THE EFFECTS OF GENE FLOW ON REINFORCEMENT

We explore the possibility that differences in the pattern of gene Row between populations may affect the evolution of reinforcement by comp aring pairs of populations undergoing one-way migration versus symmetr ic migration. The case of symmetric migration is modeled by a two-isla nd model, where the two populations exchange equal proportions of migr ants each generation. One-way migration is modeled by a continent-isla nd model, where migration is in one direction from a large continental population with a fixed genotype to an island population whose genoty pe 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 poss ible under a wide range of parameter values in a symmetric migration m odel but cannot occur in a one-way migration model. Reinforcement with one-way migration can occur only with a very strong introduced prefer ence and very strong selection against hybrids. Our results suggest th at 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.