Thermodynamic effects on the evolution of performance curves.

Models of thermal adaptation assume that warm-adapted and cold-adapted organisms can achieve the same fitness, yet recent comparative studies suggest that warm-adapted organisms outperform cold-adapted ones. We explored how this thermodynamic effect on performance might influence selective pressures on thermal physiology. In the absence of a thermodynamic effect, natural selection favors a thermal optimum for performance that closely matches the mean (or modal) body temperature. When warm-adapted organisms outperform cold-adapted organisms, natural selection can favor a thermal optimum that exceeds the mean body temperature. The optimal mismatch between the thermal optimum and the mean temperature increases as does the variation in body temperature within generations. This result holds regardless of whether performance affects fitness through fecundity or survivorship. The selective pressures generated by a thermodynamic effect might explain the substantial mismatch between thermoregulatory behavior and thermal physiology that has been observed in some species.