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{{Inc|Genus, pl. Genera (i.e., kind), is a term used in natural history to designate a group of species. As with species, so the genus is an indefinite conception, varying with the author The chief value of the conception is its use in aiding us conveniently to arrange and name plants and animals. The name of the genus is the first of the two words in the name of the plant: thus, in Brassica oleracea, Brassica designates the genus, and oleracea the particular Brassica of which we are speaking. It is difficult to trace the origin of the genus- conception in natural history, but it is usually ascribed to Konrad Gesner (Zurich, 1516-1565). {{SCH}}
}}

[[Image:Scientific classification.png|right|100px|The hierarchy of scientific classification]]
In the [[binomial nomenclature]] used worldwide, the name of an [[organism]] is composed of two parts: its '''genus''' name (always capitalized) and a [[species]] modifier (known as the "epithet"). An example is ''Homo sapiens sapiens'', the name for the [[human|human species]] (Latin for "wise wise man") which belongs to the genus ''[[Homo (genus)|Homo]]''.

Each genus must have a designated [[type species]] (see [[Type (zoology)]]). The generic name is permanently associated with the [[type specimen]] of its type species. Should this specimen turn out to be assignable to another genus, the genus name linked to it becomes a [[junior synonym]], and the remaining [[taxa]] in the now-invalid genus need to be reassessed. See [[scientific classification]] and [[nomenclature Codes]] for more details of this system. Also see [[type genus]].

The boundaries between genera are historically subjective, but with the advent of [[phylogenetics]], it is increasingly common for all taxonomic ranks (at least) below the [[class (biology)|class]] level to be restricted to demonstrably [[monophyletic]] groupings, as has been the aim since the advent of [[evolution]]ary theory. Indeed, in the better-researched groups like birds and mammals, most genera are [[clade]]s already.

Rules-of-thumb for delimiting a genus are outlined e.g. in Gill ''et al.'' (2005)<!-- A more primary references would be in order; they appear to have gotten it from somewhere but don't say from where -->. According to these, a genus should fulfill 3 criteria to be descriptively useful:
* monophyly - all descendants of an ancestral taxon are grouped together;
* reasonable compactness - a genus should not be expanded needlessly; and
* distinctness - in regards of evolutionarily relevant criteria, i.e. [[ecology]], [[Morphology (biology)|morphology]], or [[biogeography]]; note that [[DNA sequences]] are a ''consequence'' rather than a ''condition'' of diverging evolutionarily lineages except in cases where they directly inhibit [[gene flow]] (e.g. [[postzygotic barrier]]s).

Neither the [[ICZN]] nor the [[ICBN]] require such criteria for extablishment of a genus; they rather cover the formalities of what makes a description valid. Therefore, there has been for long a vigorous debate about what criteria to consider relevant for generic distinctness. At present, most of the classifications based on [[phenetics]] - overall similarity - are being gradually replaced by new ones based on [[cladistics]] (e.g., use of [[Reptilia]] and [[Amphibia]] in [[taxonomy]] is discouraged), though phenetics was only of major relevance for a comparatively short time around the [[1960s]] before it turned out to be unworkable.

The three criteria given above are almost always fulfillable for a given clade. An example where at least one is crassly violated no matter what the generic arrangement is are the [[dabbling ducks]] of the genus ''[[Anas]]'', which are [[paraphyletic]] in regard to the extremely distinct [[moa-nalo]]s. Considering them distinct genera (as is usually done) violates criterion 1, including them in ''Anas'' violates criterion 2 and 3, and splitting up ''Anas'' so that the [[mallard]] and the [[American black duck]] are in distinct genera violates criterion 3.

Many genera are divided into [[subgenus|subgenera]] (singular '''subgenus''').

A genus in one [[kingdom (biology)|kingdom]] is allowed to bear a name that is in use as a genus name or other taxon name in another kingdom. Although this is discouraged by both the [[International Code of Zoological Nomenclature]] and the [[International Code of Botanical Nomenclature]] there are some five thousand such names that are in use in more than one kingdom. For instance, ''[[Anura]]'' is the name of the [[order (biology)|order]] of [[frog]]s but also is the name of a genus of plants (although not current: it is a [[
synonym (botany)|synonym]]); and  ''[[Aotus]]'' is the genus of [[Aotus (pea)|golden peas]] and [[night monkey]]s; ''[[Oenanthe]]'' is the genus of [[wheatear]]s and [[water dropwort]]s, and ''[[Prunella]]'' is the genus of [[accentor]]s and [[self-heal]].

Obviously, within the same kingdom one generic name can apply to only one genus. This explains why the [[platypus]] genus is named ''Ornithorhynchus'' — [[George Shaw]] named it ''Platypus'' in [[1799]], but the name ''Platypus'' had already been given to the [[pinhole borer]] [[beetle]] by [[Johann Friedrich Wilhelm Herbst]] in [[1793]]. Names with the same form but applying to different taxa are called homonyms. Since beetles and platypuses are both members of the kingdom Animalia, the name ''Platypus'' could not be used for both. Johann Friedrich Blumenbach published the replacement name ''Ornithorhynchus'' in [[1800]].

==See also==
{{wiktionary}}
*[[Linnaean taxonomy]]
*[[Cladistics]]
*[[Systematics]]

==References==
* '''Gill''', Frank B.; Slikas, Beth & Sheldon, Frederick H. (2005): Phylogeny of titmice (Paridae): II. Species relationships based on sequences of the mitochondrial cytochrome-b gene. ''[[Auk (journal)|Auk]]'' '''122'''(1): 121-143. <small>[[Digital Object Identifier|DOI]]: 10.1642/0004-8038(2005)122[0121:POTPIS]2.0.CO;2</small> [http://www.bioone.org/perlserv/?request=get-abstract&doi=10.1642%2F0004-8038%282005%29122%5B0121%3APOTPIS%5D2.0.CO%3B2 HTML abstract]

==External links==
* [http://uio.mbl.edu/NomenclatorZoologicus/ Nomenclator Zoologicus]: Index of all genus and subgenus names in zoological nomenclature from 1758 to 2004.

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[[Category:Scientific classification]]
[[Category:botanical nomenclature]]
[[Category:zoological nomenclature]]