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, the name for the human species which belongs to the genus Homo. 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 above the species level to be restricted to demonstrably monophyletic groupings as has been the aim since the advent of evolutionary theory.
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.
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.
Many genera are divided into subgenera (singular subgenus).
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 particular generic name can apply to only one genus. This explains why the platypus genus is named Ornithorhynchus — it was indeed given the name Platypus, by George Shaw in 1799, but by then that name 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.
- 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: 121-143. DOI: 10.1642/0004-8038(2005)122[0121:POTPIS]2.0.CO;2 HTML abstract
- Nomenclator Zoologicus: Index of all genus and subgenus names in zoological nomenclature from 1758 to 2004.