PFENNIG, D.W.: Polyphenism and the Ecological Origins of Diversity

What are the causes of phenotypic diversity? A leading hypothesis is that divergent selection resulting from competition promotes diversification among coexisting species. This hypothesis is difficult to test, however, because there may be a long time lag between the introduction of a competitor and the evolutionary response by the focal species. Species that respond to competitors through polyphenism, the existence of environmentally cued alternative phenotypes in a population, provide a direct method to evaluate the role of competition in promoting character change. Thus, a causal link can be established experimentally between the presence of heterospecific competitors and character change if, by removing (or adding) competitors, the polyphenic species differentiates (or fails to differentiate) into an alternative morph that resembles the competitor. I tested this hypothesis in spadefoot toad tadpoles (Spea bombifrons and S. multiplicata). Depending upon their diet, individuals of both species develop into either a small-headed omnivore morph, which feeds mostly on detritus, or a large-headed carnivore morph, which specializes on shrimp. Laboratory experiments and surveys of natural ponds revealed that the two species were more dissimilar in their tendency to produce carnivores when they occurred together than when they occurred alone. Moreover, the two species became more dissimilar as their likelihood of encountering each other increased, and phenotypic plasticity, in part, caused this divergence. These results therefore demonstrate that selection to minimize competition for food may be an important cause of phenotypic divergence between coexisting species. They also illustrate how phenotypic plasticity mediates competitive interactions between species.