Distribution of phenotypic changes - Optimized population. The effect of mutations on the phenotype is quantified through the change in fitness caused upon replication. Four distributions corresponding to different values of the mutation rate μ are shown. The probability distribution Π(Δ) has been obtained by averaging over 300 generations in populations of size N = 1000. Its symmetry increases as μ grows, corresponding to populations that are more spread in the space of sequences and structures.