Countergradient Selection in the Green Frog, Rana clamitans

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Abstract
Montane populations of green frogs are larger at all stages of development, and tadpoles take a year or two longer to complete metamorphosis than do lowland populations. Complementary field and laboratory studies demonstrated that the clinal variation in morphology and development is primarily induced by the environment. However, significant genetic differences in temperature optima, and temperature sensitivity of differentiation rates, growth rates, and stage-specific growth were demonstrated. The genetic differences were counter to the observed clinal variation, e.g., in cold temperature experiments designed to simulate mountain-top conditions, montane tadpoles actually developed faster and completed metamorphosis earlier and at a smaller size than did lowland tadpoles. A greater range of developmental variation than that observed is potentially inducible by the altitudinal gradient of temperatures. Natural selection has acted to minimize the inducible variation and has favored the shortest possible larval periods given the ambient temperature constraints. Only in this sense do montane and lowland green frogs show local adaptations to their respective environments. A new model of the interactions between the genotypic and induced phenotypic components of clinal variation is presented.

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