Poleward range expansions are currently observed in many species and commonly attributed to global change. However, recent research indicates that they might be facilitated by adaptation. An example is provided by Argiope bruennichi, a widely distributed Palearctic spider, which greatly expanded its European range in the past decades. The colonization of habitats in Northern Europe was apparently enabled by a climatic niche shift. This is mirrored in an increased cold tolerance and a specific cold stress induced gene expression response of expansive Northern European spider populations. In order to unravel the genetic origin of expansive spiders, we sequenced genomes of populations throughout the species’ range. Our analysis identifies three genetic lineages in the Palearctic, an East Asian, a European and an Azorean. The Asian and European clades form a contact zone in Central Asia, from where Asian genetic variation enters the European genepool. An analysis of historical DNA indicates that this secondary contact is less than 100 years old. Interestingly, we find considerable introgression in the newly colonized invaded habitat of the
species in Northern Europe, while introgression is barely affecting native populations. We speculate that an admixture of glacial lineages might have provided the variation for a rapid genetic and evolutionary divergence of spider populations.