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within a small but critical region of XR will let us determine whether the genes that cause hybrid sterility can be separated meiotically from those that cause hybrid segregation distortion is warranted (N. Phadnis, personal communication).

It is clear, then, that hybrid males between these young subspecies suffer both sterility and segregation distortion, and that these two forms of hybrid dysfunction have similar genetic bases. We cannot, therefore, reject the possibility that at least some of the same genes cause both phenomena, as first suggested by Frank (1991) and Hurst and Pomiankowski (1991). More generally, we cannot reject the possibility that arms races between selfish genetic factors like those that cause meiotic drive contribute to the evolution of postzygotic reproductive isolation.

CONCLUSIONS

Molecular evolutionary analyses of speciation genes show that these loci are rapidly evolving, and that this evolution is often driven by positive Darwinian selection. Although the sample of genes characterized thus far by various laboratories remains small, and concentrated in the genus Drosophila, I suspect that these patterns may prove general, although likely not universal. Our recent work, along with that of several other groups, also suggests that the selection underlying the evolution of speciation genes may sometimes assume a surprising form, response to intragenomic conflicts, perhaps involving meiotic drive.

In summary, it would appear that two of Mayr’s three seminal contributions to the study of speciation were correct, or, at the least, extremely productive. First, the entire research program of the genetics of speciation over the last half-century arose out of Mayr’s Biological Species Concept. Whatever one’s views on the philosophical strengths or weaknesses of this concept, it has, as a practical matter, given rise to an extraordinarily rich research program, one that has led to a number of substantive discoveries. In a phrase, the modern genetics of speciation is a genetics of reproductive isolation. Second, there can be little doubt that this reproductive isolation typically, if not always, evolves in allopatry (Coyne and Orr, 2004; Mayr, 1963). Finally, however, recent work provides no evidence for a crucial role of genetic drift in speciation. Instead, both ecological studies (Schluter, 2000) and the genetical studies reviewed here point to an important role for positive Darwinian selection in the evolution of reproductive isolation. Although the above data do not exclude all varieties of drift-based models of speciation (founder-effect theories do, after all, feature a role for positive selection), such models seem at present unparsimonious (see also Coyne, 1994; Coyne and Orr, 2004).

It remains a testament to Mayr’s vast influence, however, that all of



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