Specialists and Generalists in Changing Environments. I. Fitness Landscapes of Thermal Sensitivity

Abstract

Animals and plants often exhibit narrow ranges of thermal preference in variable environments; fitness-enhancing activities such as reproduction and growth tend to be concentrated during times in which body temperature lies within that narrow range. The relation between fitness-associated performance and temperature is modeled by a performance curve defined by two traits: performance breadth (Tbr), a measure of thermal specialization, and the critical maximum temperature for performance (Tmax). Optimality models are used to define the fitness landscape for these two traits under several different patterns of within- and among-generation variation in temperature. In constant environments and environments in which there is significant within-generation variation, specialists with narrow preference ranges are the favored phenotype. In environments in which there is considerable among-generation but little within-generation variation, generalists with broad preference ranges are favored. Specialists in a constant environment have a mean fitness an order of magnitude higher than any phenotype in more variable environments, demonstrating that homeostatic mechanisms can confer a large fitness advantage. In contrast to previous models of environmental tolerance, these performance models suggest that increasing temporal environmental variation can favor the evolution of thermal specialization.