The imbalance of paleontological trees

One of the most extensively studied aspects of phylogenetic tree shape is b alance, which is the extent to which nodes divide a tree into clades of equ al size. Several authors have stressed the importance of tree balance for u nderstanding patterns of evolution. It has been remarked that paleontologic al studies commonly produce very unbalanced trees (also called pectinate cl adograms or "Hennigian combs"). This claim is tested here by comparing the balance of 50 paleontological trees and 50 neontological trees, all taken f rom the recent literature. Each tree was reanalyzed from the published data matrix to ensure its accuracy. The results confirm that paleontological tr ees tend to be more imbalanced than neontological trees. That paleontological trees are more imbalanced has been represented as a sh ortcoming of fossil data sets, but here it is argued that this is the expec ted result. Even under a simple Markovian model in which all speciations an d extinctions occur randomly and with equal probability in all parts of the tree, trees based on taxa from a single time period (e.g., the present day ) are generally more balanced than trees based on all taxa that ever existe d within the clade. Computer simulation is used to calculate the expected b alance and standard deviation of trees for up to 40 terminal taxa over the entire history of a model clade. The balance is measured using Coliess's in dex. Ic, and the expected balance conforms well with published paleontologi cal trees. The study underlines the difficulty of applying neontological tr ee statistics in paleontology.