Molecular correlates of reproductive isolation.

Evolution of reproductive isolation as a byproduct of genetic divergence in isolated populations is the dominant (albeit not exclusive) mode of speciation in sexual animals. But little is known about the factors linking speciation to general divergence. Several authors have argued that allopatric speciation should proceed more rapidly if isolated populations also experience divergent selection. Reproductive isolation between allopatric populations is not subject to direct selection; it can accumulate only by random drift or as a fortuitous byproduct of selection on other traits. Here I present a novel analysis of published data, demonstrating that pre- and postmating isolation of Drosophila species are more tightly correlated with allozyme divergence than with silent DNA divergence. Inasmuch as proteins are more subject to the action of natural selection than are silent DNA polymorphisms, this result provides broad support for a model of selection-mediated allopatric speciation.