RATES OF SPONTANEOUS MUTATION

Rates of spontaneous mutation per genome as measured in the laboratory are remarkably similar within broad groups of organisms but differ st rikingly among groups, Mutation rates in RNA viruses, whose genomes co ntain ca. 10(4) bases, are roughly 1 per genome per replication for ly tic viruses and roughly 0.1 per genome per replication for retroviruse s and a retrotransposon. Mutation rates in microbes with DNA-based chr omosomes are close to 1/300 per genome per replication; in this group, therefore, rates per base pair vary inversely and hugely as genome si zes vary from 6 x 10(3) to 4 x 10(7) bases or base pairs. Mutation rat es in higher eukaryotes are roughly 0.1-100 per genome per sexual gene ration but are currently indistinguishable from 1/300 per cell divisio n per effective genome (which excludes the fraction of the genome in w hich most mutations are neutral). It is now possible to specify some o f the evolutionary forces that shape these diverse mutation rates.