Oak is a tree or shrub from the genus Quercus of the beech family, Fagaceae. There are about 600 existing oak species. The common name “oak” is also found in the names of species of related genera, in particular Lithocarpus (stone oaks), as well as in the names of unrelated species such as Grevillea robusta (silky oaks) and Casuarinaceae (she oaks). The genus Quercus is native to the Northern Hemisphere and includes deciduous and evergreen species ranging from cool temperate to tropical latitudes in the Americas, Asia, Europe and North Africa. North America has the largest number of oak species, approximately 90 of which are found in the United States, and Mexico has 160 species, 109 of which are endemic. The second largest center for oak diversity is China, where there are about 100 species.
Oak trees have spirally arranged leaves with lobed edges in many species; some have jagged leaves or whole leaves with smooth edges. Many deciduous species are marcescent, without letting through dead leaves until spring. In the spring, a single oak tree produces both male flowers (in the form of catkins) and small female flowers which means that the trees are monoecious. The fruit is a nut called an acorn or oak nut borne in a cup-like structure known as a cupule; each acorn contains one seed (rarely two or three) and takes 6 to 18 months, depending on the species. Acorns and leaves contain tannic acid, which helps protect against fungi and insects. Live oak trees are characterized by the fact that they are evergreen, but in fact they are not a separate group, but are scattered throughout the genus.
The advent of molecular methods of phylogenetic analysis changed the concept of oak relationships, firstly finding by uncovering molecular support for the diphyletic division of Quercus into Old and New World clades. These methods have proven to be very useful in resolving the delicate relationships between 2-5 oak species, especially groups known to hybridize, but until recently, greater emphasis on this narrow approach has prevented systematists from making large-scale definitions of history of oak. As the sample size from the broader oak strata increases, the level resolution of the sections and species in the oak tree.
Further progress in oak taxonomy is expected to be achieved through next-generation sequencing techniques, including a recent project to sequence the entire genome of Quercus robur (the pedunculate oak). The recent completion of this genome has identified a number of mutations that may underlie the evolution of the longevity and disease resistance of oak trees. In addition, the generation of RAD-seq loci for hundreds of oak species made it possible to construct the most detailed oak phylogeny to date, although the high signal of introgression through the tree makes it difficult to obtain an unambiguous single oak history.
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