Interbreeding among species (hybridization) is an important mechanism for evolution in many plant taxa. Oaks have long been recognized for their propensity to hybridize, yet maintain distinct ecological and morphological species. This paradox raises the question of how these species can maintain distinctiveness despite successful mixing. One hypothesis is that strong divergent natural selection on ecologically relevant genes maintains differences (Gugger et al. 2015 BMC Genom). However, this raises the question of why complete reproductive barriers have not evolved. One possibility is that it depends on the strength of selection, the number of traits or genes involved, the functions of those genes, and the environmental context. A complementary hypothesis is that hybridization can be beneficial by facilitating occasional introgression of adaptive alleles from other species. We are exploring these forces across the genome and in relation to ecological function in natural systems to build on past work that considered only a small number of molecular markers in the absence of genomic information (Ortego et al. 2014 J Biogeog, 2017 New Phytol).