HORMONAL MECHANISMS AS POTENTIAL CONSTRAINTS ON EVOLUTION - EXAMPLES FROM THE ANURA

Developmental constraints are limitations on phenotypic variability re sulting from developmental mechanisms that produce biases in phenotypi c variants and hence evolution, These constraints ultimately limit the available phenotypes on which selection can act. Because hormones pla y important roles in many developmental processes, there is a great po tential for hormonal mechanisms to produce (or act as) developmental c onstraints. In the current study, I present two examples to show how h ormones may produce developmental constraints on evolution in the Anur a, One example (a universal constraint in the Anura), examines evidenc e that thyroid hormones are required for sex differentiation and repro duction 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 dichromat ism in the genus Hyperolius (Hyperoliidae) and shows how the evolution of sexual dichromatism is limited by the hormonal mechanisms regulati ng pigmentation.