Binomial nomenclature (also called binominal nomenclature or binary nomenclature) is a formal system of naming species of living things by giving each a name composed of two parts, both of which use Latin grammatical forms, although they can be based on words from other languages. Such a name is called a binomial name (which may be shortened to just "binomial"), a binomen or a scientific name; more informally it is also called a Latin name. The first part of the name identifies the genus to which the species belongs; the second part identifies the species within the genus. For example, humans belong to the genus ''Homo'' and within this genus to the species ''Homo sapiens''. The ''formal'' introduction of this system of naming species is credited to Swedish natural scientist Carl Linnaeus, effectively beginning with his work ''Species Plantarum'' in 1753.〔
The application of binomial nomenclature is now governed by various internationally agreed codes of rules, of which the two most important are the ''International Code of Zoological Nomenclature'' (''ICZN'') for animals and the ''International Code of Nomenclature for algae, fungi, and plants'' (''ICN'') for plants. Although the general principles underlying binomial nomenclature are common to these two codes, there are some differences, both in the terminology they use and in their precise rules.
In modern usage, the first letter of the first part of the name, the genus, is always capitalized in writing, while that of the second part is not, even when derived from a proper noun such as the name of a person or place. Similarly, both parts are italicized when a binomial name occurs in normal text. Thus the binomial name of the annual phlox (named after botanist Thomas Drummond) is now written as ''Phlox drummondii''.
In scientific works, the "authority" for a binomial name is usually given, at least when it is first mentioned, and the date of publication may be specified.
* "''Patella vulgata'' Linnaeus, 1758". The name "Linnaeus" tells the reader who it was that first published a description and name for this species of sea snail; 1758 is the date of the publication in which the original description can be found (in this case the 10th edition of the book ''Systema Naturae'').
*"''Passer domesticus'' (Linnaeus, 1758)". The original name given by Linnaeus was ''Fringilla domestica''; the parentheses indicate that the species is now considered to belong in a different genus. The ICZN does not require that the name of the person who changed the genus be given, nor the date on which the change was made, although nomenclatorial catalogs usually include such information.
*"''Amaranthus retroflexus'' L." – "L." is the standard abbreviation used in botany for "Linnaeus".
*"''Hyacinthoides italica'' (L.) Rothm. – Linnaeus first named this bluebell species ''Scilla italica''; Rothmaler transferred it to the genus ''Hyacinthoides''; the ICN does not require that the dates of either publication be specified.
Prior to the adoption of the modern binomial system of naming species, a scientific name consisted of a generic name combined with a specific name that was from one to several words long. Together they formed a system of polynomial nomenclature. These names had two separate functions. First, to designate or label the species, and second, to be a diagnosis or description; however these two goals were eventually found to be incompatible. In a simple genus, containing only two species, it was easy to tell them apart with a one-word genus and a one-word specific name; but as more species were discovered the names necessarily became longer and unwieldy, for instance ''Plantago foliis ovato-lanceolatus pubescentibus, spica cylindrica, scapo tereti'' (Plantain with pubescent ovate-lanceolate leaves, a cylindric spike and a terete scape), which we know today as ''Plantago media''.
Such "polynomial names" may sometimes look like binomials, but are significantly different. For example, Gerard's herbal (as amended by Johnson) describes various kinds of spiderwort: "The first is called ''Phalangium ramosum'', Branched Spiderwort; the second, ''Phalangium non ramosum'', Unbranched Spiderwort. The other ... is aptly termed ''Phalangium Ephemerum Virginianum'', Soon-Fading Spiderwort of Virginia". The Latin phrases are short descriptions, rather than identifying labels.
The Bauhins, in particular Caspar Bauhin (1560–1624), took some important steps towards the binomial system, by pruning the Latin descriptions, in many cases to two words.〔, p. v〕 The adoption by biologists of a system of strictly binomial nomenclature is due to Swedish botanist and physician Carl von Linné, more commonly known by his Latinized name Carl Linnaeus (1707–1778). It was in his 1753 ''Species Plantarum'' that he first began consistently using a one-word "trivial name" together with a generic name in a system of binomial nomenclature. This trivial name is what is now known as a specific epithet (''ICN'') or specific name (''ICZN'').〔 The Bauhins' genus names were retained in many of these, but the descriptive part was reduced to a single word.
Linnaeus's trivial names introduced an important new idea, namely that the function of a name could simply be to give a species a unique label. This meant that the name no longer need be descriptive; for example both parts could be derived from the names of people. Thus Gerard's ''phalangium ephemerum virginianum'' became ''Tradescantia virginiana'', where the genus name honoured John Tradescant the younger,〔Some sources say that both John Tradescant the younger and his father, John Tradescant the elder, were intended by Linnaeus.〕 an English botanist and gardener. A bird in the parrot family was named ''Psittacus alexandri'', meaning "Alexander's parrot", after Alexander the Great whose armies introduced eastern parakeets to Greece. Linnaeus' trivial names were much easier to remember and use than the parallel polynomial names and eventually replaced them.
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