Disruptive selection in natural populations: the roles of ecological specialization and resource competition

Disruptive selection is potentially critical in maintaining variation and initiating speciation. Yet there are few convincing examples of disruptive selection from nature. Moreover, relatively little is known about the causes of disruptive selection. Here, we document disruptive selection and its causes in natural populations of spadefoot toad tadpoles (Spea multiplicata), which are highly variable in trophic phenotype and resource use. Using a mark-recapture experiment in a natural pond, we show that selection favors extreme trophic phenotypes over intermediate individuals. We further show that such disruptive selection likely reflects both ecological specialization and resource competition. Evidence for ecological specialization comes from two field experiments, which demonstrate that extreme phenotypes forage more effectively on the main alternative resource types. Support for competition's role in disruptive selection comes from two additional experiments, which demonstrate that intermediate phenotypes, which are often the most common phenotype, compete more with each other than with extreme forms and that the intensity of disruptive selection increases with conspecific density. Generally, ecological specialization and competition are widespread, suggesting that many populations may experience some level of disruptive selection. Thus, disruptive selection may be a more common force contributing to phenotypic variation in natural populations than is currently recognized.