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International audience; In wide-ranging species, the genetic consequences of range shifts in response to climate change during the Pleistocene can be predicted to differ among different parts of the distribution area. We used amplified fragment length polymorphism data to compare the genetic structure of Arabis alpina, a widespread arctic-alpine and afro-alpine plant, in three distinct parts of its range: the North Atlantic region, which was recolonized after the last ice age, the European Alps, where range shifts were probably primarily altitudinal, and the high mountains of East Africa, where the contemporary mountain top populations result from range contraction. Genetic structure was inferred using clustering analyses and estimates of genetic diversity within and between populations. There was virtually no diversity in the vast North Atlantic region, which was probably recolonized from a single refugial population, possibly located between the Alps and the northern ice sheets. In the European mountains, genetic diversity was high and distinct genetic groups had a patchy and sometimes disjunct distribution. In the African mountains, genetic diversity was high, clearly structured and partially in accordance with a previous chloroplast phylogeography. The fragmented structure in the European and African mountains indicated that A. alpina disperses little among established populations. Occasional long-distance dispersal events were, however, suggested in all regions. The lack of genetic diversity in the north may be explained by leading-edge colonization by this pioneer plant in glacier forelands, closely following the retracting glaciers. Overall, the genetic structure observed corresponded to the expectations based on the environmental history of the different regions.

International audience; The population concept is central in evolutionary and conservation biology, but identifying the boundaries of natural populations is often challenging. Here, we present a new approach for assessing spatial genetic structure without the a priori assumptions on the locations of populations made by adopting an individual-centred approach. Our method is based on assignment tests applied in a moving window over an extensively sampled study area. For each individual, a spatially explicit probability surface is constructed, showing the estimated probability of finding its multilocus genotype across the landscape, and identifying putative migrants. Population boundaries are localized by estimating the mean slope of these probability surfaces over all individuals to identify areas with genetic discontinuities. The significance of the genetic discontinuities is assessed by permutation tests. This new approach has the potential to reveal cryptic population structure and to improve our ability to understand gene flow dynamics across landscapes. We illustrate our approach by simulations and by analysing two empirical datasets: microsatellite data of Ursus arctos in Scandinavia, and amplified fragment length polymorphism (AFLP) data of Rhododendron ferrugineum in the Alps.