Segregating sites in Wright's island model.

Expressions for the expectation and variance of the number of segregating sites in samples from an island model of population subdivision are derived. For small samples, an arbitrary number of demes can be accommodated. Results for larger samples are derived under the assumption of an infinite number of demes. However, simulations indicate that the latter results will hold quite well for the finite-island model in many cases. A new estimator of the population migration rate is proposed and is shown to outperform the widely used pairwise method.