THE EFFECTS OF GENE FLOW ON REINFORCEMENT.

We explore the possibility that differences in the pattern of gene flow between populations may affect the evolution of reinforcement by comparing pairs of populations undergoing one-way migration versus symmetric migration. The case of symmetric migration is modeled by a two-island model, where the two populations exchange equal proportions of migrants each generation. One-way migration is modeled by a continent-island model, where migration is in one direction from a large continental population with a fixed genotype to an island population whose genotype frequencies can vary. Hybrid inviability is assumed to be caused by epistatic interactions between background loci. We examine the spread of an introduced preference allele for a previously unpreferred male trait that characterizes one of the populations. Computer simulations indicate that with a weak introduced preference, reinforcement is possible under a wide range of parameter values in a symmetric migration model but cannot occur in a one-way migration model. Reinforcement with one-way migration can occur only with a very strong introduced preference and very strong selection against hybrids. Our results suggest that the speciation of a peripheral isolate, which undergoes essentially one-way migration, may be difficult to complete if secondary contact occurs before reproductive isolation is fully developed.