PATTERNS IN TREE BALANCE AMONG CLADISTIC, PHENETIC, AND RANDOMLY GENERATED PHYLOGENETIC TREES

Area of Science:

• Evolutionary biology
• Phylogenetics
• Macroevolution

Background:

• Phylogenetic trees visually represent evolutionary relationships among species.
• Tree balance, a measure of branching regularity, has been suggested to differ between cladistic and phenetic methods.
• Previous studies proposed cladistic trees are more imbalanced than phenetic trees.

Purpose of the Study:

• To investigate patterns of tree balance in cladograms and phenograms from the scientific literature.
• To compare tree imbalance between cladistic and phenetic methods.
• To evaluate the consistency of observed tree imbalance with a uniform-rate speciation model.

Main Methods:

• Analysis of 208 cladograms and phenograms from recent scientific literature.
• Development of a theoretical expression for expected tree imbalance under a uniform-rate random speciation model.
• Estimation of variances using computer simulations for the same speciation model.

Main Results:

• Tree imbalance decreases significantly with increasing tree size (number of taxa) in both theoretical models and empirical literature trees.
• When controlling for confounding variables, cladistic trees exhibit similar levels of imbalance compared to phenetic trees.
• The observed imbalance in literature trees deviates from predictions of a simple uniform-rate speciation model.

Conclusions:

• Variability in speciation and extinction rates among lineages likely explains the deviation from the uniform-rate model.
• Such rate variation is a prerequisite for macroevolutionary processes like species sorting and species selection.
• The study provides a quantitative assessment of tree balance and its relationship to speciation processes.