# Yule Model

YULE MODEL. *Eucalyptus sideroxylon*, Red ironbark.

Behind the School of Education.

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).