Evolution of Evolvability in Rapidly Evolving Populations

Mutations can affect the short-term fitness of an organism, as well as the rates and benefits of future mutations. While numerous examples of these evolvability modifiers have been observed in rapidly adapting microbial populations, existing theory struggles to predict when they will be favored by natural selection. Here, we develop a mathematical framework for predicting the fates of mutations that modify the rates and fitness benefits of future mutations. We derive analytical expressions showing how the fixation probabilities of these mutations depend on the size of the population and the diversity of competing mutations. We find that competition between linked mutations can dramatically enhance selection for modifiers that increase the benefits of future mutations, even when they impose a strong direct cost on fitness. Conversely, we find that relatively modest direct benefits can be sufficient to drive evolutionary dead-ends to fixation. Our results suggest that subtle differences in evolvability could play an important role in shaping the long-term success of genetic variants in rapidly evolving microbial populations.