THE COALESCENT PROCESS AND BACKGROUND SELECTION

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