View source for Genus

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{{Biological classification}}
A '''genus''' (from [[Ancient Greek]] '''''γένος''''' ''genos'', "offspring, family, race, gender"  - plural: genera) is a low-level [[taxonomic rank]] used in the classification of living and fossil [[organism]]s. 

Like almost all other taxonomic units, genera may sometimes be divided into [[subgenus|subgenera]], singular: subgenus. The largest main taxonomic unit below the genus is the species.

How to more precisely define a genus is a matter of continuing debate, as outlined a few paragraphs below this.

==Generic name==
{{Main|generic name}}
Generic name is a part of the Latinized name for an [[organism]]. It is a name which reflects the classification of the organism by grouping it with other closely similar organisms.

A generic name is a category name that is given to every species in a group of species which are closely related to one another. Ideally the same generic name is given to species which are all descended from a common ancestor. 

The generic name is the first part of the two-part Latin name of an organism. To take one example, for our [[human|human species]], the Latin name is ''Homo sapiens'', (''Homo'' means man, and ''sapiens'' means rational.) In this name, the generic name is ''[[Homo (genus)|Homo]]''. There are no longer any other non-extinct species in the genus ''Homo'' (although it seems that several ''Homo'' species existed in the geologically recent past).

== Taxonomy: the traditional significance of the genus==

Ever since the flowering of [[evolution|evolutionary theory]] with [[Charles Darwin]]'s writings, a genus is intended to be a name for a group of species that are very closely related to one another, by descent from a common ancestor. 
Before the age of [[DNA]] analysis, a presumed close relationship within a group of species was largely a matter of informed guesswork, based primarily on external observation, and studies of the anatomy of the organism.

==Taxonomy: the new phylogenetic approach to the genus==

Thus historically-speaking, the boundaries between genera have been rather subjective, but with the advent of [[phylogenetics]], and because of much subsequent research, it is now increasingly common for taxonomic ranks below the [[class (biology)|class]] level to be restricted to confirmed [[monophyletic]] groupings. Indeed, in the better-researched groups like birds and mammals, most genera represent [[clade]]s already.

== Types and genera==

Because of the rules of scientific naming, or "nomenclature", each genus must have a designated [[type species]] (see [[Type (zoology)]]) which defines the genus; 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]].

==One attempt to define a genus==

The 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).

==The transition to modern phylogenetic classification==

Neither the [[ICZN]] nor the [[ICBN]] require such criteria for establishment of a genus, and this is because they are concerned with the rules of nomenclature rather than the rules of taxonomy. The ICZN and ICBN rule books cover the formalities of what makes a description valid. 

Because there is no equivalent rule book for taxonomy (classification), there is an on-going vigorous debate about what criteria to consider relevant for generic distinctness. At present, most of the classifications based on the old-fashioned idea of [[phenetics]] - overall similarity - are being gradually replaced by new ones based on [[cladistics]]. For example, the use of [[Reptilia]] and [[Amphibia]] in [[taxonomy]] is now discouraged. The formal attempt to use overall similarity or 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. However, an example of a situation where at least one criterion is crassly violated no matter what the generic arrangement is the case of the [[dabbling ducks]] in the genus ''[[Anas]]''. This group is is [[paraphyletic]] in regard to the extremely distinct fossil species, [[moa-nalo]]. Considering these to be distinct genera (as is usually done) violates criterion 1, including them all in the genus ''Anas'' violates criterion 2 and 3, and splitting up the genus ''Anas'' so that the [[mallard]] and the [[American black duck]] are in distinct genera violates criterion 3.

==The problem of identical names used for different genera==

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}}
*[[Cladistics]]
*[[Phylogenetics]]
*[[Systematics]]
*[[Taxonomy]]
*[[Linnaean taxonomy]]

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

{{taxonomic ranks}}

==External links==
* [[generic name]] http://www.biolib.cz/en/glossaryterm/dir283/id2280/

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