Fitness landscapes and adaptive evolution for a pair of sites with cooperative binding. Genetic switch (left column), signal integration module (right column). (a,b) Fitness landscape F(r1, r2) without cooperativity (γ = 0). (c,d) Fitness landscape. F(r1, r2) with cooperativity (γ = 1). Next-nearest neighbour states (r1, r2) and of similar fitness are linked by compensatory mutations if the intermediate states (r1, ) and (, r2) have lower fitness. (e,f) Adaptive dynamics: ensemble averages and (thick lines), ensemble width given by (same for r2 and (thin lines); cf. fig. 1(e,f).