Competition as a Driver of Diversity Within and Between Species

Identifying the processes that generate biological diversity is a major goal of evolutionary biology. One such process is character displacement: trait divergence caused by selection to minimize resource competition. Character displacement contributes to biological diversity by enhancing phenotypic differences both between and within species, facilitating species coexistence, and promoting speciation and adaptive radiation. Despite this, the factors that influence whether and how character displacement occurs remain unclear. My dissertation research seeks to address this gap. Intraspecific character displacement may be driven by disruptive selection, which occurs when extreme phenotypes have a fitness advantage over more intermediate phenotypes. However, disruptive selection is assumed to be rare. I document disruptive selection in natural populations of spadefoot toad tadpoles (Spea multiplicata). Further, I show that this disruptive selection reflects both ecological specialization and resource competition. I suggest that because ecological specialization and competition are widespread, disruptive selection may more common than is currently recognized. Resource polymorphism - the occurrence within a population of discrete resource-use morphs - is one possible outcome of intraspecific character displacement. Yet, few studies have evaluated what ecological factors favor resource polymorphism's evolution. I combine observations of natural populations with an experiment to assess the causes of resource polymorphism in S. multiplicata tadpoles. My results suggest that ecological opportunity, along with intraspecific competition, are required for resource polymorphism to evolve. The role of maternal effects in character displacement is unclear. I investigate the role of maternal effects in the expression of resource polymorphism in S. multiplicata. I found that mothers influenced the morph determination of their offspring via condition dependent investment in eggs. In addition, my results suggest that maternal investment may mediate interspecific character displacement between S. multiplicata and a heterospecific competitor. Finally, which particular traits experience selection during character displacement can influence whether and how character displacement occurs. However, few studies identify the targets of selection during character displacement. I found that in competition with a heterospecific, selection on a single trait drove correlated selection on additional traits in S. multiplicata tadpoles. In addition, I suggest that interacting species might differ in the traits under selection during character displacement