  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 mean time
since two individuals had their most recent common ancestor ($\bar{T_s}$), and
the other is the mean number of nucleotide differences between two genes in the
sample ($\bar{d_s}$). In the case of diminishing epistasis, in which the
deleterious effect of a new mutation is attenuated, we find that the scale of
$\bar{d_s}$ with the population size depends on the mutation rate, leading then
to the onset of a sharp threshold phenomenon as N becomes large.
