Abstract
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 differentiation 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 amplified 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 differentiation was highly concordant with the pattern of morphological variation. It also provided a close fit 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. Differential 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.