Nodes in phylogenetic trees: The relation between imbalance and number of descendent species. Systematic Biology

Abstract. — The imbalance of a node in a phylogenetic tree can be defined in terms of the relative numbers of species (or higher taxa) on the branches that originate at the node. Empirically, imbalance also turns out to depend on the absolute total number of species on the branches: in a sample of large trees, nodes with more descendent species tend to be more unbalanced. Subsidiary analyses suggest that this pattern is not a result of errors in tree estimation. Instead, the increase in imbalance with species is consistent with a cumulative effect of differences in diversification rates between branches. [Equal-rates Markov model; imbalance; phylogeny shape; proportional-to-distinguishable-arrangements model.] Since the pioneering work of Savage (1983), a large body of research has been devoted to the question of what inferences about evolution can be drawn from the shape of phylogenetic trees. The property of trees most thoroughly studied is their degree of imbalance, that is, the extent to which some branches lead to many species (or higher taxa) while others lead only to a few. The ob-served degree of imbalance is typically compared to the imbalance predicted by a null model called the simpl