Genetic Draft and Quasi-Neutrality in Large Facultatively Sexual Populations

Large populations may contain numerous simultaneously segregating polymorphisms subject to natural selection. Since selection acts on individuals whose fitness depends on many loci, different loci affect each other's dynamics. This leads to stochastic fluctuations of allele frequencies above and beyond genetic drift-an effect known as genetic draft. Since recombination disrupts associations between alleles, draft is strong when recombination is rare. Here, we study a facultatively outcrossing population in a regime where the frequency of outcrossing and recombination, r, is small compared to the characteristic scale of fitness differences sigma. In this regime, fit genotypes expand clonally, leading to large fluctuations in the number of recombinant offspring genotypes. The power law tail in the distribution of the latter makes it impossible to capture the dynamics of draft by an effective neutral model. Instead, we find that the fixation time of a neutral allele increases only slowly with the population size but depends sensitively on the ratio r/sigma. The efficacy of selection is reduced dramatically and alleles behave quasi-neutrally even for Ns >> 1, provided that vertical bar s vertical bar < s(c), where s(c) depends strongly on r/sigma, but only weakly on population size N. In addition, the anomalous fluctuations due to draft change the spectrum of (quasi)-neutral alleles from f(nu) similar to nu(-1), corresponding to drift, to similar to nu(-2). Finally, draft accelerates the rate of two-step adaptations through deleterious intermediates.