The effects of competition on the strength and softness of selection

Deleterious alleles are constantly introduced into populations due to mutation. In subdivided populations, the impact of these mutations depends on the strength of selection as well as the softness of selection, that is, the extent to which fitness is governed by local rather than global competition. It is widely appreciated that the intensity and type of competition will affect selection on deleterious mutations but most empirical work has focused solely on the effects of competition on selection strength. However, competition has rarely been studied in the context of selection softness even though competition is at the conceptual root of soft selection. All other things being equal, theory predicts that inter- and intraspecific competitions have opposing effects on the softness of selection. Using Drosophila melanogaster, we estimated the strength and softness of selection in a baseline competitive environment as well as two additional competitive environments characterized by either additional intra- or interspecific competitors. We found that competitive environment had little effect on the average strength of selection. While the softness of selection was affected by the type of competition, the direction of change varied across tests of different genes, contrary to expectation. Although the hard/soft selection paradigm implicitly assumes that all individuals are equally sensitive to the local competitive environment, we found this not to be the case. Wild-type individuals were more sensitive to changes in the genetic quality of their local competitors than mutant individuals.