Evolution__3rd_Edition

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This is a “hot topic”
The Odysseus gene controls
reproductive isolation in two fruitfly
species
CHAPTER 14 / Speciation 415
(Section 14.3.2). But it may be less of a coincidence. Females may evolutionarily seize
on those male characters that contribute to superior adaptation. Natural selection
works on mate choice mechanisms as well as ecological adaptation, and the two may
become associated.
A similar association arises in some recent models of sympatric speciation
(Dieckmann & Doebeli 1999; Higashi et al. 1999; Kondrashov & Kondrashov 1999).
One theoretical problem in reinforcement is that recombination tends to break down
any association between genes for assortative mating and genes for ecological adaptation
(Section 14.6.2). But sexual selection can help to strengthen the association, making
sympatric speciation more plausible.
These two arguments are only two of several ways in which sexual selection has
recently been suggested to drive speciation. (Schluter (2000, p. 195) gives a table with
six or so additional ideas. For instance, evolutionary conflict between males and
females (Section 12.4.7, p. 336) may contribute to speciation.) Most of the arguments
are hypothetical. Sexual selection has not yet been shown to drive the evolution of
prezygotic isolation in any case of speciation, though good suggestive evidence exists.
We do not know that sexual selection is a general force of speciation. But much
research on this topic is being done.
14.12 Identification of genes that cause reproductive
isolation is another current trend in research
We have discussed the genetics of both prezygotic and postzygotic isolation, the latter
extensively in the Dobzhansky–Muller theory. Prezygotic isolation may be due to
pleiotropy and hitch-hiking; postzygotic isolation to epistatic interactions among multiple
gene loci. The discussion, however, has been abstract. Genetic crosses do provide
evidence for the Dobzhansky–Muller theory, and it is possible to explain why biological
systems fit the theory (Section 14.4.3). But in none of the work did we look at particular
examples of genes. Relatively little research has been done yet to identify genes that
contribute to pre- or postzygotic isolation. But research of this sort may provide one
way forward in studying speciation. If we can identify genes that cause prezygotic isolation,
we can see what (if anything) their pleiotropic, and hitch-hiked, effects are. If we
can identify genes causing postzygotic isolation, we can investigate what their epistatic
interactions are with other genes and why those interactions arise. Our understanding
of speciation should improve as we move from abstract theory to concrete examples.
Moreover, modern genetics has powerful techniques for identifying genes a techniques
that were not available before the “genomics” era.
As an example, consider the work of Ting et al. (1998) on a gene called Odysseus.
Drosophila simulans and D. mauritiana are two closely related fruitfly species and an
interspecific cross between them conforms to Haldane’s rule a that the male hybrids
are sterile. Ting et al. used genetic techniques to insert bits of D. mauritiana DNA into
D. simulans. They were able to show that male hybrid sterility is caused by a gene on the
X chromosome. If they inserted only this gene (Odysseus) into D. simulans males, those
males were sterilized as in an interspecies cross (Figure 14.15).