In biology, binomial nomenclature is a standard convention used for naming species. As the word 'binomial' suggests, the scientific name of a species is formed by the combination of two terms: the genus name and the species epithet or descriptor. The first term (generic name) is always capitalized, while the specific epithet (trivial "name") is not; both are to be typeset in italics, e.g. Homo sapiens. The genus name can be abbreviated to its initial letter, but never omitted, (as H. sapiens) when repeated or when several species from the same genus are being listed or discussed in the same paper or report. In rare cases this abbreviation form has spread to more general use—for example the bacterium, Escherichia coli, is often referred to as just E. coli.
Origin of names
The species descriptor should be an adjective that differentiates a species from other members of a genus. The genus name and species descriptor are usually derived from Latin, although Latin derivation is not universal. Names sometimes come from Ancient Greek, or from local languages, or from the name of the person who first discovered the species. In fact, taxonomists come up with species names from a variety of sources, including in-jokes and puns. However, names are always treated grammatically as if they were Latin words. For this reason the binomial name of a species is sometimes called its "Latin name," although this terminology is frowned upon by biologists. The term scientific name, however, is considered acceptable. There is a separate list of Latin and Greek words commonly used in systematic names.
Value and use of the binomial system
The value of the binomial system derives primarily from its economy and its widespread use:
- the same name is used in all languages, avoiding difficulties of translation;
- every species can be unambiguously identified with just two words;
- the system has been adopted internationally in botany (since 1753), zoology (since 1758) and bacteriology (since 1980¹).
The procedures associated with establishing binomial nomenclature tend to favor stability. In particular, when species are transferred between genera (as not uncommonly happens as a result of new knowledge), if possible the species descriptor is kept the same. Similarly if what were previously thought to be distinct species are found to belong to the same species, former species names may be retained as subspecies terms.
However, such stability as exists is far from absolute. A single organism may have several scientific names in circulation, depending on opinion (see synonymy), conservation according to nomenclature codes, and new findings based on molecular phylogeny. Another source of instability is the rule that nomenclature should respect priority of discovery.
Nomenclature codes rule the naming of plants (incl. Fungi, cyanobacteria) / cultivated plants / animals / bacteria / viruses. These codes differ. For example, the ICBN (plant) nomenclature does not allow tautonymy, whereas the ICZN (animal) code allows it. A BioCode has been suggested to replace several codes, but there also is debate concerning development of a PhyloCode to name clades of phylogenetic trees.
Extensions on the binomial name
When a species is further subdivided, a trinomial nomenclature is used, e.g. Astrophytum myriostigma subvar. glabrum.
In botany, a species can be further divided into any of subspecies, variety, subvariety or form, whereas in zoology, a species is only subdivided into subspecies. Trinomial names of plants therefore usually include a qualifier (such as "subvar." in the example above), whereas trinomial names of animals never do. For example, Phalacrocorax carbo novaehollandiae is the Black Shag, the subspecies of the Great Cormorant found in Australia and New Zealand, and there is no need to indicate explicitly that novaehollandiae is a subspecific term.
Authorship in scientific names
Sometimes you will see a name or abbreviation after a scientific name and even a year as well. A complete reference to a species includes not only the binomial, but the author(s) that described the species and gave it a name. This addition of authorship is usually only done once in a particular article or citation. In many cases, the name or names are abbreviated: Urtica dioica L. where "L." refers to Carolus Linnaeus. While the scientific name is italicized, the author reference is not. If at some point, a species is moved into a different genus, the original author is put in parentheses and the author responsible for publishing the "move" is then appended: Chamaecyparis nootkatensis (D. Don) Spach, originally described as Cupressus nootkatensis by D. Don. This plant has been recently moved to another genus and has now become Xanthocyparis nootkatensis (D. Don) Farjon & Hiep. Note that every such move is not retained; only authorships of the original name (description) and the latest binomial. In articles concerning the taxonomy of a species, the date of publication associated with the authorship is added as well, but this practice is rare in encyclopedic or other non-taxonomic works.
Carolus Linnaeus invented the idea of organizing species within a hierarchical classification based upon shared characteristics, a system closely associated with binomial nomenclature. It is a common misconception that Linnaeus also invented binomial nomenclature; in fact it dates back to the Bauhins, who lived nearly 200 years before Linnaeus. He was, however, the first to systematize and popularize binomial nomenclature, and it is only one aspect of his systematical achievements or misachievements (such as oversimplifying fungal systematics).
- The botanical code kept references to bacteria until 1975. A bacteriological code of nomenclature was approved at the 4th International Congress for Microbiology in 1947, but was later discarded. The official "Nomenclatural Starting Date" for the current International Code for bacteria is January 1, 1980.