Three fates of functional evolution based on tissue-specific gene expression patterns, where circles denote genes, squares denote tissues, and edges denote that genes are expressed in tissues. Immediately after gene duplication, both duplicates retain the same set of ancestral functions denoted by the same set of edges. Under pure SF a complementary set of ancestral edges are retained by each duplicate. Under pure CF both duplicates retain the same set of ancestral edges. Under pure NF one duplicate retains the ancestral edges while the other duplicate develops new edges with other tissues.