EVOLUTION OF RESISTANCE TO HIGH-TEMPERATURE IN ECTOTHERMS

Body temperature influences the performance and fitness of ectotherms. How thermal sensitivity responds to selection for resistance to high temperature is broadly relevant in evolutionary physiology and also has practical implications. We review several complementary approaches to studying the evolution of thermal sensitivity. First, we analyze comparative data that illustrate the historical evolution of thermal sensitivity of locomotion in iguanid lizards. Taxa that experience high body temperatures in nature have evolved high optimal temperatures for sprinting. Critical thermal maxima are coadapted with optimal temperatures but not with critical thermal minima. Thus some but not all aspects of thermal sensitivity are coadapted. Second, we describe selection experiments that help reveal potential genetic constraints on the future evolution of thermal sensitivity in Drosophila. Thermal sensitivity responds rapidly both to laboratory natural selection and artificial selection, and tolerance of extreme high temperature appears genetically correlated with performance at intermediate temperature. Third, applying a recent model by Lynch and Lande, we describe how the shape of thermal performance curves may affect evolutionary responses of thermal sensitivity to a gradual shift in the thermal environment. Our theoretical predictions depend crucially on the relationship-between the genetic variation in optimal temperature and the performance breadth. If genetic variation is independent of breadth, then populations with an intermediate value of performance breadth will tolerate the greatest rate of environmental change. Moreover, if a trade-off exists between maximum performance and breadth of performance, then thermal specialists will be favored over thermal generalists in a rapidly changing environment. On the other hand, if genetic variation increases with increasing breadth, then populations of thermal generalists will tolerate the greatest rates of environmental change.