Is phenotypic plasticity adaptive?

Phenotypic plasticity is the tendency for organisms with the same genotype to produce different, but repeatable, phenotypes under different environmental conditions. Human behavior genetics is concerned with partitioning human variation into genetic versus non-genetic variation, and the non-genetic variation is due in part to phenotypic plasticity. Here we examine human phenotypic plasticity from the perspective of evolutionary biology. The question-to what degree is phenotypic plasticity adaptive-has been studied intensively during the past 15 years, but it has been studied almost exclusively in plants and non-human animals. Although the question has usually been asked of examples of plasticity that seem particularly likely to be adaptive, the results of empirical tests have been mixed. These mixed results suggest that many cases of apparently adaptive plasticity might fail to support the adaptive hypothesis when currently accepted methodology is applied. Here we describe the techniques that have been developed to test the adaptive hypothesis, and consider whether any of them can be successfully applied to the study of human phenotypic plasticity. We conclude that while many of the techniques of experimental evolutionary biology cannot be applied to humans, there are methods by which students of human behavior can successfully test the adaptive plasticity hypothesis.