One of the good pieces in today’s New York Times is the article by Carol Yoon on speciation, “Genes offer new clues in old debate on species’ origin.“ Maybe I’m biased, since speciation is my own area of research, but I think it’s nice that such an important area of evolutionary biology (after all, biodiversity requires both changes within lineages and the generation of new lineages) is singled out during Darwin Week. Carol’s article was inspired by my grand-student Daven Presgraves’s and his colleague Shanwu Tang’s new paper in Science describing a gene causing inviability in species hybrids. Tang and Presgraves found that two genes producing nucleoporins (proteins involved in transport of substances across the nuclear membrane) caused the death of hybrids between two species of fruit flies (Drosophila). Apparently the genes had diverged so much between the species that they do not work together when present together in the genome of a hybrid.
It is a surprise that nuceloporins would be involved in the death of species hybrids, and we have no idea why this happens. One of the areas of speciation that we know almost nothing about is which genes have diverged to produce the reproductive barriers between species. We know of only nine at present, all producing either the death or sterility of hybrids. And all of these genes bear, in their DNA, the traces of natural selection, so we can at least say that natural selection—as opposed to other evolutionary forces—was involved in this case of evolutionary divergence. My only quibble with this discussion is that Yoon repeatedly calls genes like this “speciation genes.” But we don’t know if these genes were actually involved in speciation: their divergence may have occurred after the reproductive barriers between species (which, after all, can be caused by divergence in mating behavior or ecological preference) had already evolved. That is, the sterility or death of hybrids may represent examples of POST-speciation evolution. Nevertheless, they are still quite interesting, because they can explain some of the regularities of evolution, such as “Haldane’s rule,” (the preferential death or sterility of those hybrids having sex chromosomes that are not alike—males in most species but females in birds and butterflies).
Kudos to Ms. Yoon for adhering to the biological species concept (BSC) in her article: the view that a “species” is a group of interbreeding organisms separated from other such groups by genetically enoded barriers to reproduction. There are many other species concepts, but none that has yielded a productive research program on speciation. As far as I know, every single paper studying the process of speciation in real organisms is concerned with studying the origin of reproductive barriers.