Logo: to the web site of Uppsala University

Most animals undergo ontogenetic niche shifts during their life. Yet, standard ecological theory builds on models that ignore this complexity. Here, we study how complex life cycles, where juvenile and adult individuals each feed on different sets of resources, affect community richness. Two different modes of community assembly are considered: gradual adaptive evolution and immigration of new species with randomly selected phenotypes. We find that under gradual evolution complex life cycles can lead to both higher and lower species richness when compared to a model of species with simple life cycles that lack an ontogenetic niche shift. Thus, complex life cycles do not per se increase the scope for gradual adaptive diversification. However, complex life cycles can lead to significantly higher species richness when communities are assembled trough immigration, as immigrants can occupy isolated peaks of the dynamic fitness landscape that are not accessible via gradual evolution.

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.

Consumers regularly experience trade-offs in their ability to find, handle, and digest different resources. Evolutionary ecologists recognized the significance of this observation for the evolution and maintenance of biological diversity long ago and continue to elaborate on the conditions under which to expect one or several specialists, generalists, or combinations thereof. Existing theory based on a single evolving trait predicts that specialization requires strong trade-offs such that generalists perform relatively poorly, while weak trade-offs favor a single generalist. Here, we show that this simple dichotomy does not hold true under joint evolution of two or more foraging traits. In this case, the boundary between trade-offs resulting in resource specialists and resource generalists is shifted toward weaker trade-off curvatures. In particular, weak trade-offs can result in evolutionary branching, leading to the evolution of two coexisting resource specialists, while the evolution of a single resource generalist requires particularly weak trade-offs. These findings are explained by performance benefits due to epistatic trait interactions enjoyed by phenotypes that are specialized in more than one trait for the same resource.