On the Evolution of Reproductive Isolation and the Origin of Species

Summary

Species, in sexually reproducing organisms, are arrays of interbreeding natural populations that are reproductively isolated from other such arrays. The origin of species may be seen as one important consequence of the evolution of reproductive isolating mechanisms (RIMs) between populations, or groups thereof. Some recent authors have argued that new species arise rather by the evolution of mutual recognition systems; and that species should be defined as populations sharing in a common fertilization system. These two views differ concerning the evolutionary sequence of events leading to new species. The first view implies that postzygotic RIMs are likely to evolve first, as a consequence of genetic divergence between geographically separated populations; whereas sexual isolation and other prezygotic RIMs are likely to evolve later. The evolution of prezygotic RIMs will be directly promoted by natural selection whenever populations exhibiting postzygotic RIMs become sympatric. The second hypothesis postulates, on the contrary, that mechanisms of mutual recognition readily evolve among geographically separated populations. A series of tests with several widely–distributed species of Drosophila, support the first hypothesis.