Concerted evolution is the tendency of the different genes in a gene family or cluster to evolve in concert. This means that each gene locus in the family comes to have the same genetic variant.
The a globin genes of primates illustrates the principle:
All primates have two alpha globins; we can therefore assume that the common ancestor of primates had two alpha globin genes. The sequence of each alpha globin gene differs between primate species; in the great apes any two species differ by about 2.5 amino acid substitutions in each gene. If one gene accumulates about 2.5 amino acid changes in the time between two species, then two different genes (alpha1 and alpha2) which have been separated for maybe 300 million years should have accumulated many more changes if they have been evolving independently. The conclusion is that they have not evolved independently; they have evolved in concert.
Figure: a phylogeny of the human globin genes. The genes multiplied by gene duplications, and the dates on the figure are the times of the duplications as inferred from the molecular clock. In fact, as the text explains, the inferred dates are untrustworthy. From Jeffreys et al. (1983).
Why does concerted evolution take place?