The expected length of pendant and interior edges of a Yule tree

The Yule (pure-birth)model is the simplest nullmodel of speciation; each lineage gives rise to a new lineage independentlywith the same rateλ.We investigate the expected length of an edge chosen at random from the resulting evolutionary tree. In particular, we compare the expected length of a randomly selected edge with the expected length of a randomly selected pendant edge.We provide some exact formulae, and showhowour results depend slightly on whether the depth of the tree or the number of leaves is conditioned on, and whether λ is known or is estimated using maximum likelihood. © 2010 Elsevier Ltd. All rights reserved.