  We derive a Poisson random field model for population site polymorphisms
differences within and between two species that share a relatively recent
common ancestor. The model can be either equilibrium or time inhomogeneous. We
first consider a random field of Markov chains that describes the fate of a set
of individual mutations. This field is approximated by a Poisson random field
from which we can make inferences about the amounts of mutation and selection
that have occurred in the history of observed aligned DNA sequences.
