How Extreme Temperatures Impact Organisms and the Evolution of their Thermal Tolerance

Synopsis Understanding the biological impacts of extreme temperatures requires translating meteorological estimates into organismal responses, but that translation is complex. In general, the physiological stress induced by a given thermal extreme should increase with the extreme's magnitude and duration, though acclimation may buffer that stress. However, organisms can differ strikingly in their exposure to and tolerance of a given extreme temperatures. Moreover, their sensitivity to extremes can vary during ontogeny, across seasons, and among species; and that sensitivity and its variation should be subject to selection. We use a simple quantitative genetic model and demonstrate that thermal extremes-even when at low frequency-can substantially influence the evolution of thermal sensitivity, particularly when the extremes cause mortality or persistent physiological injury, or when organisms are unable to use behavior to buffer exposure to extremes. Thermal extremes can drive organisms in temperate and tropical sites to have similar thermal tolerances despite major differences in mean temperatures. Indeed, the model correctly predicts that Australian Drosophila should have shallower latitudinal gradients in thermal tolerance than would be expected based only on gradients in mean conditions. Predicting responses to climate change requires understanding not only how past selection to tolerate thermal extremes has helped establish existing geographic gradients in thermal tolerances, but also how increasing the incidence of thermal extremes will alter geographic gradients in the future.