A series of simulated metabolic networks after (a) 10, (b) 40, (c) 50, and (d) 100 generations. Yellow squares represent enzymes, gray circles represent metabolites. An edge leading from a metabolite to an enzyme indicates that the respective metabolite is an educt in the reaction. An edge from an enzyme to a metabolite marks it as a product. The size of the nodes and the width of the edges encode for the number of minimal pathways in which the respective object is involved. The colors in panel (d) encode the age of the reactions, where red stands for old and blue for newer reactions.