The elementary processes of protein network evolution. The progression of time is symbolized by arrows. (a) Link attachment and (b) link detachment occur through nucleotide substitutions in the gene encoding an existing protein. These processes affect the connectivities of the protein whose coding sequence undergoes mutation (shown in black) and of one of its binding partners (shown in gray). Empirical data shows that attachment occurs preferentially towards partners of high connectivity, cf. fig. 3. (c) Gene duplication usually produces a pair of nodes (shown in black) with initially identical binding partners (shown in gray). Empirical data suggests duplications occur at a much lower rate than link dynamics and that redundant links are lost subsequently (often in an asymmetric fashion), which affects the connectivities of the duplicate pair and of all its binding partners [22, 25, 38].