Figure 1

The number of viable metabolisms V ( n ) decreases as the number of reactions n decreases. (A) The vertical axis (note the logarithmic scale) shows the number of genotypes, and the horizontal axis shows the number n of reactions in a potential metabolism. Black circles represent the number of genotypes in genotype space Ω(n) (regardless of viability), grey circles show the number of potential metabolisms viable on glucose, whereas the blue circles denote the number of potential metabolisms viable on all 10 carbon sources. (B) The vertical axis (note the logarithmic scale) shows the fraction |V(n)| /|Ω(n)|. The grey circles show the fraction of genotypes viable on glucose relative to the number of possible metabolisms, whereas the blue circles denote the fraction of genotypes viable on 10 carbon sources relative to the number of possible metabolisms. Note that viable genotypes become extremely rare as the number of reactions in a metabolism decreases. Data for both figures is based on all viable metabolisms for each n (Additional file 3 and Additional file 8).