Faster rates of microevolution have been recorded for plants and marine foraminifera occupying warmer low latitude environments relative to those occurring at higher latitudes. By contrast, because this rate heterogeneity has been attributed to a relationship between thermal habit and mutagenesis via a body temperature linkage, it has been assumed that microevolution in mammals should not also vary systematically with environmental temperature. However, this assumption has not previously been empirically examined. In this study, we tested for a thermally mediated influence on the tempo of microevolution among mammals using a comprehensive global dataset that included 260 mammal species, from 10 orders and 29 families. In contrast to theoretical predictions, we found that substitution rates in the cytochrome b gene have been substantially faster for species living in warmer latitudes and elevations relative to sister species living in cooler habitats. These results could not be attributed to factors otherwise thought to influence rates of microevolution, such as body mass differentials or genetic drift. Instead, the results indicate that the tempo of microevolution among mammals is either responding directly to the thermal environment or indirectly via an ecological mechanism such as the 'Red Queen' effect.
