Seasonal evolution and its effects on competition

Abstract Rapid evolution can influence competitive dynamics. While theory and past work have focused on the competitive consequences of rapid evolution in response to competitors (e.g., character displacement), abiotic environmental factors are also responsible for much of the rapid evolution observed in nature. Yet, we lack knowledge of how evolution in response to a variable abiotic environment shapes competition between evolving species. Our knowledge of these effects is limited not only by the absence of direct quantification of environment‐driven evolution's effects on competition, but also by the rarity of systems in which environment‐driven evolution itself has been measured across multiple competitors. Here, we used an outdoor mesocosm experiment with three species of naturally co‐occurring drosophilid fly competitors to quantify seasonal evolution of stress tolerance and reproductive traits and the effects of this evolution on demographic performance under competition. We found that rapid seasonal evolution occurred across all three species, across all measured traits, and across every sampling interval. Throughout the course of their growing season, species first converged on similar phenotypic profiles (e.g., high heat tolerance and low fecundity), then evolved in parallel. This seasonal evolution had clear impacts on pairwise competition between species. In competition trials in a common garden greenhouse environment, seasonal evolution had the potential to alter competitive fitness, measured as species' demographic performance during one full generation of competition, by over 30%. Under seasonally varying field conditions over the summer, in one species pair, seasonal evolution combined with changing environmental conditions had 7x stronger effects on competitive fitness than did the changing environmental conditions alone. Our work highlights that the rapid, environment‐driven evolution increasingly documented by evolutionary biologists can have an underappreciated influence on ecological competitive dynamics.