in a population remains controversial.
Classical duplication-degeneration-complementation/
subfunctionalization models do
not invoke positive selection, but stipulate
a higher retention rate of duplicate genes in
small rather than larger populations. Considerably
more retentions and fewer losses of
duplicate genes in rodents as compared with
humans indicate that positive selection may
play a more important role than originally
anticipated (91). If two redundant gene copies
were retained in the genome without significant
functional divergence, the organism may
acquire increased genetic robustness against
harmful mutations (Figure 1c). In multigene
families descended from a common ancestor,
individual genes in the group exert similar
functions and have similar DNA sequences
(67, 68). One concept, concerted evolution,
applies particularly to localized and typically
tandem copies of a gene. The concept
posits that all genes in a given group evolve
coordinately, and that homogenization is
the result of gene conversion (Figure 1d ).
For most multigene families, the currently
favored model is birth-and-death evolution