Linking local species interactions to rates of speciation in communities

Regional species diversity limits the diversity of local communities by defining the pool of species that are available to colonize sites. Biogeographical processes that influence speciation and extinction rates determine the size and composition of this regional species pool. Community ecologists are beginning to recognize the importance of these biogeographical processes in influencing diversity patterns among local communities, but the potential for local interactions to influence biogeographical processes, especially speciation, has been little considered. In this paper I discuss one mechanism by which variation in the strengths of local interactions can influence the potential for population differentiation and thus for influencing speciation rates in the allopatric model of speciation. When more than one selective agent acts on the phenotype, the shape of the overall fitness surface depends critically on the relative strengths of the various selective agents. If one selective agent, which imparts strong selection, acts in all populations of a species, population differentiation via adaptation to local ecological conditions or via differentiation of sexual systems is retarded because the overall fitness surface in all populations strongly resembles the shape of the strongest selective agent. Consequently, the potential for speciation is reduced. Alternatively, if selective agents in various populations impart relatively equitable strengths of selection, the potential for differences in the overall fitness surfaces among populations is enhanced, which will enhance the potential for population differentiation and thus speciation. Analogous results obtain when multiple selective agents impact genetically correlated characters. Because the strength of selection generated by a species interaction should increase with the strength of the interaction, and because fewer species can usually coexist when the strengths of interactions are greater, the number of coexisting species and the potential for speciation in component taxa may covary among communities. This analysis indicates that the relative strengths of interactions can be as important to diversification in communities as the number of niche dimensions along which differentiation can occur.