Strength of selection potentiates distinct adaptive responses in an evolution experiment with outcrossing yeast. Saccharomyces cerevisiae

Combining experimental evolution with whole-genome sequencing is now a well-established method for studying the genetics of adaptation and complex traits. In this type of work that features sexually-reproducing populations, studies consistently find that adaptation is highly polygenic and fueled by standing genetic variation. Less consistency is observed with respect to general evolutionary dynamics however; for example, investigators remain ambivalent about whether selection produces repeatable versus idiosyncratic responses, or whether small shifts in allele frequencies at many loci drive adaptation, versus selective sweeps at fewer loci. Resolving these open questions is a crucial next step as we move toward extrapolating findings from laboratory evolution experiments to populations inhabiting natural environments. We propose that subtle differences in experimental parameters between studies can influence evolutionary dynamics in meaningful ways. We subject populations of outcrossing Saccharomyces cerevisiae to zero, moderate, and high ethanol stress for 200 generations and ask: 1) does stronger selection intensity lead to greater changes in allele frequencies, and a higher likelihood of selective sweeps at sites driving adaptation; and (2) do targets of selection vary with selection intensity? We find some evidence for positive correlations between selection intensity and allele frequency change, but no evidence for more sweep-like patterns at high intensity. While we do find genomic regions that suggest some shared genetic architecture across treatments, we also identify distinct adaptive responses in each selection treatment. Combined with evidence of phenotypic trade-offs between treatments, our findings support the hypothesis that selection intensity might influence evolutionary outcomes by actuating pleiotropic and epistatic interactions.