Divergent outcomes of reinforcement speciation: the relative importance of assortative mating and migration modification

Most studies of reinforcement speciation focus on the evolution of assortative mating, but R. A. Fisher argued that migration modification is likely to be a common alternative mechanism. Despite previous models showing that assortative mating and migration modification may both be involved in reinforcement, no one has determined their relative evolutionary importance. This is surprising because understanding the biological conditions favoring these mechanisms may explain why certain pairs of species exhibit abutting, nonoverlapping geographical ranges with habitat fidelity while other pairs coexist in sympatry with sexual isolation. In this article, we explicitly model the evolution of both mechanisms simultaneously. First, we explore how these mechanisms differ in their evolutionary dynamics. Second, we ask how they affect each other's evolution and whether the interaction alters their relative importance in reinforcement. Our results reveal that assortative mating may evolve faster and under a broader range of biological conditions than migration modification. However, direct evolutionary interactions favor migration modification when populations experience strong divergent selection. Depending on the nature of postmating isolation, these mechanisms may either interfere with each other's evolution or coevolve in the same system. These results illustrate the importance of studying multiple mechanisms of speciation simultaneously in future speciation models.