The double-edged effect of environmental fluctuations on evolutionary rescue

Recent studies revealed that contemporary evolution can be rapid enough to prevent population extinction in deteriorating environments through "evolutionary rescue". Researchers have investigated how evolutionary rescue is affected by various factors such as initial population sizes, the amount of genetic variation, and the speed of environmental changes, but few studies focused on environmental fluctuations. As the ongoing global changes are influencing the mean and variance of many environmental variables, it is crucial to explore how environmental fluctuations affect evolutionary rescue for understanding eco-evolutionary dynamics in the wild. Here we show that increasing the amplitude of environmental fluctuations around long-term deteriorating trends has negative and positive effects on evolutionary rescue by analyzing a mathematical model and conducting laboratory experiments on the green alga Chlorella vulgaris under increasing salinity. Increasing the amplitude of environmental fluctuations produces an episode of a huge environmental change, thereby increasing the adaptation lag between the optimal trait value and the population trait mean, which eventually causes population extinction in model simulations. On the other hand, large environmental fluctuations can increase trait variance within a population, potentially promoting adaptive evolution. Indeed, we observed that algal strains that experienced large environmental fluctuations could grow in a harsh environment whereas strains that experienced smaller or no environmental fluctuations could not. These results suggest that we will be able to promote or prohibit evolutionary rescue in nature by carefully considering the double-edged effect of environmental fluctuations on evolutionary rescue.