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FIGURE 2.1 The rate of increase in “total” reproductive isolation with genetic distance in Drosophila. Postzygotic isolation is measured such that a value of 0 means no reproductive isolation (either pre- or postzygotic) and a value of 1 means complete reproductive isolation. Genetic distance is measured by Nei’s genetic distance, which increases approximately linearly with time over the values shown. Data include both allopatric and sympatric species pairs. Although the measure of total reproductive isolation shown includes aspects of both pre-and postzygotic isolation, it is imperfect and generally excludes forms of reproductive isolation (e.g., ecological isolation) that are not readily measured in the laboratory. Data are based on Coyne and Orr (1989, 1997). [Reproduced with permission from Coyne and Orr, 1997 (Copyright 1997, International Journal of Organic Evolution).]


Biologists who do not specialize in speciation are invariably surprised to learn that evolutionists have identified very few genes causing reproductive isolation. The reason for this slow progress is, however, simple. If species are taxa that are reproductively isolated, a genetics of speciation must, almost by definition, be a genetics where such a thing is not possible, between organisms that do not exchange genes. The consequence of this methodological dilemma is that evolutionists have been unable to address a large set of fundamental questions about the genes that cause reproductive isolation, so-called speciation genes. (This perhaps unfortunate term, which is now entrenched in the literature, refers to any locus that causes reproductive isolation, whether in F1 or later-generation hybrids, and whether the gene was among the first to cause isolation or not.)

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