When is sympatric speciation truly adaptive? An analysis of the joint evolution of resource utilization and assortative mating

The plausibility of sympatric speciation has long been debated among evolutionary ecologists. The process necessarily involves two key elements: the stable coexistence of at least two ecologically distinct types and the emergence of reproductive isolation. Recent theoretical studies within the theoretical framework of adaptive dynamics have shown how both these processes can be driven by natural selection. In the standard scenario, a population first evolves to an evolutionary branching point, next, disruptive selection promotes ecological diversification within the population, and, finally, the fitness disadvantage of intermediate types induces a selection pressure for assortative mating behaviour, which leads to reproductive isolation and full speciation. However, the full speciation process has been mostly studied through computer simulations and only analysed in part. Here I present a complete analysis of the whole speciation process by allowing for the simultaneous evolution of the branching ecological trait as well as a continuous trait controlling mating behaviour. I show how the joint evolution can be understood in terms of a gradient landscape, where the plausibility of different evolutionary paths can be evaluated graphically. I find sympatric speciation unlikely for scenarios with a continuous, unimodal, distribution of resources. Rather, ecological settings where the fitness inferiority of intermediate types is preserved during the ecological branching are more likely to provide opportunity for adaptive, sympatric speciation. Such scenarios include speciation due to predator avoidance or specialization on discrete resources.