Patterns of parapatric speciation

Geographic variation may ultimately lead to the splitting of a subdivided p opulation into reproductively isolated units in spite of migration. Here, w e consider how the waiting time until the first split and its location depe nd on different evolutionary factors including mutation, migration, random genetic drift, genetic architecture, and the geometric structure of the hab itat. We perform large-scale, individual-based simulations using a simple m odel of reproductive isolation based on a classical view that reproductive isolation evolves as a by-product of genetic divergence. We show that rapid parapatric speciation on the time scale of a few hundred to a few thousand generations is plausible even when neighboring subpopulations exchange sev eral individuals each generation. Divergent selection for local adaptation is not required for rapid speciation. Our results substantiates the claims that species with smaller range sizes (which are characterized by smaller l ocal densities and reduced dispersal ability) should have higher speciation rates. If mutation rate is small, local abundances are low, or substantial genetic changes are required for reproductive isolation, then central popu lations should be the place where most splits take place. With high mutatio n rates, high local densities, or with moderate genetic changes sufficient for reproductive isolation, speciation events are expected to involve mainl y peripheral populations.