Comparison of Undirected Phylogenetic Trees Based on Subtrees of Four Evolutionary Units

Abstract

A method to measure how similar are 2 phologenetic trees for the same collection of evolutionary units (EU) is described. The measure does not change with changes in the direction of evolution in either of the 2 trees being compared; it depends only on the convexity and proximity of the groups of EU within the collection under study. It can also be used with only partially resolved phylogenetic trees. The measure is based on the idea that a group of 4 EU is the smallest group for which there is more than 1 possible distinct undirected phylogenetic tree. For a given phylogenetic tree, the undirected subtree inherited by a group of 4 EU is the tree that results when all the branches containing only EU not in this group of 4 are removed. Every group of 4 EU either inherits 1 of 3 distinct types of undirected phylogenetic trees or is unresolved. Two phylogenetic trees can be compared on the basis of which groups of 4 EU inherit the same type of undirected phylogenetic tree. The ideas can be extended to comparisons of 3 or more trees.