Partial reproductive isolation of subspecies of the California vole, Microtus californicus

Summary

A partial reproductive barrier found between subspecies of the California vole indicates that there may be obstacles to free interbreeding within this species in nature. Both the reduced fertility between subspecies and the sterility of hybrid males found in laboratory crosses would be obstacles to gene exchange between contiguous subspecies populations. Combative behavior engaged in by approximately half of the mixed subspecies pairs in the laboratory indicate that there might also be behavioral constraints to their breeding. Substantial differentiation has already occurred between M.c. californicus and stephensi in some of the faster evolving structural gene loci coding for enzymes not involved in glucose metabolism. Due to significant differences in body size, skull characteristics, and organ weights, the two subspecies are clearly distinguishable on the basis of these morphophysiological characters at this stage of reproductive isolation. Thus, there is good reason to believe that these populations of the California vole are diverging to the species level. I am continuing the investigation of the intraspecific divergence with a comparative study of the geographically intervening subspecies M.c. sanctidiegi, to determine its interfertility with californicus and stephensi. The speciation process has been studied extensively in drosophilids (Ayala, 1975) and there have been many studies of geographical variation and speciation in the genus Peromyscus (Dice, 1940, 1968), but it is not likely that the same model of speciation will apply equally to all organisms (White, 1973; Dobzhansky, 1972). The California vole promises to provide another important case study of the speciation process and of the nature of genetic changes involved in it.