Genus

Genus (plural, genera) is a category of biological classification ranking in the hierarchical system below family and above species and including one or more species, with the species being so grouped generally exhibiting similar characteristics (anatomy, physiology) or assumed evolutionary relatedness. The genus is also the first part of the binomial nomenclature used worldwide, whereby 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 species (Latin for "wise wise man"), which belongs to the genus Homo.

Scientific classification or biological classification is how biologists group and categorize extinct and living species of organisms. Modern classification has its roots in the system of Carolus Linnaeus, who grouped species according to shared physical characteristics. Groupings have been revised since Linnaeus to reflect the Darwinian principle of common descent. Molecular systematics, which uses genomic DNA analysis, has driven many recent revisions and is likely to continue to do so.

Scientific classifications, or taxonomies, are frequently hierarchical in structure. Between family and species, other categories are also sometimes used, such as subfamily (above genus), subgenus (below genus), and so forth.

Delineating genera

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 level to be restricted to demonstrably monophyletic groupings, as has been the aim since the advent of evolutionary theory. Indeed, in the better-researched groups like birds and mammals, most genera are clades already.

Rules-of-thumb for delimiting a genus are outlined e.g. in Gill et al. (2005). 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, 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 barriers).

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-nalos. 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.

Use of genus

A genus in one 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 of frogs but also is the name of a genus of plants (although not current: it is a synonym); and Aotus is the genus of golden peas and night monkeys; Oenanthe is the genus of wheatears and water dropworts, and Prunella is the genus of accentors 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.

Type species

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.

• A type species is the nominal species that is the name-bearing type of a nominal genus or subgenus.

Each genus must have a designated type species (the term "genotype" was once used for this but has been abandoned because the word has been co-opted for use in genetics, and is much better known in that context). The description of a genus is usually based primarily on its type species, modified and expanded by the features of other included species. The generic name is permanently associated with the name-bearing type of its type species.

Ideally, a type species best exemplifies the essential characteristics of the genus to which it belongs, but this is subjective and, ultimately, technically irrelevant, as it is not a requirement of the Code. If the type species proves, upon closer examination, to belong to a pre-existing genus (a common occurrence), then all of the constituent species must be either moved into the pre-existing genus, or disassociated from the original type species and given a new generic name; the old generic name passes into synonymy, and is abandoned, unless there is a pressing need to make an exception (decided case-by-case, via petition to the International Commission on Zoological Nomenclature).

Type genus

A type genus is that genus from which the name of a family or subfamily is formed. As with type species, the type genus is not necessarily the most representative, but is usually the earliest described, largest or best known genus. It is not uncommon for the name of a family to be based upon the name of a type genus which has passed into synonymy; the family name does not need to be changed in such a situation.

• A type genus is the nominal genus that is the name-bearing type of a nominal family-group taxon.

See also

• Linnaean taxonomy
• Cladistics
• Systematics

References

ISBN links support NWE through referral fees

• 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 122(1): 121-143. DOI: 10.1642/0004-8038(2005)122[0121:POTPIS]2.0.CO;2 HTML abstract

External links

• Nomenclator Zoologicus: Index of all genus and subgenus names in zoological nomenclature from 1758 to 2004.

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