Evolutionary analyses of morphological and physiological plasticity in thermally variable environments

Morphological and physiological plasticity is often thought to represent an adaptive response to variable environments. However, determining whether a given pattern of plasticity is in fact adaptive is analytically challenging, as is evaluating the degree of and limits to adaptive plasticity. Here we describe a general methodological framework for studying the evolution of plastic responses. This framework synthesizes recent analytical advances from both evolutionary ecology and functional biology, and it does so by integrating field experiments, functional and physiological analyses, environmental data, and genetic studies of plasticity. We argue that studies of plasticity in response to the thermal environment may be particularly valuable in understanding the role of environmental variation in the evolution of plasticity: not only can thermally-relevant traits often be mechanistically and physiologically linked to the thermal environment, but also the variability and predictability of the thermal environment itself can be quantified on ecologically relevant time scales. We illustrate this approach by reviewing a case study of seasonal plasticity in the extent of wing melanization in Western White Butterflies (Pontia occidentalis). This review demonstrates that 1) wing melanin plasticity is heritable, 2) plasticity does increase fitness in nature, but the effect varies between seasons and between years, 3) selection on existing variation in the magnitude of plasticity favors increased plasticity in one melanin trait that affects thermoregulation, but 4) the marked unpredictability of short-term (within-season) weather patterns substantially limits the capacity of plasticity to match optimal wing phenotypes to the weather conditions actually experienced. We complement the above case study with a casual review of selected aspects of thermal acclimation responses. The magnitude of thermal acclimation ("flexibility") is demonstrably modest rather than fully compensatory. The magnitude of genetic variation (crucial to evolutionary responses to selection) in thermal acclimation responses has been investigated in only a few species to date. In conclusion, we suggest that an understanding of selection and evolution of thermal acclimation will be enhanced by experimental examinations of mechanistic links between traits and environments, of the physiological bases and functional consequences of acclimation, of patterns of environmental variability and predictability, of the fitness consequences of acclimation in nature, and of potential genetic constraints.