Phenotypic plasticity - the ability of one genotype to produce many phenotypes in response to environmental change - may play a critical role in allowing populations to endure environmental transitions. Moreover, selection on genetic variation underlying plastic phenotypes may modify and refine these traits such that they are adaptive in novel environments. Yet the prevalence of such genetic variation in natural populations, and the proximate mechanisms underlying its expression, remain relatively uncharacterized. My research sought to address this gap by assessing the role of environmentally-dependent phenotypes and their underlying genetic variation in the origins and diversification of feeding strategies among spadefoot toad larvae. While most spadefoot larvae feed on a range of diets, two lineages specialize on specific resources. Tadpoles of the genus Spea can develop as a carnivore morph that preys on, and is induced by fairy shrimp. In contrast, Sc. couchii, which cooccurs with Spea, specializes on detritus. Using a comparative approach, I determined that Sc. couchii have been excluded from the shrimp resource because of predation pressure imposed by Spea, and that Sc. couchii minimize this predation risk via behavioral plasticity. Furthermore, this dietary exclusion has been followed by evolutionary modification of Sc. couchii's behavioral, physiological and morphological responses to shrimp. Relaxed selection on traits that are no longer used - such as those involved with consuming shrimp in Sc. couchii - can result in the accumulation of neutral population genetic variation. When these populations encounter a new environment (or revisit an old one), such genetic variation may be expressed and is potentially adaptive. I found that feeding Sc. couchii shrimp amplifies the expression of genetic variation in a trait that is adaptive for consuming shrimp (as seen in Spea). Additionally, the hormone corticosterone, which is elevated in Sc. couchii fed the shrimp diet, can mediate the expression of this variation. In summary, ecological interactions can influence genetic variation underlying environmentally-dependent phenotypes and, by extension, phenotypes that are expressed and potentially adaptive when populations transition to novel environments. Furthermore, characterizing environmentally-dependent physiological processes may help us to better understand when and how such phenotypes are expressed
