Molecular Evolution: Rates

Abstract The rate of molecular evolution varies dramatically between taxa, for example some viruses have a rate of genome evolution a million times faster than mammals. Although some rate variation may be due to random fluctuations or locus‐specific effects, studies have revealed strong and predictable patterns in the differences in the rate of molecular evolution between species. In particular, large, long‐lived organisms with low reproductive output tend to have slower rates of molecular evolution than related species with shorter lives, faster generations or higher fecundity. Studies of the variation in the rate of molecular evolution between species may reveal the mechanisms underlying these differences and can inform analyses that seek to derive information on evolutionary history and processes from molecular data. Key Concepts The number of genetic differences between lineages increases with the time since their last common ancestor, but differences do not accrue in the same rate in all lineages. Variation in the rate of molecular evolution can be compared between species by comparing absolute rates, derived from genome sequences of related individuals, or estimated by comparing sequences between different species where the age of the divergence is known. A more common approach is to compare the relative rate differences between species by comparing the number of sequence changes that have accumulated since they last shared a common ancestor or inferring branch lengths along a phylogeny. Mutation rate varies between species, and selection can influence the mutation rate by adjusting the balance between the competing costs of DNA repair and mutation. Many mutations arise from DNA replication errors, so mutation rate can be influenced by the number of times DNA is copied per unit time. Rates of molecular evolution in many taxa scale with body size, possibly because smaller bodied taxa go through more genome replications per unit time, a hypothesis referred to as the generation time effect. Natural selection might play a role in fine‐tuning mutation rates to fit different life history strategies, for example reducing mutation rates in large, long‐lived organisms.