Fig. 2. Hypothetical example of the effects of positive directional selection favoring individuals with higher values for a particular trait on the mean value of that trait (A) and on the plasticity of that trait or of a subordinate trait (B). (A) The standard expectation for the effects of positive directional selection on the distribution of a trait (for example, heat tolerance) across several generations. During generation one, a selective event – high temperature lasting for several days – kills a majority of the individuals in the population (G₁) before they can breed. The survivors (S₁) of this selective event then breed and the mean heat tolerance in their offspring (G₂) is somewhat higher than for their parents (G₁). The difference in population mean phenotype between generations one (G₁) and two (G₂) indicates that evolution has occurred (assuming that the environment in which the organisms are living has not changed in a way that causes the altered phenotypes via direct environmental effects). This process continues for several generations such that the mean value of the trait in generation five (G₅) is substantially higher than in generation one. (B) A hypothesis regarding the correlated evolution of the plasticity of heat tolerance or of a subordinate trait that supports heat tolerance (e.g. expression of heat shock proteins). In the original population, exposure to high temperatures for a few hours or days causes some individuals to increase in heat tolerance (which would probably be adaptive if the high temperatures continued) while an equal number of other individuals actually exhibit a decrease in heat tolerance, which would be maladaptive (inappropriate) if high temperatures persisted. For the population as a whole, the average plastic response is zero. Following a selective event and subsequent breeding of the survivors (S₁), which produces the next generation (G₂), the average plastic response in this new generation tends to be an increase in heat tolerance. Thus, natural selection has caused an evolutionary increase in both the average `innate' (or `constitutive' or `intrinsic') heat tolerance (A) and a shift in the average plasticity of individuals (B) such that, on average, they become more heat tolerant when exposed (acutely) to high temperatures. This constitutes the evolution of adaptive plasticity. See text for discussion of possible genetic mechanisms of such a correlated response to selection.