Speciation and body size evolution operate independently in the mammalian family tree. The findings, reported in Nature by SFI External Professor Mark Pagel and collaborators from the University of Hull and the University of Reading, could explain dramatic size differences between closely related mammals.

Adaptive radiation assumes that species diverge rapidly early in evolution, accompanied by a high rate of morphological evolution. This is followed by a longer period of slower evolutionary fine-tuning, ensuring that ecological niches are filled. Using novel statistical methods to analyze a comprehensive mammalian phylogeny and dataset of body sizes of over 3,000 extant mammal species, Pagel and colleagues show that this is not the case for mammalian body size evolution.

The authors find that rates of speciation and morphological evolution are decoupled in mammals, and there is no evidence for an early burst of evolutionary rates or a subsequent slowing down; instead, body size diversification operates in sporadic bursts across the phylogenetic tree. This means that the musk ox, for example, is much bigger than its closest relative, the goral. The results highlight natural selection’s role as a precise sculptor of mammalian size diversity, able to produce rapid body size changes in specific parts of the tree.

Read the PlanetEarth Online article (October 26, 2011)

Read the Nature paper (published online October 19, 2011)