HYPOTHESES FOR SELECTIVITY. “A number of nonmutually exclusive explanations have been forwarded to account for these patterns, including differences in the propensity of genes to experience neutral or selectively imposed subfunctionalization or neofunctionalization, constraints imposed by dosage considerations, and the need for stoichiometric balance in multiprotein complexes.”
http://www.amjbot.org/content/early/2015/10/09/ajb.1500320