A genus (plural: genera) is a relatively low-level taxonomic unit, used in the classification of living and fossil organisms. (Like almost all other taxonomic units, genera may sometimes be divided into subgenera, singular: subgenus.) What exactly constitutes a genus is a matter of continuing debate, as outlined a few paragraphs below this.
But in any case, even though the actual definition of a genus is in dispute, up to the present day, a genus is an essential part of the two-part system biologists use for naming organisms. This system is known as binomial nomenclature, and it was invented by Carl Linnaeus in the 18th century.
The genus is the first part of the two-part Latin name of an organism. To take one relevant example, for our human species the Latin name is Homo sapiens, (Homo means man, and sapiens means rational.) In this name, the genus is Homo. There are no longer any other non-extinct species in the genus Homo (although dozens of species of phylum Homo existed over the past ~ 2 million years).
Ever since the flowering of 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. But 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.
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 level to be restricted to confirmed monophyletic groupings. Indeed, in the better-researched groups like birds and mammals, most genera represent clades already.
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.
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, and this is because they are concerned with the rules of nomenclature rather than the rules of taxomony. The ICZN and ICBN rule books cover the formalities of what makes a description valid. Because there is no equivalent rule book for taxomony (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.
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.
- Nomenclator Zoologicus: Index of all genus and subgenus names in zoological nomenclature from 1758 to 2004.