Abstract
Understanding the distribution of fitness effects of new mutations is central to predicting adaptive evolution. Short-term experiments provide a snapshot of this distribution, but observing how it changes as organisms adapt is challenging. Here we use saturated, genome-wide insertion libraries to quantify how the fitness effects of new mutations changed in two E. coli populations that adapted to a constant environment for 15,000 generations. The proportions of neutral and deleterious mutations remained constant, despite fitness gains of ∼50%. In contrast, the beneficial fraction declined rapidly and became exponentially distributed, with genetic interactions profoundly reshuffling the loci subject to beneficial mutations. Despite this volatility, the ancestral distribution predicts many of the alleles that become dominant in the long-term experiment, even after the initial period of rapid adaptation. Overall, our results suggest that short-term adaptation can be idiosyncratic but empirically reproducible, and that long-term dynamics can be described by simple statistical principles.
Competing Interest Statement
The authors have declared no competing interest.