Adaptations and maladaptations due to evolution at constant, but not at fluctuating temperatures

Climate change exposes populations to more frequent periods of extreme temperatures and faster temperature fluctuations. Theoretical models suggest that different types of adaptations should occur in constant versus fluctuating environments of varying frequency. Furthermore, it has been hypothesized that the number and severity of fitness trade-offs evolving in fluctuating environments depend on population size. To evaluate whether specific types of adaptations evolve at fluctuating temperatures and how population size might affect the evolution of trade-offs, we performed an evolution experiment with fission yeast (Schizosaccharomyces pombe). Small and large populations were evolved for 500 generations at constant and fluctuating temperatures, after which the evolved strains were competed against ancestral strains in their respective selection environments, and in alternative environments to detect fitness trade-offs. We observed significant adaptation and maladaptation only to constant heat, but not to fluctuating temperatures. Moreover, population size did not have significant effect on capacity of adaptation or trade-offs in alternative environments. Our results suggest that constant extreme temperatures may act as stronger selective pressures than temperature variations, and that fitness trade-offs are unlikely to constrain adaptation to fluctuating temperatures. Future experiments in fluctuating temperatures should consider that the number of generations required for populations to adapt may be longer than for constant environments.