H. ALLEN ORR*
Recent studies of the genetics of speciation in Drosophila have focused on two problems: (i) identifying and characterizing the genes that cause reproductive isolation, and (ii) determining the evolutionary forces that drove the divergence of these “speciation genes.” Here, I review this work. I conclude that speciation genes correspond to ordinary loci having normal functions within species. These genes fall into several functional classes, although a role in transcriptional regulation could prove particularly common. More important, speciation genes are typically very rapidly evolving, and this divergence is often driven by positive Darwinian selection. Finally, I review recent work in Drosophila pseudoobscura on the possible role of meiotic drive in the evolution of the genes that cause postzygotic isolation.
Ernst Mayr made at least three contributions that are fundamental to the genetic study of speciation. The first, and surely most important, was his codification of what we mean by species and thus by speciation. In his book, Systematics and the Origin of Species, Mayr (1942) famously argued that species are best defined by the Biological Species Concept: species are groups of actually or potentially interbreeding natural populations that are reproductively isolated from other such groups. Reproductive isolation is thus the sine qua non of good species.
Mayr’s second contribution, elaborated in his book, Animal Species