Patterns of genetic variation in northern red oak (Quercus rubra L.) populations in midwestern United States

The distribution of genetic variation within and between plant populations reflects past evolutionary events as well as the potential for future evolutionary change. Such information is valuable to scientists interested in evolutionary biology, forest tree breeding, and conservation genetics. The major objective of this study was to evaluate patterns of genetic variation in 28 midwestern populations of Northern Red Oak (Quercus rubra L.), a major North American forest tree species, on macrogeographic and microgeographic scales.; Using as genetic markers enzymes identified through starch gel electrophoresis, results from the macrogeographic survey demonstrated that this species has high amounts of genetic variation and high rates of gene flow among populations. However, the finding that northern populations were genetically differentiated from southern populations suggests that selection has had a sufficient impact to overcome the high rates of gene flow. The microgeographic analysis was conducted by comparing the scale of genetic differentiation between subpopulations located on adjacent slopes within a site with that of widely disparate populations. The findings indicate that subpopulations in different microhabitats within a site can show considerable genetic differentiation, most likely due to the impact of natural selection.; This investigation demonstrates how the opposing evolutionary forces of natural selection and gene flow contribute to patterns of genetic variation within a species. Such knowledge on this economically important species will be useful to those studying tree breeding as well as to those interested in the conservation of our increasingly fragmented forests.