In his 1989 book, Wonderful Life, Stephen Jay Gould proposed the following thought experiment: Rewind the tape of life and let evolution play out a second time. Does the replay produce anything like what we see today? In other words, is evolution reproducible, or do chance events (perhaps inconsequential at the time) cause evolutionary paths to diverge? Using experimental evolution, we can perform Gould's thought experiment in the laboratory by evolving hundreds of replicate populations.
We evolved ~600 replicate populations of the budding yeast, Saccharomyces cerevisiae, for 1,000 generations in rich glucose medium. We used whole-genome whole-population sequencing to examine the dynamics of genome sequence. Combining experimental evolution and quantitative genetics, we quantify the fitness effects of all mutations in 11 lineages and we identify genetic interactions.
Our results show that patterns of genome sequence evolution are driven by a balance between chance effects, which increase stochastic variation in evolutionary outcomes, and the deterministic action of selection on individual mutations, which favors parallel evolutionary solutions in replicate populations.