English: Fates of mutations in a heterogeneous population. This schematic shows the fates (“fix”, fixation; “ext” extinction) of mutations with different selective effects, as indicated in each panel, depending on the fitness of the genotype they occur in. The light gray area shows the distribution of fitnesses in the population. Small gray arrows show how natural selection changes the distribution of genotypes: fitter than average genotypes increase in frequency, less fit than average genotypes decrease in frequency. Colored arrows show the effect size of new mutations: neutral mutations (effect size ) are shown in blue (panel a.), adaptive mutations (effect size ) are shown in orange (panels b. weakly beneficial, panel c. strongly beneficial, panel d. intermediate beneficial effect). Mutations that are destined for fixation (extinction) are shown with solid (dashed) arrows. Mutations with a large beneficial effect can fix if they arise in any background (panel c), whereas mutations with a small beneficial effect can fix only if they arise in a very fit background (panels a., b., and d.). The resulting effective population size for mutations with a given effect size is shown as red shading.