Figure 4.

Mutation bias tends to increase ρ, such that even the global solution breaks down in sufficiently small populations. P_+ρ is the probability that a mutation increases ρ, and P_−ρ is the probability of a decrease. Each data point, (except those taken from Figure 1 with P_+ρ:P_−ρ = 1:1 and N = 10^3.6–10^6.0), is pooled from five replicates of high-ρ initial conditions and five replicates of low-ρ initial conditions. Because we assume multiplicative mutational effects to ρ, its value converges even for extremely small N. i.e., as ρ increases, the additive effect size Δρ of a typical mutation also increases, preventing it from passing through the drift barrier. For (A–C), data are shown as mean $\pm$SD. For (D), data are shown as mean $\pm$SE. For (A) and (D), these apply to log-transformed values. L = 600 and σ_E = 2.25.