High correlation of mutational robustnesses in one environment for genotypes that are viable in two environments. The horizontal axis shows the mutational robustness with respect to a first environment and the vertical axis shows the mutational robustness with respect to a second environment. Data are based on 1000 sampled genotypes with n = 831 reactions, equal to the number of reactions in E. coli. Genotypes are viable in a) glucose and acetate, b) glucose and succinate, and c) acetate and succinate environments. In all three cases, we can see a high positive association between robustnesses for each environment.