromosomal mutations.4 De Vries probably included these two cases among his list of single variations because
Thus, in the special case of translocation heterozygotes, ordinary meiotic recombination can produce effects suggesting the discontinuity of stabilization processes. So it is not surprising de Vries classed both of these mutations as single variations, since they both resulted in the abrupt production of a permanent new form. But, in general, his single variations were brought about by chromosomal mutations. The line he drew between “individual variability” and “discontinuous variability” roughly corresponded to a line between ordinary (intrachromoset) meiotic recombination and stabilization processes involving chromosomal mutations (see Section 4).
De Vries eventually presented his results and ideas in The Mutation Theory (1901–1903) and in Species and Varieties: Their Origin through Mutation (1905). In these books he specifically rejects the idea that forms treated as species arise gradually in isolation through natural selection. Instead, he proposes that the production of such forms is typically an abrupt event producing a new and subsequently stable type. Although he thought Darwin had been right in asserting a parallel exists between natural and artificial selection, de Vries was convinced Darwin had misunderstood the methods actually used by breeders. As de Vries put it:
He saw “single variations” as the main source of new types of organisms. In his estimation, “individual variability” was “of very subordinate importance,” both in nature and in the greenhouse (Hugo de Vries 1901–1903, 1905: 799, 802). Moreover, he realized his single variations were closely associated with hybridization. For example, de Vries (1901–1903: vol. I, 76) claimed
1. Allen (1969), describing Hugo de Vries.
2. The O. lata plants were an exception. They made no pollen, and could produce seed only when fertilized by the pollen from other types of plants. O. scintillans also was exceptional. Only a percentage of its offspring were again of the scintillans type. The rest were like O. lamarckiana. But the other five new forms described by Hugo de Vries did make pollen and produced exclusively offspring of their own kind when self-fertilized.
3. In addition to single variation, the term usually used by Hugo de Vries when writing in English, Darwin used the terms, sport, spontaneous variation, sudden variation, and bud-variation to refer to a saltation. Indeed, he used single variation only once in each of the first four editions of the Origin, where the more common term was sport. Thus, in the first edition, he says (1859: 32), “No one supposes that our choicest productions have been produced by a single variation from the aboriginal stock,” where he seems literally to mean “a single large change.” In Variation of Animals and Plants under Domestication, the usual terms used for the same phenomenon were spontaneous variation, sudden variation, and bud-variation. Darwin used the word mutation only in the old, general sense of “change.”
4. Emerson (1935). These resulted from what Muller (quoted in Babcock 1918: 117) called “the emergence in a state of homozygosis, through crossing over, of recessive factors constantly present in the heterozygous stock.”
5. That is, meiotic crossing-over between the Renner complexes in permanent translocation heterozygotes is rare (Grant 1981: 386).
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