TABLE 4

The effects of background selection on weakly selected mutations with and without back mutation

Without back mutationUs = 0
Ubackssk/k0 (2 SE)f0bD (2 SE)Pow 1 (%)k/k0 (2 SE)D (2 SE)Pow 1 (%)
00.0010.127 (0.032)0.115–0.76 (0.24)220.130 (0.029)–0.46 (0.23)8
00.00050.102 (0.025)0.115–0.61 (0.26)21
With back mutationUs = 0
Ubackssk/k0 (2 SE)f0bD (2 SE)Pow 1 (%)k/k0 (2 SE)D (2 SE)Pow 1 (%)
0.0010.00050.115 (0.027)0.115–0.46 (0.29)140.130 (0.029)–0.46 (0.23)8
0.010.00050.146 (0.029)0.115–0.39 (0.22)8

Uback is the mutation rate to back mutations, with respect to the small-effect mutational type. Power is based on forward simulations with θ = 6. Other symbols are as in Table 1. Parameter values are N = 3000, Ub = 0.09, sb = 0.04, Us = 0.01. The simulation results due simply to the presence of mutations with effect sb at rate Ub (i.e., without weak mutations) are presented in the last column. The value of Ub was chosen to make the effect of background selection sufficiently strong that the weak mutations become effectively neutral in the presence of the strong mutations.