Oaks can provide a model to study hybridization as they often maintain species integrity and distinct environmental adaptations despite recurrent gene flow. Lind-Riehl et al. identify candidate genes potentially involved in local adaptation in the interfertile species Quercus rubra and Q. ellipsoidalis, which are characterized by contrasting adaptations to drought. They find that a CONSTANS-like (COL) gene is nearly fixed on alternative alleles in both species, as reflected by high FST values (55–80 %) between neighbouring Q. rubra and Q. ellipsoidalis populations. All other markers show low interspecific differentiation (approx. 5 %). They conclude that this COL gene may play a role in adaptive divergence and reproductive isolation (via flowering time), making it a promising candidate speciation gene.