Selection against migrants contributes to the rapid evolution of ecologically dependent reproductive isolation

Ecologically dependent reproductive isolation (EDRI) is the decrease in gene flow that occurs when adaptive divergence reduces the fitness of individuals that move between ecological environments. A particularly simple and perhaps almost universal mechanism contributing to EDRI is natural selection against migrants themselves (i.e. before any selection against their offspring). Here I use single-locus and quantitative-genetic models to examine how quickly this mechanism can contribute to EDRI after the founding of a new population. I find that as long as divergent selection is relatively strong, selection against migrants can make a substantial contribution to EDRI in less than 30 generations. This mechanism is only one of several that might influence EDRI. The addition of other mechanisms, such as habitat choice or selection against hybrids, would presumably accelerate the accumulation of EDRI. Although EDRI is neither complete (some gene flow may still occur) nor unconditional (it depends on the ecological context), it may reduce gene flow to the point that absolute and unconditional reproductive isolation can evolve through other mechanisms. Whenever organisms colonize new ecological environments, selection against migrants may contribute to rapid reductions in gene flow. Future studies of ecological speciation may profit from particular attention to this mechanism of reproductive isolation.