Evolutionary fate of single gene duplications (a–c), and duplication of multigene families (d–e). Single
gene duplication most often results in a nonfunctional duplicate gene copy (a, nonfunctionalization).
(b) In rare instances, the functional duplicate gene copy and the ancestral gene diverge in function;
neofunctionalization means that one of the two genes retains the original function, while the other
evolves a new, often beneficial function. Subfunctionalization implies that both the original and the
duplicate genes mutate and evolve to fulfill complementary functions already present in the original gene.
Duplication via retrotransposition represents a particular case of sub- or neofunctionalization. Multigene
families evolve in a coordinated fashion, such that the DNA coding sequences and function of the single
members of a family remain close to that of the ancestral gene (d–e). (d ) Concerted evolution: After
multiple rounds of duplication, gene conversion homogenizes the DNA sequences of the individual
members. (e) Birth-and-death evolution invokes a process of equilibrium between inactivating mutations
and ongoing duplication of functional gene copies.