A new method for evaluating the shape of large phylogenies

The symmetry of branching of evolutionary trees is considered to be informative of the evolutionary process. Most recent methods for measuring this symmetry (“imbalance”) measure only one aspect of tree shape. The method we present here provides an almost complete description of tree shape and allows calculation of imbalance parameters for very large phylogenies. The method can detect different patterns of radiation, both among nodes within a tree and among trees. Preliminary tests of the method suggest that bird and angiosperm cladograms have similar tree balance if only the resolved topology of the tree is considered, but very different balance if the dimensions of terminal taxa, measured as number of species, are also included.