Ecological determinants of mutation load and inbreeding depression in subdivided populations.

Population structure can dramatically alter mutation load, but the magnitude and direction of this effect depend on whether selection is soft or hard. The abstract nature of previous load models obscured the importance of ecological details in determining the softness of selection. Under the more ecologically explicit model used here, I show that genes related to intrinsic growth rate tend to experience hard selection whereas those genes affecting competitive ability experience softer selection. Although resource limitation determines the strength of selection on competitive ability, it only affects the softness of selection, and thus the load, of mutations with pleiotropic effects on intrinsic growth rate. While competition is typically considered necessary for soft selection, the model shows that interspecific competition hardens selection and can thereby lead to reduced load. Finally, genetic variation in the consumption rates of individuals who die before reproduction can cause supersoft selection, resulting in large increases in load. The same conditions that increase mutation load also increase inbreeding depression.