Fig. 6. Mutational origins of the Eurasian hard sweeps.

The probability that a sweep was caused by an SGV relative to a de novo mutation conditional on a sweep of either type occurring within a fixed time interval (left panel), following equations in ref. ⁵⁷ (and assuming standard human generation times⁵⁰ and mutation rates¹⁰²). SGVs are assumed to have been under some degree of purifying selection (denoted by different coloured symbols) before the environmental shift that leads to a strong bottleneck (that is, a tenfold reduction in population size; see Supplementary Fig. 21 for results from models with less severe bottlenecks) and initiates the beneficial selection phase (symbol shapes). Fixation from SGVs was highly likely (>75%) for constrained time intervals (≤20 kyrs) when the beneficial selection strength (Sel. ben.) was moderate (s ~0.01), both being plausible values for Eurasian sweeps, provided that purifying selection (Sel. del.) before the environmental shift was weak (s ≤ 0.001). Factoring in the probability that fixed SGVs all descend from a single copy at the time of environmental shift results in consistently high probabilities (≥50%) that selection resulted in a hard sweep pattern regardless of the mutational source, provided that SGVs had previously been deleterious (right panel). Notably, the fixation of variants that had previously been strongly deleterious required large mutational target (Mut. target) sizes (>1000 possible mutations) to ensure fixation within a 40,000-year interval when the beneficial selection strength was ~0.01, which may be implausibly large for many traits.