Limits to adaptation in partially selfing species
Single locus theory indicates that selfing species are more able than outcrossing ones to fix emerging recessive beneficial mutations, as they are not masked as heterozygotes. However, partially selfing organisms suffer from relaxed recombination, which reduces overall selection efficiency. Although the effect of linked deleterious alleles on adaptation has previously been studied, the extent to which multiple adaptations interfere in partially selfing organisms is currently unknown. We derive branching-process models to quantify the extent that emergence of a second beneficial allele is obstructed by an existing selective sweep. We consider both the potential loss of the second beneficial mutation if it has a weaker advantage than the first sweep (the `stochastic interference effect’), and also the potential replacement of the first sweep if the second mutant is fitter (`replacement effect’). Overall, the stochastic interference effect has a larger impact on preventing fixation of both adaptive alleles in highly selfing organisms, but the replacement effect can be stronger with multiple mutations. Interference has two opposing effects on Haldane’s Sieve. First, recessive mutants are disproportionally likely to be lost, so it is more likely that only dominant mutations will emerge in outcrossers. Second, with frequent rates of adaptive evolution, outcrossing organisms are more able to fix weak beneficial mutations of any dominance value, contrary to the predictions of Haldane’s Sieve. Our analysis shows that even under low rates of adaptive mutation, interference can be sufficiently strong to greatly limit adaptation in selfing organisms.