Examples of at least four different modes of GRN evolution have been described. Each panel depicts cells (hexagons) organized into segments or compartments (light versus dark shading). Cells in the ancestral organism (left) that will experience GRN evolution are outlined in bold. (A) GRN evolution occurs “in place” when the ancestral GRN (red) changes to create a GRN with modified activity (blue) and thus a new phenotype in directly homologous cells or tissues. Alternatively, the ancestral state can be maintained while the GRN is co-opted via one or more changes to produce a modified serial homolog of the original structure (B), or a novel structure or phenotype in a non-homologous group of cells (C). Finally, a “repeal, replace, and redeploy” scenario can occur in which the GRN (red) evolves such that the ancestral function is lost from its original location but new activity is gained in a non-homologous group of cells (blue). The original function may be lost altogether or replaced by a different GRN (purple). Adapted from [6].