On the structure of genealogical trees in the presence of selection

We investigate through numerical simulations the effect of selection on two summary statistics for nucleotide variation in a sample of two genes from a population of N asexually reproducing haploid individuals. One is the mea n time since two individuals had their most recent common ancestor (<(T)ove r bar(s)>), and the other is the mean number of nucleotide differences betw een two genes in the sample (<(d)over bar(s)>). In the case of diminishing epistasis, in which the deleterious effect of a new mutation is attenuated, we find that the scale of <(d)over bar(s)> with the population size depend s on the mutation rate, leading then to the onset of a sharp threshold phen omenon as N becomes large. (C) 2000 Elsevier Science B.V. All rights reserv ed.