The evolution of thermal physiology in ectotherms

During the last quarter of a century, the evolution of the thermal sensitivity of performance in ectotherms has become a major focus of research programs in evolutionary physiology. Graphical and mathematical models describe how the relationship between body temperature and performance, termed the performance function, should evolve in response to the thermal environment. Interspecific comparisons of the thermal sensitivity of locomotor performance have revealed that the performance function is evolutionarily labile in some taxa but is more conservative in others. A lack of heritable variation or weak selection on performance may explain the conservation of thermal physiology in certain groups, but evolutionary trade-offs do not appear to have been important constraints. Other aspects of thermal physiology, such as the thermal sensitivity of growth rate, have evolved rapidly in ectotherms. Despite the apparent lability of thermal physiology in some taxa, there is limited evidence that thermoregulatory behavior and thermal physiology are coadapted. Future studies should broaden taxonomic and phenotypic foci, while paying close attention to the assumptions of current theories. r 2002 Elsevier Science Ltd. All rights reserved.