Evidence for higher rates of nucleotide substitution in rodents than in man (molecular clock/generation-time effect/synonymous substitution/nonsynonymous substitution/neutral theory)

When the coding regions of 11 genes from ro- dents (mouse or rat) and man are compared with those from another mammalian species (usually bovine), it is found that rodents evolve significantly faster than man. The ratio of the number of nucleotide substitutions in the rodent lineage to that in the human lineage since their divergence is 2.0 for synony- mous substitutions and 1.3 for nonsynonymous substitutions. Rodents also evolve faster in the 5' and 3' untranslated regions of five different mRNAs; the ratios are 2.6 and 3.1, respective- ly. The numbers of nucleotide substitutions between members of the j3-globin gene family that were duplicated before the man-mouse split are also higher in mouse than in man. The difference is, again, greater for synonymous substitutions than for nonsynonymous substitutions. This tendency is more con- sistent with the neutralist view of molecular evolution than with the selectionist view. A simple explanation for the higher rates in rodents is that rodents have shorter generation times and, thus, higher mutation rates. The implication of our find- idgs for the study of molecular phylogeny is discussed.