Functional divergence is the process by which genes, after gene duplication, shift in function from an ancestral function. Functional divergence can result in either subfunctionalization, where a paralog specializes one of several ancestral functions, or neofunctionalization, where a totally new functional capability evolves. It is thought that this process of gene duplication and functional divergence is a major originator of molecular novelty and has produced the many large protein families that exist today [1][2].
Many well known protein families are the result of this process, such as the ancient gene duplication event that led to the divergence of hemoglobin and myoglobin, the more recent duplication events that led to the various subunit expansions (alpha and beta) of vertebrate hemoglobins [3], or the expansion of G-protein alpha subunits [4]
Gu X. A simple statistical method for estimating type-II (cluster-specific) functional divergence of protein sequences. Mol Biol Evol. 2006 Oct;23(10):1937-45. Epub 24 July 2006.