How and why ecological communities change their species membership over time and space is a central issue in ecology and evolution. Phylogeographic approaches based on animal mitochondrial DNA sequences have been important for revealing historical patterns of individual species and can provide qualitative comparisons among species. Exciting new methods, particularly implementing approximate Bayesian computation (ABC –Beaumont et al. 2002), now allow model-based quantitative comparisons among species and permit the probabilistic exploration of alternative community-level hypotheses (see review by Hickerson et al. 2010). In this issue of Molecular Ecology, Ilves et al. (2010) use an ABC approach to bring fresh insights into the well-studied question of how North Atlantic coastal species contracted and expanded their ranges in response to late Pleistocene/Holocene climate fluctuations.