Why do closely related, coexisting species typically differ in phenotypic features associated with resource use? One answer to this question is that such differences might generally evolve in allopatry, as different species adapt to divergent environmental conditions, and any differences that thereby accumulate might subsequently enable coexistence in sympatry. Alternatively, coexisting species might generally diverge in sympatry, because of selection to reduce competition for food (character displacement). Here we evaluated these two causes of character divergence by asking which hypothesis better explains differences in feeding morphology between tadpoles of two species of spadefoot toads, Spea bombifrons and S. multiplicata. We found that, in natural ponds containing both species, S. multiplicata almost always developed into a smaller, round-bodied tadpole with normal sized jaw muscles used for feeding on detritus at the pond bottom (the omnivore morph), whereas S. bombifrons almost always developed into a larger, flat-headed tadpole with greatly enlarged jaw muscles used for feeding on crustaceans in open water (the carnivore morph). By contrast, in all but one of 18 similar ponds containing a single species, both species expressed both phenotypes. Divergence between species in morph production appears to have evolved in sympatry: when we compared population means for each of four key trophic characters, we found that no allopatric population of S. bombifrons was as carnivore-like as the sympatric S. bombifrons, and, for three of four characters, no allopatric population of S. multiplicata was as omnivore-like as the sympatric S. multiplicata. In contrast to significant differences in trophic characters, we found no divergence between allopatric and sympatric populations in a character not directly involved in feeding on detritus or crustaceans (overall body size). These data, together with our earlier experimental work, reveal that coexisting S. bombifrons and S. multiplicata have diverged from one another in resource use and in phenotypic features associated with resource use because of selection to reduce competition for food (i.e., character displacement). Spea tadpoles, therefore, are one of few systems for which both experimental and observational evidence link phenotypic divergence to resource competition, thereby providing a model for understanding why coexisting species often differ phenotypically
