Historical and ongoing hybridization between the oak species Quercus petraea and Q. robur leads to difficulty in identifying the taxonomic status of individual trees.
Despite this, Beatty et al. using genetic and morphometric analysis of populations from Northern Ireland, which represents the northwestern limit of the species’ ranges, identified ‘pure’ individuals of both species, as well as a range of hybrid classes. Furthermore, analysis of chloroplast DNA indicates that unidirectional introgression followed by backcrossing may have facilitated long-distance recolonization after the ice ages.
Gemma E. Beatty, W. Ian Montgomery, Florentine Spaans, David G. Tosh, Jim Provan, 2016, 'Pure species in a continuum of genetic and morphological variation: sympatric oaks at the edge of their range', Annals of Botany, vol. 117, no. 4, pp. 541-549 http://dx.doi.org/10.1093/aob/mcw002