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Large populations may contain numerous simultaneously segregatingpolymorphisms subject to natural selection. Since selection acts on individualswhose fitness depends on many loci, different loci affect each other'sdynamics. This leads to stochastic fluctuations of allele frequencies above andbeyond genetic drift - an effect known as genetic draft. Since recombinationdisrupts associations between alleles, draft is strong when recombination israre. Here, we study a facultatively outcrossing population in a regime wherethe frequency of out-crossing and recombination, r, is small compared to thecharacteristic scale of fitness differences \sigma. In this regime, fitgenotypes expand clonally, leading to large fluctuations in the number ofrecombinant offspring genotypes. The power law tail in the distribution of thelatter makes it impossible to capture the dynamics of draft by an effectiveneutral model. Instead, we find that the fixation time of a neutral alleleincreases only slowly with the population size but depends sensitively on theratio r/\sigma. The efficacy of selection is reduced dramatically and allelesbehave "quasi-neutrally" even for Ns>> 1, provided that |s|< s_c, where s_cdepends strongly on r/\sigma, but only weakly on population size N. Inaddition, the anomalous fluctuations due to draft change the spectrum of(quasi)-neutral alleles from f(\nu)\sim 1/\nu, corresponding to drift, to\sim1/\nu^2. Finally, draft accelerates the rate of two step adaptationsthrough deleterious intermediates.

Genetic Draft and Quasi-Neutrality in Large Facultatively Sexual Populations