An example can be seen in bacteria: Escherichia coli has seven operons encoding various Ribosomal RNA. For each of these genes, rDNA sequences are essentially identical among all of the seven operons (sequence divergence of only 0.195%). In a closely related species, Haemophilus influenzae its six ribosomal RNA operons are entirely identical. When the 2 species are compared together however, the sequence divergence of the 16S rRNA gene between them is 5.90%.
1. Amplification of a gene, usually assisted by recombination events in IS elements (in bacteria) or in similar high copy number repetitive genetic elements (ERV, LINE, SINE, etc.). This increases the copy number of the gene. In sexual reproducing organisms unequal crossing over during meiosis may be responsible for amplification due to misalignment of repeated sequences.
2. Redistribution of genes requires transposition, probably assisted by the same repetitive genetic elements as in 1).
3. Homogenization of alleles by gene conversion may play a role in sexual reproducing organisms.
Some genes have a bias and are more prone to gene conversion than others, thus reinforcing the unity of the genes within a gene family of a species.