  The Yule (pure-birth) model is the simplest null model of speciation; each
lineage gives rise to a new lineage independently with the same rate $\lambda$.
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 show how our results depend slightly
on whether the depth of the tree or the number of leaves is conditioned on, and
whether $\lambda$ is known or is estimated using maximum likelihood.
