SPECIATION BY REINFORCEMENT - A MODEL DERIVED FROM STUDIES OF DROSOPHILA

Reinforcement is an increase in premating reproductive isolation betwe en taxa resulting from selection against hybrids. We present a model o f reinforcement with a novel type of selection on female mating behavi or. Previous models of reinforcement have focused on the divergence of female mating preferences between nascent species. We suggest that an increase in the level of female mating discrimination can yield reinf orcement without further divergence of either male characters or femal e preferences. This model indicates that selection on mating discrimin ation is a viable mechanism for reinforcement and may allow speciation under less stringent conditions than selection on female preference. This model also incorporates empirical results from genetic studies of hybrid fitness determination in Drosophila species. We find that the details of inheritance, which include sex-linked transmission, sex-lim ited fertility reduction, and X-autosome epistasis, have important eff ects on the likelihood of reinforcement. In particular, X-autosome epi stasis for hybrid fitness determination facilitates reinforcement when hybrid fertility reduction occurs in males, but hinders the process w hen it occurs in females. HALDANE's rule indicates that hybrid sterili ty will generally evolve in males prior to females within nascent spec ies. Thus, HALDANE's rule and X-autosome epistasis provide conditions that are surprisingly favorable for reinforcement in Drosophila.