Editing Oak (section)

=== Hybridisation ===

[[File:Quercus stellata.jpg|thumb|left|upright=0.9 |A hybrid white oak, possibly ''[[Post oak|Quercus stellata]]'' × ''[[Chinkapin oak|Q. muehlenbergii]]'']]

[[Interspecific hybridization]] is quite common among oaks, but usually between species within the same section only,<ref>{{Cite journal |last=Valen |first=Leigh Van |year=1976 |title=Ecological Species, Multispecies, and Oaks |journal=Taxon |volume=25 |issue=2/3 |pages=233–239 |doi=10.2307/1219444 |issn=0040-0262 |jstor=1219444|bibcode=1976Taxon..25..233V }}</ref> and most common in the white oak group. White oaks cannot discriminate against pollination by other species in the same section. Because they are [[wind pollinated]] and have weak internal barriers to hybridization, hybridization produces functional seeds and fertile hybrid offspring. Ecological stresses, especially near habitat margins, can also cause a breakdown of mate recognition as well as a reduction of male function (pollen quantity and quality) in one parent species.<ref>{{Cite journal|last1=Williams |first1=Joseph H. |first2=William J. |last2=Boecklen |first3=Daniel J. |last3=Howard |year=2001 |title=Reproductive processes in two oak (''Quercus'') contact zones with different levels of hybridization |journal=Heredity |volume=87 |issue=6 |pages=680–690 |doi=10.1046/j.1365-2540.2001.00968.x|pmid=11903563 |doi-access=free |bibcode=2001Hered..87..680W }}</ref><ref>{{Cite book|last=Arnold |first=M. L. |year=1997 |title=Natural Hybridization and Evolution |publisher=Oxford University Press |location=New York |isbn=0-19-509974-5}}</ref>

Frequent hybridization among oaks has consequences for oak populations around the world; most notably, hybridization has produced large populations of hybrids with much [[introgression]] and the [[evolution]] of new species.<ref name=r2>{{Cite journal|last1=Conte |first1=L. |last2=Cotti |first2=C. |last3=Cristofolini |first3=G. |year=2007 |title=Molecular evidence for hybrid origin of ''Quercus crenata'' Lam. (Fagaceae) from ''Q-cerris'' L. and ''Q-suber'' L. |journal=Plant Biosystems |volume=141 |issue=2 |pages=181–193 |doi=10.1080/11263500701401463|bibcode=2007PBios.141..181C |s2cid=83882998 }}</ref> Introgression has caused different species in the same populations to share up to 50% of their genetic information.<ref name=r3>{{cite journal |last1=Gomory |first1=D. |last2=Schmidtova |first2=J. |year=2007 |title=Extent of nuclear genome sharing among white oak species (''Quercus'' L. subgen. ''Lepidobalanus'' (Endl.) Oerst.) in Slovakia estimated by allozymes |journal=Plant Systematics and Evolution |volume=266 |issue=3–4 |pages=253–264 |doi=10.1007/s00606-007-0535-0|bibcode=2007PSyEv.266..253G |s2cid=23587024 }}</ref> As a result, genetic data often does not differentiate between clearly morphologically distinct species, but instead differentiates populations.<ref name=r4>{{Cite journal |last1=Kelleher |first1=C. T. |first2=T. R. |last2=Hodkinson |first3=G. C. |last3=Douglas |first4=D. L. |last4=Kelly |year=2005 |title=Species distinction in Irish populations of ''Quercus petraea'' and ''Q. robur'': Morphological versus molecular analyses |journal=Annals of Botany |pmid=16199484 |volume=96 |issue=7 |pages=1237–1246 |doi=10.1093/aob/mci275|pmc=4247074 }}</ref> The maintenance of particular loci for adaptation to ecological niches may explain the retention of species identity despite significant gene flow.<ref>{{Cite journal |last1=Zhang |first1=Ruhua |last2=Hipp |first2=Andrew L. |last3=Gailing |first3=Oliver |date= October 2015 |title=Sharing of chloroplast haplotypes among red oak species suggests interspecific gene flow between neighboring populations |journal=Botany |volume=93 |issue=10 |pages=691–700 |doi=10.1139/cjb-2014-0261 |issn=1916-2790 |hdl=1807/69792 |hdl-access=free}}</ref>

The [[Fagaceae]], or beech family, to which the oaks belong, is a slowly-evolving [[clade]] compared to other [[angiosperm]]s,<ref>{{Cite journal|doi=10.1139/g93-089 |last1=Frascaria |first1=N. |first2=L. |last2=Maggia |first3=M. |last3=Michaud |first4=J. |last4=Bousquet |year=1993 |title=The RBCL Gene Sequence from Chestnut Indicates a Slow Rate of Evolution in the Fagaceae |journal=Genome |volume=36 |issue=4 |pages=668–671 |pmid=8405983|url=http://agritrop.cirad.fr/590955/1/1993_frascaria%20et%20al.pdf }}</ref><ref>{{cite journal |last1=Manos |first1=P. S. |first2=A. M. |last2=Stanford |year=2001 |title=The historical biogeography of Fagaceae: Tracking the tertiary history of temperate and subtropical forests of the Northern Hemisphere |journal=International Journal of Plant Sciences |volume=162 |pages=S77–S93 |issue=Suppl. 6 |doi=10.1086/323280 |bibcode=2001IJPlS.162S..77M |s2cid=84936653 }}</ref> and the patterns of hybridization and introgression in ''Quercus'' pose a significant challenge to the [[species concept|concept of a species]] as a group of "actually or potentially interbreeding populations which are reproductively isolated from other such groups."<ref name="Raven 2005"/> By this definition, many species of ''Quercus'' would be lumped together according to their geographic and ecological habitat, despite clear distinctions in morphology and genetic data.<ref name="Raven 2005">{{cite book |last1=Raven |first1=Peter H. |first2=George B. |last2=Johnson |first3=Jonathan B. |last3=Losos |first4=Susan R. |last4=Singer |title=Biology |url=https://archive.org/details/biology00pete |url-access=registration |edition=Seventh |publisher=McGraw Hill |location=New York |year=2005 |isbn=0-07-111182-4}}</ref>