Phylogeographic structure of white oaks throughout the European continent.

Patterns of chloroplast DNA (cpDNA) variation were studied in eight white oak species by sampling 345 populations throughout Europe. The detection of polymorphisms by restriction analysis of PCR-amplified cpDNA fragments allowed the identification of 23 haplotypes that were phylogenetically ordered. A systematic hybridization and introgression between the eight species studied is evident. The levels of subdivision for unordered (GST) and ordered (NST) alleles are very high and close (0.83 and 0.85). A new statistical approach to the quantitative study of phylogeography is presented, which relies on the coefficients of differentiation GST and NST and the Mantel's test. Based on pairwise comparisons between populations, the significance of the difference between both coefficients is evaluated at a global and a local scale. The mapped distribution of the haplotypes indicates the probable routes of postglacial recolonization followed by oak populations that had persisted in southern refugia, especially in the Iberian peninsula, Italy and the Balkans. Most cpDNA polymorphisms appear to be anterior to the beginning of the last recolonization. A subset of the preexisting haplotypes have merely expanded north, while others were left behind in the south.