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African elephants form the genus Loxodonta, a widely accepted taxon.

In biology, a taxon (plural taxa; back-formation from taxonomy) is a group of one or more populations of an organism or organisms seen by taxonomists to form a unit. Although neither is required, a taxon is usually known by a particular name and given a particular ranking, especially if and when it is accepted or becomes established. It is not uncommon, however, for taxonomists to remain at odds over what belongs to a taxon and the criteria used for inclusion. If a taxon is given a formal scientific name, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping.

Although preceded by Linnaeus's system in Systema Naturae (10th edition, 1758)^[1] and unpublished work by Bernard and Antoine Laurent de Jussieu, the notion of a unit-based "natural system" of biological classification was first made widely available in 1805 through the publication, as the introduction to the third edition of Jean-Baptiste Lamarck's Flore françoise, of Augustin Pyramus de Candolle's Principes élémentaires de botanique, an exposition of a system for the "natural classification" of plants. Since then, systematists have striven to construct an accurate classification encompassing the diversity of life; today, a "good" or "useful" taxon is commonly taken to be one that reflects evolutionary relationships.^{[n 1]}

Many modern systematists, such as advocates of phylogenetic nomenclature, use cladistic methods that require taxa to be monophyletic (all descendants of some ancestor). Their basic unit, therefore, is the clade rather than the taxon. Similarly, among those contemporary taxonomists working with the traditional Linnean (binomial) nomenclature, few propose taxa they know to be paraphyletic.^[2] An example of a well-established taxon that is not also a clade is the class Reptilia, the reptiles; birds are descendants of reptiles but are not included in the Reptilia.

History

The term taxon was first used in 1926 by Adolf Meyer-Abich for animal groups, as a backformation from the word Taxonomy; the word Taxonomy had been coined a century before from the Greek components τάξις (taxis, meaning arrangement) and -νομία (-nomia meaning method).^[3][4] For plants, it was proposed by Herman Johannes Lam in 1948, and it was adopted at the VII International Botanical Congress, held in 1950.^[5]

Definition

The Glossary of the International Code of Zoological Nomenclature (1999) defines^[6] a

• "taxon, (pl. taxa), n.

A taxonomic unit, whether named or not: i.e. a population, or group of populations of organisms which are usually inferred to be phylogenetically related and which have characters in common which differentiate (q.v.) the unit (e.g. a geographic population, a genus, a family, an order) from other such units. A taxon encompasses all included taxa of lower rank (q.v.) and individual organisms. [...]"

Ranks