Figure 2. The effect of demographic history on the accumulation of deleterious mutations.

To study the expected value of R_{WestAfrica/Europe} stratified by selection coefficient, we simulated a previously published model of the joint history of West Africans and Europeans⁶, for a range of selection coefficients, assuming both additive (h=0.5) and recessive (h=0) models of selection. For the additive case, R_{WestAfrica/Europe} dips below a confidently detectable ratio of 0.95 (given the standard errors of our empirical measurements) for s ∈ (−0.0004, −0.004). Real distributions of selection coefficients may include a large fraction of their density outside this range, and thus a true signal may be difficult to detect. We also simulated a published model of the history of Denisovans and Neanderthals¹⁵. The simulations predict similar curves for R′_{WestAfrica/Denisova} and R′_{WestAfrica/Neanderthal} reflecting their similar inferred demographic histories (we use a normalized R’ statistic to correct for the effects of branch shortening in these ancient genomes). The simulations show that R′_{WestAfrica/Denisova} is expected to be below a detectable ratio of 0.95 for s ∈ (−0.00002, −0.03) and that R′_{WestAfrica/Neanderthal} is expected to be below 0.95 for s ∈ (−0.00002, −0.09). For recessively acting alleles, the directionality of the effects are opposite.