The limits of mtDNA phylogeography: complex patterns of population history in a highly structured Iberian lizard are only revealed by the use of nuclear markers

Phylogeographic analyses based on the sole use of the mitochondrial DNA (mtDNA) molecule reveal only a small part of the evolutionary history of a species or a set of related species. In this study, we have combined the application of slow- and fast-evolving nuclear markers (proteins and microsatellites, respectively) together with the analysis of two-gene genealogies to further understand the history of the Iberian endemic Schreiber's green lizard, Lacerta schreiberi, a species for which a well established phylogeographical scenario is available. In sharp contrast with the observation of four divergent and almost allopatric mtDNA clades, our nuclear data revealed how two groups of populations diverged, persisted and began to admix along the mountains of the Iberian Central System. In addition, the combination of mtDNA and nuclear data showed how the core area of the species distribution responded to ice ages, first by relatively old processes of population expansion to the south followed by episodes of contraction that are at the origin of present-day isolates, and more recently by a postglacial expansion to the Iberian Northwest where new habitats were made available after climatic amelioration. Taken together with recently published results for a variety of other organisms, our results suggest that complex processes of fragmentation, expansion and admixture can only be properly addressed through the use of several and complementary types of molecular markers. Finally, we also suggest that southern European refugia are both hotspots and melting pots of genetic diversity.