Haploids adapt faster than diploids across a range of environments

Despite a great deal of theoretical attention, we have limited empirical data about how ploidy influences the rate of adaptation. We evolved isogenic haploid
and diploid populations of Saccharomyces cerevisiae for 200 generations in seven different environments. We measured the competitive fitness of all ancestral and
evolved lines against a common competitor and find that in all seven environments haploid lines adapted faster than diploids, significantly so in three environments. We
apply theory that relates the rates of adaptation and measured effective population sizes to the properties of beneficial mutations. We obtained rough estimates of the
average selection coefficients in haploids between 2-10% for these first selected mutations. Results were consistent with semi-dominant to dominant mutations in four
environments and recessive to additive mutations in two other environments. These results are consistent with theory that predicts haploids should evolve faster than
diploids at large population sizes.