Although many studies testing the beneficial acclimation hypothesis have rejected it, what these rejections imply about the adaptive value of physiological change remains unclear. Uncertainty arises because the hypothesis focuses on the relative performance of organisms exposed to one environment versus another, whereas the raw material available to evolution is variation in acclimation responses of individual traits. This mismatch is problematic when organisms are exposed to poor environments. In poor environments, the adaptive or maladaptive value of changes in individual traits may be obscured by long-term decrements in organismal condition. A better match between the evolutionary pressures shaping acclimation and the tests used to examine them can be achieved by focusing on the fitness consequences of acclimation changes in individual traits.