Figure 3

Selection threshold for mutations affecting fitness. The number of logic operations evolved or maintained is shown as a function of the beneficial mutational fitness effect used. For beneficial mutations, the end-of-experiment average number of operations was reported; e.g., when logic operations had fitness effects of 0.25, an average of 5.8 operations evolved by positive selection. For deleterious mutations, the number of operations remaining after evolution with alternative fitness effects was used; e.g., when logic operations had beneficial fitness effects of 0.25, an average of 7.65 were maintained by purifying selection. In both cases, the number of operations evolved or maintained was reported relative to the beneficial fitness effect of an operation-creating mutation for simplicity. Deleterious mutations therefore correspond to the reversal of beneficial mutations with the fitness effects indicated on the x-axis. No operations evolved or were maintained for fitness effects of ≤ 0.075. Half of the operations evolved or were maintained at a fitness effect of approximately 0.2.