Comparative Rates of Evolution in Large Mammals according to Range Size and Latitude

Abstract

Of all the relationships in ecology and evolution, the positive correlation between higher biodiversity at lower latitudes may have received the most attention. This relationship is almost ubiquitous across the domains of life, since it applies to both terrestrial and aquatic habitats, and has existed for at least several hundred million years. Despite over two centuries of enquiry, debate continues as to what drives this, and other, gradients in biodiversity. Of the many hypotheses that have sprung up surrounding diversity gradients, the Integrated Evolutionary Speed Hypothesis (IESH) proposes that the variables of temperature, water, and population size, and additionally habitat heterogeneity and time, are the drivers of the latitudinal diversity gradient. The IESH predicts (among other things) that diversification rates are dependent on rates of microevolution, and so those variables that drive genetic rates of change may ultimately drive evolution. A relationship between genetic rates of change and latitude has since been shown for various ectothermic organisms, and additionally for small mammals. This study adds to previous works that test the IESH, while also being unique. It introduces range size contrasts as a proxy for population size differences; and contrasts animal pairs from mostly contiguous, as opposed to disjunct, landmasses. Genetic rates of change in the cytochrome b gene were contrasted for 52 pairs of geographically and genetically proximate large mammal species using two variables: latitude (23) and range size (29). I found no relationship between genetic rates of change and latitude, in contrast with several prior studies. It might be that the mechanisms controlling genetic rates of change are different for large mammals, or alternately it may reflect inadequacies in this study e.g. small sample size, or variation between latitudinal positions was too small. However, and notably I found a positive relationship between genetic rates of change and range size for mammals, in alignment with previous work finding faster rates of change for larger populations of continental bird species in contrast to smaller populations of their island congeners. These findings have important implications for conservation ecology as a species' adaptive (survival) potential may be dependent on the size of the refuge provided for it, and/or the size of the founding population for said refuge.