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The most recent classification of Quercus divides the genus into two subgenera and eight sections. These divisions support the evolutionary diversification of oaks among two distinct clades: the "Old World" clade, for oaks diversifying mainly in Eurasia; and the "New World" clade, for oaks diversifying mainly in the Americas.

Subgenus Quercus



 * Sect. Quercus (synonyms Lepidobalanus and Leucobalanus), the white oaks of Europe, Asia and North America. Styles are short; acorns mature in 6 months and taste sweet or slightly bitter; the inside of an acorn shell is hairless. The leaves mostly lack a bristle on their lobe tips, which are usually rounded. The type species is Quercus robur.
 * Sect. Protobalanus, the canyon live oak and its relatives, in the southwestern United States and northwest Mexico. Styles short, acorns mature in 18 months and taste very bitter. The inside of the acorn shell appears woolly. Leaves typically have sharp lobe tips, with bristles at the lobe tip.
 * Sect. Ponticae, a disjunct including just two species. Styles short, acorns maturing in 12 months. Leaves with large stipules, high secondary venation, highly toothrd.
 * Sect. Virentes, the southern live oaks of the Americas. Styles short, acorns maturing in 12 months.  Leaves evergreen or subevergreen.
 * ct. Lobatae (synonym Erythrobalanus), the red oaks of North America, Central America and northern South America. Styles long; acorns mature in 18 months and taste very bitter. The inside of the acorn shell appears woolly. The actual nut is encased in a thin, clinging, papery skin. Leaves typically have sharp lobe tips, with spiny bristles at the lobe.

Subgenus Cerris



 * Sect. Cyclobalanopsis, the ring-cupped oaks of eastern and southeastern Asia. Evergreen trees growing 10 – tall. They are distinct from subgenus Quercus in that they have acorns with distinctive cups bearing concrescent rings of scales; they commonly also have densely clustered acorns, though this does not apply to all of the species.Species of Cyclobalanopsis are common in the evergreen subtropical laurel forests which extend from southern Japan, southern Korea, and Taiwan across southern China and northern Indochina to the eastern Himalayas, in association with trees of genus Castanopsis and the laurel family (Lauraceae).
 * Sect. Cerris, the Turkey oak and its relatives of Europe and Asia. Styles long; acorns mature in 18 months and taste very bitter. The inside of the acorn's shell is hairless. Its leaves typically have sharp lobe tips, with bristles at the lobe tip.
 * Sect. Ilex, the Ilex oak and its relatives of Eurasia and northern Africa. Styles medium-long; acorns mature in 12-24 months, appearing hairy on the inside. Leaves evergreen, with bristle-like extensions on the teeth.

Oak phylogenetics
The advent of molecular techniques for phylogenetic analysis transformed understanding of oak relationships, initially by uncovering molecular support for the diphyletic division of Quercus into Old World and New World clades. These techniques have proved highly useful in resolving fine-scale relationships among 2-5 oak species, particularly groups known to hybridize, but until recently the larger emphasis on this narrow approach prevented systematists from making large-scale determinations about oak history. As the capacity for sampling across wider swaths of oak species rose, so has resolution at the section and species level across the oak tree.

Further advances in oak systematics are expected to arise from next-generation sequencing techniques, including a recent project to sequence the entire genome of Quercus robur (the pedunculate oak). The recent completion of that genome has uncovered an array of mutations that may underlie the evolution of longevity and disease resistance in oaks. In addition, the generation of RAD-seq loci for hundreds of oak species has allowed for the construction of the most highly detailed oak phylogeny to date, although the high signal of introgression across the tree poses difficulties for deriving an unambiguous, unitary history of oaks.