Predictions of gene family distributions in microbial genomes: Evolution by gene duplication and modification

A universal property of microbial genomes is the considerable fraction of g enes that are homologous to other genes within the same genome. The process by which these homologues are generated is not well understood, but sequen ce analysis of 20 microbial genomes unveils a recurrent distribution of gen e family sizes. We show that a simple evolutionary model based on random ge ne duplication and point mutations fully accounts for these distributions a nd permits predictions for the number of gene families in genomes not yet c omplete. Our findings are consistent with the notion that a genome evolves from a set of precursor genes to a mature size by gene duplications and inc reasing modifications.