Rapid evolution can select for fitness tradeoffs in fluctuating environments

In fluctuating environments, the fates of new mutations depend on the fitness tradeoffs they experience across multiple environmental conditions. Selection on these tradeoffs is well-understood when mutations compete in isolation, but much less is known in the empirically relevant case where multiple mutations compete at the same time. Here, we develop a theory to predict how rapidly adapting populations select on fitness tradeoffs at many linked genetic loci. We derive analytical expressions showing how the fixation probabilities of these mutations depend on their underlying fitness tradeoffs, the timescales of environmental variation, and the future mutations they produce over time. We find that in large populations, competition between linked mutations can strongly favor mutations that carry larger fitness tradeoffs, even when they have a lower geometric mean fitness. We show that this "specialist advantage" arises because transient benefits enhance the rate of producing future mutations -- an effect that continues to compound over time even when local conditions shift. We also demonstrate that successful lineages acquire well-timed mutations that appear to anticipate future environments, which can boost the rate of adaptation. These results show how large populations balance competing demands in temporally fluctuating environments, and provide a baseline for interpreting fitness tradeoffs in many natural and experimental settings.