Figure 2.—

Idiosyncratic fitness effects of individual fixed beneficial mutations in different genetic backgrounds. This figure shows the mean (±SEM; n ≥ 4) fitness effect of beneficial mutations across genetic backgrounds. (A) Mutations that were fixed in both A455T (high initial fitness) and A455G (low initial fitness). (B) Mutations that were fixed in both C1550T (intermediate initial fitness) and A455G (low initial fitness). (C) The fitness effects of mutations that were fixed in the A455T and C1550T backgrounds. The dashed line is a plot of y = x; mutations that fall along this line have additive effects across backgrounds, mutations that fall above the line show diminishing returns, and mutations that fall below this line show escalating returns.