Hybridization within the genus Quercus L appears to be extensive and reports vary from sightings of individual hybrid trees to small numbers of individual hybrid trees within populations to populations with characteristics of small-scale (eg Quercus robur and Quercus petraea in Hurepoix, France) and large-scale introgression (eg Quercus robur and Quercus petraea in Scotland) and, in some cases, the occurrence of hybrid swarms (eg Quercus douglasii and Quercus turbinella subsp californica in California). This has peruaded some authorities to question the current formal species concept in the genus and to suggest alternatives. The evidence supporting these cases of hybridization is examined in detail. The majority of the reports of hybrids between species of Quercus are based on an analysis of morphological data alone using a variety of univariate, bivariate and, more effectively, multivariate statistics, while other forms of evidence, such as estimates of fertility in the putative hybrids, resynthesis of hybrids, habitat characteristics of the putative hybrids and F2 segregation of parental types, have only been used occasionally. Data from chemotaxonomic investigations of suspected Quercus hybrids (mainly isozymes and phenolic components) in some instances support the morphological evidence but in other instances are contradictory; chemical data are also shown to be variable and possibly related to environmental variation which will limit their usefulness. It is concluded that, before any radical revision of the genus is attempted in which the specific limits are redefined, a wider application of the possible techniques for the study of hybrids be applied in order to clarify the true extent of gene flow between Qercus species.
