The Specialization Hypothesis for Phenotypic Plasticity in Plants

Adaptive plasticity in plants is commonly interpreted for fitness estimates like size and fecundity. The specialization hypothesis, however, predicts that plasticity in such characters is not a product of selection but, rather, a product of specialized (i.e., ecotypic) adaptation to particular environmental conditions. In response to a recent test of this hypothesis (Emery et al. 1994), we refine its predictions to recognize that the evolution of specialized ecotypes may be accompanied by an increase, decrease, or no change in the plasticity of size or fecundity. These predictions depend on whether specialization is associated with the less favorable or more favorable end of an environmental gradient and on whether specialization to one end of the gradient comes at a cost of reduced performance at the other end. We argue that, for size or fecundity characters, a plastic response to environmental deterioration is adaptive only if the alternative is dormancy or death and is generally less adaptive than phenotypic stability. Based on analysis of reaction norms for reciprocal transplants, we illustrate how it is possible to reject the specialization hypothesis and how to recognize results that are consistent with this hypothesis.

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