In biology, a taxon ( back-formation from '' taxonomy''; plural taxa) 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 very common, 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 In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

, its use is then governed by one of the nomenclature codes specifying which scientific name is correct for a particular grouping. Initial attempts at classifying and ordering organisms (plants and animals) were set forth in Carl Linnaeus's

system A system is a group of Interaction, interacting or interrelated elements that act according to a set of rules to form a unified whole. A system, surrounded and influenced by its environment (systems), environment, is described by its boundaries, ...

in ''

Systema Naturae ' (originally in Latin written ' with the ligature æ) is one of the major works of the Swedish botanist, zoologist and physician Carl Linnaeus (1707–1778) and introduced the Linnaean taxonomy. Although the system, now known as binomial nomen ...

'', 10th edition (1758), as well as an unpublished work by Bernard and

Antoine Laurent de Jussieu Antoine Laurent de Jussieu (; 12 April 1748 – 17 September 1836) was a French botanist, notable as the first to publish a natural classification of flowering plants; much of his system remains in use today. His classification was based on an e ...

. The idea of a unit-based system of biological classification was first made widely available in 1805 in the introduction of Jean-Baptiste Lamarck's ''Flore françoise'', and Augustin Pyramus de Candolle's ''Principes élémentaires de botanique''. Lamarck set out a system for the "natural classification" of plants. Since then,

systematists Biological systematics is the study of the diversification of living forms, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees (synonyms: cladograms, phylogenetic tr ...

continue to construct accurate classifications encompassing the diversity of life; today, a "good" or "useful" taxon is commonly taken to be one that reflects evolutionary relationships. Many modern systematists, such as advocates of

phylogenetic nomenclature Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...

, use cladistic methods that require taxa to be

monophyletic In cladistics for a group of organisms, monophyly is the condition of being a clade—that is, a group of taxa composed only of a common ancestor (or more precisely an ancestral population) and all of its lineal descendants. Monophyletic gro ...

(all descendants of some ancestor). Their basic unit, therefore, the

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

is equivalent to the taxon, assuming that taxa should reflect evolutionary relationships. Similarly, among those contemporary taxonomists working with the traditional Linnean (binomial) nomenclature, few propose taxa they know to be

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

. An example of a long-established taxon that is not also a clade is the class Reptilia, the reptiles; birds and mammals are the descendants of animals traditionally classed as reptiles, but neither are included in the Reptilia (birds are traditionally placed in the class

Aves Birds are a group of warm-blooded vertebrates constituting the class Aves (), characterised by feathers, toothless beaked jaws, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a strong yet lightweigh ...

, and mammals in the class Mammalia).

History

The term ''taxon'' was first used in 1926 by Adolf Meyer-Abich for animal groups, as a back-formation 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). For plants, it was proposed by Herman Johannes Lam in 1948, and it was adopted at the VII International Botanical Congress, held in 1950.

Definition

The glossary of the '' International Code of Zoological Nomenclature'' (1999) definesICZN (1999
International Code of Zoological Nomenclature. Glossary
. International Commission on Zoological Nomenclature. 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

A taxon can be assigned a taxonomic rank, usually (but not necessarily) when it is given a formal name. "

Phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

" applies formally to any biological domain, but traditionally it was always used for animals, whereas "division" was traditionally often used for plants, fungi, etc. A prefix is used to indicate a ranking of lesser importance. The prefix ''super-'' indicates a rank above, the prefix ''sub-'' indicates a rank below. In zoology, the prefix ''infra-'' indicates a rank below ''sub-''. For instance, among the additional ranks of class are superclass, subclass and infraclass. Rank is relative, and restricted to a particular systematic schema. For example,

liverworts The Marchantiophyta () are a division of non-vascular land plants commonly referred to as hepatics or liverworts. Like mosses and hornworts, they have a gametophyte-dominant life cycle, in which cells of the plant carry only a single set of ge ...

have been grouped, in various systems of classification, as a family, order, class, or division (phylum). The use of a narrow set of ranks is challenged by users of cladistics; for example, the mere 10 ranks traditionally used between animal families (governed by the International Code of Zoological Nomenclature (ICZN)) and animal phyla (usually the highest relevant rank in taxonomic work) often cannot adequately represent the evolutionary history as more about a lineage's phylogeny becomes known. In addition, the class rank is quite often not an evolutionary but a phenetic or

group and as opposed to those ranks governed by the ICZN (family-level, genus-level and species-level taxa), can usually not be made monophyletic by exchanging the taxa contained therein. This has given rise to

phylogenetic taxonomy Phylogenetic nomenclature is a method of nomenclature for taxa in biology that uses phylogenetic definitions for taxon names as explained below. This contrasts with the traditional approach, in which taxon names are defined by a '' type'', which ...

and the ongoing development of the '' PhyloCode'', which has been proposed as a new alternative to replace Linnean classification and govern the application of names to

s. Many cladists do not see any need to depart from traditional nomenclature as governed by the ICZN,

International Code of Nomenclature for algae, fungi, and plants The ''International Code of Nomenclature for algae, fungi, and plants'' (ICN) is the set of rules and recommendations dealing with the formal botanical names that are given to plants, fungi and a few other groups of organisms, all those "trad ...

, etc.

Notes

References

External links