Sinervo 9702033 Adaptive phenotypic plasticity is fundamentally a developmental phenomenon; expression of developmental pathways is modified in response to environmental cues, generating continuous trait variation or discrete, alternative morphologies. Understanding the proximate developmental basis of such plasticity can reveal much about the general role of development in evolution. Experimental manipulation of four species of spadefoot toad tadpoles will be used to investigate the proximate developmental basis underlying the production of a derived, inducible larval morphology. When typical, omnivorous larvae of some species in this clade consume prey high in thyroid hormone (TH), they may undergo transition to an adaptive, carnivorous tadpole. Manipulating TH levels in larvae from these species tests hypotheses addressing the role of TH in inducing the alternative morph. Similar manipulations of larvae from species that do not naturally become carnivorous tests hypotheses addressing how environmental factors (diet high in TH) may have perturbed development in the common ancestor, initiating evolution of the fully adapted plastic response seen today in the more derived species. Such an integrative approach may elucidate how proximate developmental mechanisms influence the route by which novel traits are produced, predisposing the production of certain phenotypes and influencing the path of evolutionary change.
