Revisiting the impacts of non-random extinction on the tree-of-life

The tree-of-life represents the diversity of living organisms. Species extinction and the concomitant loss of branches from the tree-of-life is therefore a major conservation concern. There is increasing evidence indicating that extinction is phylogenetically non-random, such that if one species is vulnerable to extinction so too are its close relatives. However, the impact of non-random extinctions on the tree-of-life has been a matter of recent debate. Here, we combine simulations with empirical data on extinction risk in mammals. We demonstrate that phylogenetically clustered extinction leads to a disproportionate loss of branches from the tree-of-life, but that the loss of their summed lengths is indistinguishable from random extinction. We argue that under a speciational model of evolution, the number of branches lost might be of equal or greater consequences than the loss of summed branch lengths. We therefore suggest that the impact of non-random extinction on the tree-of-life may have been underestimated.