Phylogeny imbalance: taxonomic level matters.

Two lines of evidence indicate that the degree of symmetry in phylogenetic topologies differs at different hierarchical levels. First, in a set of 61 phylogenies with superspecific taxa as their terminals, trees were on average more unbalanced (asymmetric) when the species richness of terminals was considered than when it was not. Second, nodes with a given number of higher taxa descended from them were on average more unbalanced than were nodes with the same number of species as descendants. There are several possible reasons--some biological, some artifactual--for the differences. Whatever the reason, these results caution against treating species-level and higher level phylogenies as equivalent when considering tree shape. The imbalance measure adopted here permits the use of trees that contain polytomies, facilitating a larger sample than has been achieved previously.