Many examples of both morphological plasticity (which is fixed during development) and physiological plasticity (which is reversible, and has traditionally been termed acclimation) are thought to be adaptive responses to variation in the environment. The beneficial acclimation hypothesis proposes that acclimation has a fitness benefit in the environment that caused it, but the adaptive significance of acclimation has often been questioned (Section 5.2.1). Field experiments can show whether plasticity is adaptive (Kingsolver and Huey 1998). Seasonal polyphenism is a form of phenotypic plasticity, and seasonal changes in the wing colour of butterflies are frequently cited as examples. The plasticity of wing melanization and its fitness-related effects have been investigated in detail in western white butterflies, Pontia occidentalis (for review see Kingsolver and Huey 1998), using measurement (and sometimes manipulation) of a variety of quantitative traits, combined with tests of flight performance in natural environments. Melanization of the hindwings (the ventral surface and the base of the dorsal surface) is sensitive to developmental photoperiod and increases in spring. Basal melanin directly affects Tb achieved during basking, and the beneficial acclimation hypothesis would argue that darker wings provide an advantage by increasing activity time in cool seasons, while paler wings will reduce the risk of overheating in summer.
Alternative wing phenotypes were generated either by rearing P. occidentalis under short-day and long-day conditions, or by painting the wings of wild-collected individuals. Butterflies were then released into the field for subsequent monitoring of the effects on mating success and survival (Kingsolver 1995a, 1996). In the first study, mark-recapture of the paler butterflies reared under long-day conditions showed better survival during summer. The second study, using butterflies with painted wings, served to eliminate any phenotypic
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