Distribution of M and of the number of modules s for genotypes of phenotype with V
= 89 in an ensemble and comparison with E. coli. The horizontal axis shows the modularity index M in (a) and the number of modules s in (b). The vertical axis shows the frequency of genotypes with the corresponding value of M (panel a) and s (panel b) in a random sample of 1000 genotypes (n = 831 reactions each, as in the in silico E. coli metabolic model) that are viable in V
= 89 different minimal environments. In both panels, the histogram is displayed along with estimates of M and s for E. coli. From (a), we can reject at a p-value of 0.028 the hypothesis that the modularity index M of E. coli is drawn from this same distribution. Thus, E. coli can be considered as atypically modular.