Several taxonomical classifications of the Genus Quercus have been developed in the past, and all were based entirely on morphological characters, so they were not really reliable. With the advent of new technologies (such as scanning electron microscope, molecular genetics and so on) our view has considerably changed, especially with the scientifics works from 2001*. The traditional concepts have been replaced by that of two major clades in the Genus Quercus, leading to the new (2017) classification that you can see HERE (pdf).
See below a short summary giving a broad outline of the updated classification.
The Genus Quercus is composed of two subgenera with eight sections (See the Phylogeny of the major Oak clades below)
1/ Subgenus Cerris, with three sections :
a) Section Cyclobalanopsis (ca 90 species in tropical and sub-tropical Asia)
b) Section Cerris (10-12 species in Eurasia and North Africa)
c) Section Ilex (ca 35 species in Eurasia and North Africa)
2/ Subgenus Quercus, with five sections
d) Section Quercus (ca 150 species in North America, Mexico, Central America, Western Eurasia, East Asia and North Africa) with 7 Series (Roburoid, Lobatoid, Prinoid, Albae, Stellatae, Polymorphae, Leucomexicanae).
e) Section Ponticae (2 species in Eurasia and Western North America)
f) Section Virentes (7 species in North America, Mexico, Cuba, Central America)
g) Section Lobatae (ca 120 species in North America, Mexico, Central America, Colombia) with 5 Series (Agrifoliae, Palustres, Coccineae, Phellos, Erythromexicanae)
h) Section Protobalanus (5 species in North America and Mexico)
- Significance of Pollen Characteristics for Infrageneric Classification and Phylogeny in Quercus (Fagaceae), 2009
- The Oaks of Western Eurasia : Traditional Classifications and Evidence from two Nuclear Markers, 2010