A genus (plural: genera, from Γένος; Latin genus "descent, family, type, gender") is a low-level taxonomic rank used in the classification of living and fossil organisms. The taxonomic ranks are life, domain, kingdom, phylum, class, order, family, genus, and species
Like almost all other taxonomic units, genera may sometimes be divided into 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.
The generic name is the first component of an organism's binomial scientific nomenclature, classifying an organism with like organisms; the second component of a scientific nomenclature is species. The generic name is written with an initial majuscule letter, the species name is in lower case, e.g. Canis lupus is the Grey wolf's scientific name, Canis (dog) and lupus (wolf).
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.Binomial Nomenclature
The present system of binomial nomenclature identifies each species by a scientific name of two words, Latin in form and usually derived from Greek or Latin roots. The first name (capitalized) is the genus of the organism, the second (not capitalized) is its species. The scientific name of the white oak is Quercus alba, while red oak is Quercus rubra. The first name applies to all species of the genus—Quercus is the name of all oaks—but the entire binomial applies only to a single species. Many scientific names describe some characteristic of the organism (alba=white; rubra=red); many are derived from the name of the discoverer or the geographic location of the organism. Genus and species names are always italicized when printed; the names of other taxa (families, etc.) are not. When a species (or several species of the same genus) is mentioned repeatedly, the genus may be abbreviated after its first mention, as in Q. alba. Subspecies are indicated by a trinomial; for example, the southern bald eagle is Haliaeetus leucocephalus leucocephalus, as distinguished from the northern bald eagle, H. leucocephalus washingtoniensis.
The advantages of scientific over common names are that they are accepted by speakers of all languages, that each name applies only to one species, and that each species has only one name. This avoids the confusion that often arises from the use of a common name to designate different things in different places (example elk), or from the existence of several common names for a single species. There are two international organizations for the determination of the rules of nomenclature and the recording of specific names, one for zoology and one for botany. According to the rules they have established, the first name to be published (from the work of Linnaeus on) is the correct name of any organism unless it is reclassified in such a way as to affect that name (for example, if it is moved from one genus to another). In such a case definite rules of priority also apply.
The 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:
Neither the International Code of Zoological Nomenclature (ICZN) nor the International Code of Botanical Nomenclature (ICBN) require such criteria for establishing a genus, because these organisations are concerned with the nomenclature rules, not taxonomy rules. The ICZN and ICBN formally establish a valid nomenclature.
Thes three criteria are most always fulfilable for a given clade, however, an example of a criterion violation, despite the generic arrangement is the dabbling ducks in the genus Anas. This group is a paraphyletic and distinct fossil species, moa-nalo. Considering these as distinct genera violates criterion 1, including all species the Anas genus violates criteria 2 and 3, and splitting the genus so that the mallard duck 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); 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.
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.