The duplication-degeneration-complementation (DDC) model. (a) A gene with four regulatory elements (black boxes), each controlling independent functions such as expression domains, is duplicated. Random null mutations in the regulatory elements (open boxes) through degeneration lead to subfunctionalization, where the regulatory elements complement each other to achieve the full ancestral repertoire. (b) Temporal subfunctionalization, illustrated here by the temporal expression patterns of a hypothetical ancestor and two evolved duplicates. The expression level of the duplicates has evolved such that the ancestral expression pattern is maintained in complementary temporal domains via the combined expression of the two duplicates.