Abstract
Rates of molecular evolution are known to vary across species, often deviating from the classical expectation of a strict molecular clock. In many cases, the rate of molecular evolution has been found to correlate to generation time, an effect that could be explained if species with shorter generation times have higher mutation rates per year. We investigate this hypothesis using direct estimates of the mutation rate for 133 eukaryotic species from diverse taxonomic groups. Using a phylogenetic comparative approach, we find a strong negative correlation between mutation rate per year and generation time, consistent across all phylogenetic groups. Our results provide a simple explanation for why generation time plays a pivotal role in driving rates of molecular evolution across eukaryotes.