  We consider a model of a population of fixed size $N$ undergoing selection.
Each individual acquires beneficial mutations at rate $\mu_N$, and each
beneficial mutation increases the individual's fitness by $s_N$. Each
individual dies at rate one, and when a death occurs, an individual is chosen
with probability proportional to the individual's fitness to give birth. Under
certain conditions on the parameters $\mu_N$ and $s_N$, we show that the
genealogy of the population can be described by the Bolthausen-Sznitman
coalescent. This result confirms predictions of Desai, Walczak, and Fisher
(2013), and Neher and Hallatschek (2013).
