Logotyp: till Uppsala universitets webbplats

Negative frequency-dependent disruptive selection, which arises due to the interplay between organisms of a population and their environment, is an important element driving phenotypic diversification and even speciation. Such selection regime can result from frequency- and density-dependent interactions between the organisms and their environment, so that the fitness landscape itself changes as the population evolves. This can result in the population evolving towards a fitness minimum, at which point a population experiences disruptive selection. Under this regime, more extreme phenotypes are favored over intermediate ones, which in turn leads to phenotypic diversification. Branching points occur in many models in which fitness is derived from ecological scenarios that account for resource competition, predation, pathogens. This phenomenon is well understood in simple cases where interactions are mediated by a single quantitative trait in an unstructured life-cycle. This thesis, then, provides a theoretical exploration of the effects of complexity, as represented by the joint evolution of consumer traits involved in resource acquisition, on the potential for phenotypic diversification as instantiated in the process of evolutionary branching. We use the mathematical modeling framework of adaptive dynamics -- which incorporates ecological details into evolutionary processes -- to conduct our investigations, with additional help from computer simulations. We find that the effects of complexity on the potential for diversification are not straightforward, and that these depend on the specificities of the ecological scenario one is investigating. In Paper I we find that joint evolution of consumer traits involved in resource acquisition result in epistatic interactions which make it more likely that the consumer population will evolve to become a single specialist. In Paper II, we show that adding a plasticity modifier trait to the co-evolution of resource acquisition traits has mild effects in facilitating evolutionary branching, and that plasticity itself is driven to low levels by the aforementioned epistatic interactions between traits. In Paper III we find that the joint evolution of juvenile and adult specific feeding efficiencies in an organism with a complex life-cycle generally facilitates evolutionary branching because the life-stage with a higher population density is often under a regime of frequency-dependent disruptive selection. And in Paper IV we find that the joint evolution of juvenile and adult resource acquisition traits in an organism with a complex life-cycle does not itself increase the potential for evolutionary branching, but it can lead to significantly higher community richness when communities are assembled trough immigration.