Geographical structuring of mitochondrial DNA in Chalcides sexlineatus within the island of

The skink Chalcides sexlineatus shows clear north^south ecology-correlated variation in morphology within the island of Gran Canaria. Detailed study of this variation previously supported the hypothesis of in situ di¡erentiation arising through climate-mediated variation in selection pressures. The more recent discovery of substantial within-island mitochondrial DNA (mtDNA) sequence divergence suggested an alternative historical vicariance scenario. This paper provides a detailed analysis of geographical structuring in mtDNA, based on a fragment ampli¢ed from the 12S rRNA region in 96 individuals. A very clear phylogeographic pattern was detected: three `deep ' lineages were evident corresponding to northern, south-eastern and south-western parts of Gran Canaria with generally low levels of mtDNA introgression. The pattern of among-site di¡erentiation was highly concordant with the pattern of morphological variation. It also provided a close ¢t to a simple microevolutionary model based on population vicariance during volcanic eruptions known to have occurred during the past 2.8 million years in the north of the island. The minimum number of historical migration events required to explain the mtDNA tree was calculated and a novel randomization test used to show that there were a lower number of putative migrations across the morphology transition zone, relative to migrations within northern and southern zones. Di¡erential adaptation to the two major habitat types and selection against hybrids may explain why the morphology transition zone coincides with the main ecotone and is relatively narrow, respectively