A genus

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.

Generic name

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).

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.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.