Yule Model

The Yule model of evolutionary branching is a probabilistic model in which species proceed forward in time, and each of a group of species is equally likely to be the next one to divide in two: to speciate. Equivalently, considering species backward in time, each pair of species is equally likely to be the next to coalesce. The Yule model is used to predict the properties of phylogenetic trees under a sensible set of assumptions for how evolution takes place.

The British statistician G. Udny Yule was an early researcher interested in the shapes of evolutionary trees, and his “A mathematical theory of evolution, based on the conclusions of Dr. J. C. Willis, F. R. S.” (Philosophical Transactions of the Royal Society of London 213: 21-87, 1925) is a founding paper in mathematical phylogenetics.

Professor Yule is memorialized both by his eponymous model of random evolution and by the shape of this red ironbark tree (the first two letters of his last name, at least).