David N. Reznick

Summary The role of phenotypic plasticity in evolution has historically been a contentious issue because of debate over whether plasticity shields genotypes from selection or generates novel opportunities for selection to act. Because plasticity encompasses diverse adaptive and non‐adaptive responses to environmental variation, no single conceptual framework adequately predicts the diverse roles of plasticity in evolutionary change. Different types of phenotypic plasticity can uniquely contribute to adaptive evolution when populations are faced with new or altered environments. Adaptive plasticity should promote establishment and persistence in a new environment, but depending on how close the plastic response is to the new favoured phenotypic optimum dictates whether directional selection will cause adaptive divergence between populations. Further, non‐adaptive plasticity in response to stressful environments can result in a mean phenotypic response being further away from the favoured optimum or alternatively increase the variance around the mean due to the expression of cryptic genetic variation. The expression of cryptic genetic variation can facilitate adaptive evolution if by chance it results in a fitter phenotype. We conclude that adaptive plasticity that places populations close enough to a new phenotypic optimum for directional selection to act is the only plasticity that predictably enhances fitness and is most likely to facilitate adaptive evolution on ecological time‐scales in new environments. However, this type of plasticity is likely to be the product of past selection on variation that may have been initially non‐adaptive. We end with suggestions on how future empirical studies can be designed to better test the importance of different kinds of plasticity to adaptive evolution.

Theoretical studies of life history evolution almost unanimously incorporate some form of tradeoff between life history variables, such as reduced parental survival due to increased reproductive effort. These tradeoffs, or of reproduction, are fundamental in predicting the optimal life history in any given environment. The importance of such in theoretical models has motivated researchers to check that they exist by seeking an association between some component of reproductive effort, such as fecundity, and some form of cost, such as parental survival. Four methods have been used for making these evaluations: 1) Phenotypic correlations, where some index of reproductive effort is correlated with a potential cost, 2) Experimental manipulations, where reproduction is directly manipulated and a cost-related response is measured, 3) Genetic correlations, where formal quantitative genetic designs are used to estimate the genetic correlation between some component of reproductive effort and some component of cost, and 4) Correlated responses to selection, where artificial selection causes correlated changes in reproductive effort and some component of Because the theoretical studies are models of evolution, I argue that a relevant empirical study must deal with genetically based tradeoffs. By this criterion, only the last two methods (which represent a small minority of the reviewed studies) properly address the costs dealt with by theoreticians; however, this evaluation is not intended as a litmus test for the merits of a given approach to the problem. I discuss both the important contributions of the first two methods to our understanding of the of reproduction and the limitations of the last two methods in assessing costs.

Natural populations of guppies were subjected to an episode of directional selection that mimicked natural processes. The resulting rate of evolution of age and size at maturity was similar to rates typically obtained for traits subjected to artificial selection in laboratory settings and up to seven orders of magnitude greater than rates inferred from the paleontological record. Male traits evolved more rapidly than female traits largely because males had more genetic variation upon which natural selection could act. These results are considered in light of the ongoing debate about the importance of natural selection versus other processes in the paleontological record of evolution.

Theoretical studies have demonstrated the potential importance of demographic selection in molding life history patterns. Some of the factors which have been examined include density-dependent versus density-independent mortality (Mac-