Figure 2

A higher modularity index M and a greater number of modules s are by-products of increasing environmental versatility. The Environmental Versatility Index (V _env , horizontal axis in both (a) and (b)) denotes the number of minimal environments in which a genotype is forced to be viable. The modularity index M (vertical axis in (a)) for a genotype gives the number of reactions contained in the FCSs of that genotype. The number of FCSs (modules) in a metabolic network genotype is denoted by s (vertical axis in (b)). The figure shows that with increasing V _env , both M and the number of modules s in a genotype increase. The data shown here are based on MCMC sampled genotypes with n = 831 reactions (as in the in silico E. coli metabolic model), and 10 different choices for nested sets of environments when requiring viability on more and more environments. Each choice of nested set is displayed with a different color and symbol in (a) and (b). Each of the 10 nested sets, as well as their average (line shown for visual guidance), show a clear rise in the average of M (panel a) and the average of s (panel b) as one increases V _env .