Test on heterogeneity in population differentiation among central vs. marginal populations. Shown are the mean FST (+/- s.d.) in the recent expansion area (exp), the Holocene expansion area (hol) and the LGM refugia (ref). There is a significantly stronger structure in the expansion area than in both other areas (1000 simulations, p < 0.001 for both comparisons). The comparison between hol and ref was not significantly different.