Dispensable genes

Viruses and bacteria with comparatively few gene loci have spontaneous mutatiion rates of the order of 10 −3 and 10 −4 per base pair per year, respectively. By contrast, the number of gene loci in vertebrates approaches 10 5 and the spontaneous mutation rate is of the order of 10 −9 per base pair per year. This inverse relationship between the spontaneous mutation rates (largely determined by the accuracy of the species' DNA polymerase and DNA repair mechanisms) and the total number of gene loci has an ironic consequence. Species with DNA polymerases which are too accurate and editing mechanisms that are too good will not easily discard dispensable genes from their genomes. The average half-life of fish enzyme loci that have become redundant (therefore, dispensable) is roughly 50 million years. Accordingly, one would expect a substantial fraction of the still-functioning mammalian genes at any given moment in evolution to represent dispensable kinds.