Genetic diversity tends to decrease and genetic differentiation tends to increase towards the periphery of a species’ range, but this has rarely been tested for plants of azonal habitats such as rocky slopes or screes. Pouget et al. study a narrow endemic Mediterranean plant, Arenaria provincialis (Caryophyllaceae), across its geographic range and find that despite its narrow distribution it has a high level of molecular variation. As predicted by the central–marginal theory, the areas characterized by the highest genetic diversity are centrally located. The current range size and abundance patterns are not sufficient to predict the organization of genetic diversity, which can only be explained by phylogeographic analysis of the long-term history of migrations and persistence.