  Phylogenetic trees represent the evolutionary relationships between extant
lineages, omitting extinct or non-sampled lineages. Extending the work of
Stadler and collaborators, this paper focuses on the distribution of branch
lengths in phylogenetic trees arising under a constant-rate birth-death model.
We derive branch length distributions of interior branches with and without
random sampling of individuals of the extant population. We establish that
branches connected to the tree leaves and interior branches behave very
differently under sampling: pendant branches get longer as the sampling
probability reaches zero, whereas the distribution of interior branches quickly
reaches an asymptotic state.
