The distribution of beneficial mutant effects under strong selection.

For a general theory of adaptation, it is essential to know the distribution of fitness effects of beneficial mutations. Recent theoretical and empirical studies have made considerable progress in determining the characteristics of this distribution. To date, the experiments have largely verified the theoretical predictions. Despite the fact that the theoretical work has assumed small selection coefficients, strong selection has been observed in some experiments, especially those involving novel environments. Here, we derive the distribution of fitness effects among fixed beneficial mutants without the restriction of low selection coefficients. The fate of strongly favored alleles is less affected by stochastic drift while rare, causing the distribution of fitness effects among fixed beneficial mutations to reflect more closely the distribution among all newly arising beneficial mutations. We also find that when many alleles compete for fixation within an asexual population (clonal interference), the beneficial effects of a newly fixed mutant cannot be well estimated because of the high number of subsequent mutations that arise within the genome, regardless of whether selection is strong or weak.