(A) Modular gene network where gene X, at the top of a regulatory network, directs expression of gene Y, which in turn directs expression of gene Z. All these genes are expressed in the tip of an appendage (e.g., leg) depicted on the right. (B) The modular gene network is co-opted into a new tissue by the evolution of a novel CRE in gene X. The Y and Z genes, which only receive inputs from X and Y genes, respectively, are also turned on in the new tissue (e.g., eyespot centers in a butterfly larval wing). The CREs of the Y and Z genes now have a dual function in directing gene expression in two separate developmental contexts, e.g., they are pleiotropic. (C) De novo network evolution where elements of the same non-modular gene network, X, Y, and Z, each evolve a separate CRE that drives gene expression in the novel developmental context.