The coalescent process and background selection.

Some statistical properties of gene trees are described for a model with background deleterious mutations. It is argued that the history of a small sample of genes under this model is a continuous time Markov chain that quickly reaches stationarity. This observation leads to simple expressions for the expected nucleotide diversity and suggests that the frequency spectrum in small samples should be approximately the same as under a strict neutral model. The results concerning expected nucleotide diversity are compared with observed variation on the third chromosome of Drosophila melanogaster.