Hormonal Mechanisms as Potential Constraints on Evolution: Examples from the Anura

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Abstract
Developmental constraints are limitations on phenotypic variability resulting from developmental mechanisms that produce biases in phenotypic variants and hence evolution. These constraints ultimately limit the available phenotypes on which selection can act. Because hormones play important roles in many developmental processes, there is a great potential for hormonal mechanisms to produce (or act as) developmental constraints. In the current study, I present two examples to show how hormones may produce developmental constraints on evolution in the Anura. One example (a universal constraint in the Anura), examines evidence that thyroid hormones are required for sex differentiation and reproduction in frogs. The thyroid hormone requirement for these processes may prevent the evolution of neoteny in anurans. The second example (a local constraint) examines the mechanisms underlying sexual dichromatism in the genus Hyperolius (Hyperoliidae) and shows how the evolution of sexual dichromatism is limited by the hormonal mechanisms regulating pigmentation

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