Figure 1.

Evolutionary dynamics are bistable in the absence of variation in gene expression (σ_E = 0), but not with variation in gene expression (σ_E = 2.25–3.5). We calculated the average values of ρ, D, and L_del toward the end of the simulations, and then measured the genotype evolvability after changing the optimal trait value (see Methods for details). For each value of N, 20 simulations were initialized at high-ρ conditions, and 15 at low-ρ conditions. For σ_E = 2.25–3.5, simulations from the two initial conditions reached indistinguishable endpoints (Figure A in File S1), so the results were pooled. The increment in N is 10^0.1 between 10^4.4 and 10^5.2 to increase resolution, and is 10^0.2 elsewhere. At σ_E = 0, D is indistinguishable from zero for N ≥ 10^5.2 under high-ρ conditions, and for N ≥ 10^4.7 under low-ρ conditions, corresponding to L_del being effectively zero. In contrast, when σ_E = 2.25 or 3.5, because the weakness of selection on low-expression genes prevents L_del from falling all the way to zero, D never quite reaches zero either, despite appearing superimposable in B. For (A–C), data are shown as mean $\pm \text{SD.}$ For evolvability (D), data are shown as mean $\pm \text{SE.}$ For (A) and (D), these apply to log-transformed values. Evolvability is based on time to fitness recovery; see Figure C in File S1 for similar results based on time to trait recovery. L = 600.