The evolution of ecological specialization is a major theme in evolutionary history. The evolutionary consequences of such specialization have received considerable attention, but have rarely been studied using knowledge of phylogenetic relationships of the taxa involved, a prerequisite for answering many kinds of evolutionary questions. This research will involve an investigation of the evolutionary consequences of trophic specialization in garter snakes (genus Thamnophis) using an explicitly phylogenetic approach. The first part of the study tests hypotheses that certain traits (head shapes and visual characteristics) are adaptations for a specialized underwater foraging mode. The investigation will provide an especially powerful examination of adaptive hypotheses by combining within a phylogenetic context (1) comparative tests of the evolutionary association of head shape and visual traits with feeding mode, and (2) experimental analyses of trait function. The second part of the study will involve rigorous phylogenetic and statistical tests of the idea that specialist taxa are constrained in their subsequent evolution. These tests will focus on trophic niche transitions among terrestrial specialists, aquatic specialists, and generalist garter snakes. This examination of constraint will complement the tests of adaptation, because contraints on niche transitions provide the evolutionary boundaries within which the adaptive changes occur.
