The genus Quercus represents an important component of the scrublands and low forests of the Four Corners area of northeastern Arizona, southeastern Utah, southwestern Colorado, and northwestern New Mexico. The area is dominated by the widespread Quercus gambelii, although relictual and marginal populations of Q. turbinella and Q. havardii are also present. Hybridization among these 3 species is common, leading to much variability in phenotypic traits, and this hybridization forms part of the widespread complex of hybrid oaks recognized as Q. x undulata. Multivariate analysis of leaf morphology was performed in 9 populations, representing the various taxa and levels of hybridization. This analysis was intended to determine the distinctiveness conferred by, and extent of influence from, hybridization and successive introgression. It was found that, though common, hybridization seems to act at very restricted spatial scales and not in the production of a region-wide hybrid swarm. Most hybridization occurs between the predominant Q. gambelii and Q. turbinella. Quercus havardii appears to maintain itself as a fairly discrete taxon, with little gene exchange among congener species. Though morphological introgression actively occurs only at the local population level, it seems to have significantly altered the morphology of the parental species. This seems to be supporting evidence for extensive introgression toward the southern Q. turbinella when its distribution was more widespread. This asymmetric introgression toward Q. turbinella suggests a pattern of character coherence and the presence of a suite of advantageous traits, which may allow the species to enlarge its niche via hybridization.