Comparative population genetic and phylogenetic analyses were used to study historical and recent gene flow between two colour variants of the spider Eresus cinnaberinus, in order to explain variant distributions in Northern and Central Europe. Recently, the colour variants have been assigned to two species, E. cinnaberinus and E. sandaliatus, the latter found isolated in Denmark and in Bavaria. Explaining Eresus's distributions thus poses a twofold problem: (i) clarifying species limits at a population-species transition and (ii) explaining noncontinuous distributions in a postglacially colonized area. Combined allozyme and mtDNA data suggest that disjunct distributions of E. sandaliatus in Bavaria and Denmark were caused by introgression of E. cinnaberinus into a E. sandaliatus background, giving rise to E. cinnaberinus phenotypes, rather than competitive exclusion of a genetically independent species by the other. Introgression caused mtDNA paraphyly of the derived E. sandaliatus whereas paraphyly of E. cinnaberinus outside the putative introgression zone may be associated with lineage sorting. Allozymes reveal local and extant gene flow processes better than mtDNA, but, because of the divided population structure, allozymes have limited power in making inferences about historical gene flow and the speed of postglacial colonization. Mitochondrial DNA distributions indicate that postglacial colonization of Northern Europe occurred rapidly and in several waves from different source populations.
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