Differences in rate of cytochrome-b evolution among species of rodents

Although molecular evolution often appears to proceed in a clocklike fashio n, examples to the contrary are increasing in number. Our study compares ra te of cytochrome-b evolution in 21 rodent species, each of which belongs to a different genus. In these comparisons, substitutions at synonymous sites appear to be saturated, precluding inferences about rate of synonymous sub stitution. Rate of nonsynonymous substitution differs significantly among m any of the rodents studied. However, the cause or causes of these differenc es in substitution rate remains in question. Differences in generation time , body size, or metabolic rate do not seem to be associated with rate of no nsynonymous substitution in these rodents. Effective population size remain s a viable explanation of the observed rate heterogeneity. However, we sugg est that a search for simple causes of differences in rate of molecular evo lution may be difficult in light of numerous aspects of an organism's biolo gy that may together influence evolutionary rates over space and time.