Genotype-phenotype map of metabolic networks. Different representations of a hypothetical metabolic network (A), as a node in a genotype network (B), or as a binary vector (C) listing the reactions in the network. Each genotype (circles) on the genotype network in (B) has 1221 neighbors (not all edges are drawn) that differ by a single mutation. Neighbors in (B) are connected by edges. The colors of the genotypes represent different phenotypes. The phenotypes of the metabolic networks are computed using flux balance analysis applied to 124 environments with different sulfur sources. Two hypothetical phenotypes are represented in (D) as binary vectors listing the environments a genotype is viable in (D). Random evolutionary walks can be seen as paths on a genotype network that stay on genotypes with the same phenotype (represented as the genotype color). "Mutations" correspond to additions or deletions of individual reactions from the metabolic network. The number of genotypes in the genotype space is 21221.