Signatures of random and selective mass extinctions in phylogenetic tree balance.

Current models of diversification with evolving speciation rates have trouble mimicking the extreme imbalance seen in estimated phylogenies. However, these models have not incorporated extinction. Here, we report on a simple simulation model that includes heritable and evolving speciation rates coupled with mass extinctions, Random (but not selective) mass extinctions, coupled with evolving among-lineage variation in speciation rates, increase imbalance of postrecovery clades. Thus, random mass extinctions are plausible contributors to the imbalance of modern clades. Paleontological evidence suggests that mass extinctions are often random with respect to ecological and morphological traits, consistent with our simulations. In contrast, evidence that the current anthropogenic mass extinction is phylogenetically selective suggests that the current extinction episode may be qualitatively different from past ones in the way it reshapes future biotas.