Although some of the rst genes on Earth were the products of rare interactions of groups of nucleotides, mutation, and selection, these processes required a great deal of time. Recombination accelerated the generation of new genes by joining existing coding regions together, but the evolution of completely new genes for each new function still was a slow process. However, once a few successful genes and gene combinations had evolved, faster processes could occur. One of the most successful was gene duplication followed by mutation and divergence. A single gene can serve as starting material for many other genes that eventually act to diversify the functions in the organism. This process probably is responsible for many (or most) of the gene diversity present today. The presence of similar regions in very different genes and proteins is one piece of evidence for this. Multigene families provide additional direct evidence that this process has occurred often and continues to occur. Many genes within genomes, and especially eukaryotic genomes, are members of multigene families. These consist of two or more genes that were duplications of a progenitor gene. The duplicated genes then mutated such that they had different characteristics. This was advantageous in most cases, since entirely new genes did not have to evolve, but copies of existing genes could mutate and selection then acted to retain the advantageous versions. Many multigene families have been described (Table 11.1), including ribosomal RNA genes (already discussed), immunoglobulin genes, histones, and others.