Are rates of molecular evolution in mammals substantially accelerated in warmer environments? Reply

1. INTRODUCTION In a previous study, Weir & Schluter [1] (hereafter WS2007) argue that smaller average genetic distances between species in cooler latitudes represent faster rates of speciation relative to tropical species. Such an interpretation assumes clades evolve at constant rates across latitudes so that a hypothetical molecular clock ticks at the same rate, regardless of climate. By contrast, our research shows that their assumed relationship between genetic distance and time is confounded by climate [2]. A number of other studies have also found microevolutionary rate variation to be mediated by differences in population size, elevation, latitude, water availability and ocean depth [3–7]. In a later investigation, Weir and Schluter themselves found a greater than fourfold variation in the avian molecular clock that was not readily explained by stochastic variation in substitution rates [8]. While Weir & Schluter [9] (hereafter WS2010) agree that warm latitude species are evolving faster, they claim that the size of this difference is insufficient to confound their earlier results. However, several techniques employed in their reanalysis are unsuitable to the questions being considered and their analyses lack statistical validity. Moreover, their phylogenetic approach is found to be less suitable than our original approach in 95 per cent of cases, as evaluated by the Akaike Information Criterion (AIC).