The determination of whole genome sequences allowed the identification of all of the gene families related by primary sequence homology within a specific organism. Figure 2.1 shows a cluster analysis of the proteins encoded by the Arabidopsis genome (Thomas Girke, University of California Riverside, personal communication). For example, of the ~27,000 individual proteins in Arabidopsis, ~80% of proteins are members of homology-related families, whereas only ~20% represent unique sequences. The distribution shows that approximately half of the genes are members of groups consisting of >11 members and that nearly one quarter of proteins belong to groups of >100 members. The larger families include large numbers of protein kinases and cytochrome P450s. This clearly illustrates that new proteins evolved one from another and that divergent evolution is a primary mechanism for achieving novel functionality.
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