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Isozyme variability was examined in populations representing the red oak complex (Quercussubg. Erythrobalanus) on an island archipelago and adjoining peninsula in Lake Superior, near Bayfield, Wisconsin. A concomitant study of morphometric variation described in the companion manuscript, revealed a continuum in leaf morphology extending from an interior mainland site to the outermost island. The existence of this clinal variation presented an ideal opportunity to examine the genetic structure of a hybrid population along with the putative progenitor species. Dormant leaf bud samples were collected from specimens of Quercus rubraL., Q. ellipsoidalisHill, and their putative hybrids from three islands and two locations on the peninsula. Acorns were collected from some of these same trees from one peninsula location and two islands. Twelve putative enzyme loci from six enzyme systems were analyzed. Allele frequency data indicated little differentiation between populations. Mean FSTvalues for the adult trees and acorns were 0.042 and 0.020. Genetic identities according to Nei ranged from 0.958 to 0.999. Despite these high levels of genetic similarity, the populations appeared to be highly inbred as indicated by positive mean FSTvalues of 0.183 and 0.373 for the adult trees and acorns. Estimates of migration rate per generation (Nm) for the adult trees was 5.70, a value that is low when compared to estimates for other plant species with similar life history characteristics.

The Apostle Islands in Lake Superior are populated by trees that are clearly related to Quercus rubraL. However, several islands have trees with morphological characteristics suggestive of hybridization with Q. ellipsoidalisHill. Leaf specimens were collected from trees in five locations: the outermost island, an intermediate island, the nearest‐shore island, the northeast shoreline, and an inland forest about 24 km from the shoreline. Seventeen landmarks were digitized for two to five leaves per tree. These landmarks were used to generate nine linear characters and three angles. These characters, along with the number of bristle tips per leaf, were used in various combinations for several principal component analyses. In addition, the landmark configurations were examined using rotational‐fit methods. The patterns observed in both types of analysis indicate phenotypic variation coincident with a line connecting the two most distant sample sites. The location nearest the geographic center of this line is also nearest the center of the two‐dimensional view of phenotypic variation. Trees at each site illustrate a distinctive pattern in the rotational‐fit analyses, and patterns of co‐variation in the morphometric characters are different for each site. The observed morphometric variation is consistent with the hypothesis that there is hybridization between these two species, most likely in the form of introgression from Q. ellipsoidalisinto Q. rubra.