Population differentiation among three species of white oak in northeastern Illinois

We used microsatellite DNA analysis to examine population differentiation among three species of white oak, Quercus alba L., Quercus bicolor Willd., and Quercus macrocarpa Michx., occurring in both pure and mixed stands in northeastern Illinois. Using individual-based Bayesian clustering or principal components analyses, no strong genetic groupings of individuals were detected. This suggests that the three species do not represent distinct and differentiated genetic entities. Nevertheless, traditional approaches where individuals are pre-assigned to species and populations, including F statistics, allele frequency analysis, and Nei's genetic distance, revealed low, but significant genetic differentiation. Pairwise F statistics showed that some intraspecific comparisons were as genetically differentiated as interspecific comparisons, with the two populations of Q. alba exhibiting the highest level of genetic differentiation (theta = 0.1156). A neighbor-joining tree also showed that the two populations of Q. alba are distinct from one another and from the two other species, while Q. bicolor and Q. macrocarpa were genetically more similar. Pure stands of Q. macrocarpa did not show a higher degree of genetic differentiation than mixed stands.