Fig. 3.

Selectability of error-minimizing mode of control in the face of an existing mechanism with the opposite mode. The region marked “Activator” (“Repressor”) is a region in which a mutant bearing an activator (repressor) regulatory system can become fixed in a wild-type population carrying the opposite mode of control. In the region marked “Historical precedent,” mutants with optimal regulatory mechanisms do not have sufficient fitness advantage to take over a population with an existing, suboptimal mode of regulation. The x axis is s_min/(Δf₀ + Δf₁), where s_min is the minimal selection advantage needed for fixation, Δf₀ is the reduction in fitness due to errors in the free state of an activator site, and Δf₁ is the reduction in fitness due to errors in the free state of a repressor site. In this plot, the ratio Δf₁/Δf₀ is constant, equal to 1.5.