Evolution and plasticity of thermal performance: An analysis of variation in thermal tolerance and fitness in 22 Drosophila species

The thermal biology of ectotherms is often used to infer species’ responses to changes in temperature. It is often proposed that temperate species are more cold-tolerant, less heat-tolerant, more plastic, have broader thermal performance curves (TPCs), and lower optimal temperatures, when compared to tropical species; however, relatively little empirical work has investigated these expectations. Here we measure thermal tolerance limits and thermal performance of the fitness components viability, developmental speed, and fecundity at seven temperatures in 22 species of Drosophila reared at a common temperature. For 10 of these species, we also measured thermal tolerance and thermal performance following developmental acclimation to three additional temperatures. Using these data, we test several fundamental hypotheses about the evolution and plasticity of heat and cold resistance and thermal performance. We find that cold (and to a lesser degree heat) tolerance varied between the species in a predictable pattern. This confirms the marked effect of evolutionary adaptation and acclimation on thermal tolerance. However, contrary to expectation, find that the breadth of thermal performance in fitness traits were similar in temperate, widespread and tropical species and we also find that the plasticity of TPCs is constrained. These data support the idea that thermal tolerance limits have evolved in response to extreme environmental conditions that limits species persistence whereas the temperature range for optimal thermal performance is likely under selection by the temperatures that prevail during the more benign seasons. Further, it calls to question the utility of TPCs in inferring ecological adaptation. We conclude that the thermal performance of fitness traits in Drosophila species are temporally and spatially stable and that thermal tolerance varies considerably across latitudes.