Figure 2.

The ancestral fitness distribution as a function of position. In both left and right, μ = 10⁻⁸, N = 10⁴, and r = 4 × 10⁻⁸. The focal site is located at the center of a genome of length L = 10⁶. Our theoretical results are shown as solid lines, while the simulations are represented with circles. On the left, s = 10⁻², such that ${\mathit{Nse}}^{-U_{\text{d}}/\left(s+R/2\right)}=71.7$. On the right, s = 10⁻³, such that ${\mathit{Nse}}^{-U_{\text{d}}/\left(s+R/2\right)}=6.2$. We see that as N_es becomes smaller, the deterministic approximation begins to break down, and fluctuations in the population occasionally allow lineages carrying deleterious mutations to become a large fraction of the population. This, in turn, leads to less-fit ancestral lineages than predicted by our theoretical results.