Evolutionary branching and sympatric speciation caused by different types of ecological interactions

Evolutionary branching occurs when frequency-dependent selection splits a p henotypically monomorphic population into two distinct phenotypic dusters. A prerequisite for evolutionary branching is that directional selection dri ves the population toward a fitness minimum in phenotype space. This articl e demonstrates that selection regimes leading to evolutionary branching rea dily arise fi-om a nide variety of different ecological interactions within and between species. We use classical ecological models for symmetric and asymmetric competition, for mutualism, and for predator-prey interactions t o describe evolving populations with continuously varying characters. For t hese models, we investigate the ecological and evolutionary conditions that allow for evolutionary branching and establish that branching is a generic and robust phenomenon. Evolutionary branching becomes a model for sympatri c speciation when population genetics and mating mechanisms are incorporate d into ecological models. In sexual populations with random mating, the con tinual production of intermediate phenotypes from two incipient branches pr events evolutionary branching. In contrast, when mating is assortative for the ecological characters under study, evolutionary branching is possible i n sexual populations and can lead to speciation. Therefore, we also study t he evolution of assortative mating as a quantitative character. We show tha t evolution under branching conditions selects for assortativeness and thus allows sexual populations to escape from fitness minima. We conclude that evolutionary branching offers a general basis for understanding adaptive sp eciation and radiation under a wide range of different ecological condition s.