In their discussion of the ways in which evolutionary hypotheses of acclimation can be rigorously tested, Huey and Berrigan (1996) present a useful framework for understanding phenotypic plasticity in the context of evolutionary physiology. In their terminology

(1) phenotypic plasticity refers to the malleability of an organism's phenotype in response to, or in anticipation of, environmental conditions experienced (or to be experienced) by the organism. Thus, acclimation responses (or acclimatization in the field) are examples of phenotypic plasticity in physiological traits;

(2) the norm of reaction (commonly also termed the reaction norm) is the form that the phenotypic effect takes;

(3) developmental switches are a subset of plastic responses and involve an irreversible change in the phenotype in response to environmental conditions experienced during a critical developmental phase;

(4) cross-generational effects are phenotypic modifications transmitted either maternally or paternally (e.g. Crill et al. 1996);

(5) developmental pathologies are environmentally induced pathological modifications of the pheno-type that occur during development (e.g. Roberts and Feder 1999);

(6) labile effects are acute, rapid modifications of performance as a function of the organism's immediate environment. Performance curves are often used to illustrate these effects, and these

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