244W Poster - Population Genetics
Wednesday June 08, 8:30 PM - 9:15 PM

Author:
Daniel Schrider

Affiliation: University of North Carolina, Chapel Hill, NC

Keywords:
Natural selection

The prominence of positive selection, in which beneficial mutations are favored by natural selection and rapidly increase in frequency, is a subject of intense debate. Positive selection can result in selective sweeps, in which the haplotype(s) bearing the adaptive allele “sweep” through the population, thereby removing much of the genetic diversity from the region surrounding the target of selection. Two models of selective sweeps have been proposed: classical sweeps, or “hard sweeps”, in which a single copy of the adaptive allele sweeps to fixation, and “soft sweeps”, in which multiple distinct copies of the adaptive allele leave descendants after the sweep. Soft sweeps can occur because of recurrent mutation to the adaptive allele, or the presence of standing genetic variation consisting of multiple copies of the adaptive allele prior to the onset of selection. Importantly, soft sweeps will occur when populations can rapidly adapt to novel selective pressures, either because of a high mutation rate or because adaptive alleles are already present. The prevalence of soft sweeps is especially controversial, and it has been noted that even when multiple copies of the adaptive allele are present at or near the onset of selection, only one copy may end up surviving the sweep. Thus, selection on standing variation or recurrent mutations may not always result in hard sweeps. Here, we show that the inverse is true: selection on single-origin de novo mutations may often result in soft sweeps. This is made possible by allelic gene conversion, which “softens” hard sweeps by copying the adaptive allele onto multiple genetic backgrounds. We carried out a simulation study examining the impact of gene conversion on sweeps from a single de novo variant in models of human, Drosophila, and Arabidopsis populations. The fraction of simulations in which gene conversion had produced multiple haplotypes with the adaptive allele upon fixation was appreciable. Indeed, under realistic demographic histories and gene conversion rates, soft sweeps are more likely than hard sweeps, even if selection always acts on a single-origin mutation. Thus, even when the mutation rate is low or there is no standing variation, soft sweeps may be common, and hard sweeps are expected to be the exception rather than the rule in species with high rates of gene conversion. These results also imply that the presence of soft sweeps does not necessarily mean that adaptation has been especially rapid.