In biology, a taxon 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Although preceded by Linnaeus's system in Systema Naturae and unpublished work by Bernard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.
Many modern systematists, such as advocates of phylogenetic nomenclature, use cladistic
methods that require taxa to be monophyletic.
Their basic unit, therefore, is the clade rather than the taxon.
Among those contemporary taxonomists working with the traditional Linnean nomenclature,
however, few propose taxa they know to be paraphyletic.
An example of a well-established taxon that is not also a clade is the class Reptilia,
the reptiles.
History The term taxon was first used in 1926 by Adolf
Bernhard Meyer for animal groups.
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 defines a "taxon,, 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 the unit from other such units.
A taxon encompasses all included taxa of lower rank and individual organisms.
[...]"
But there are other definitions.
Ranks
A taxon can be assigned a taxonomic rank, usually when it is given a formal name.
"Phylum" 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 have been grouped, in various systems of classification, as a
family, order, class, or division.
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 and animal phyla 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 paraphyletic
group and as opposed to those ranks governed by the ICZN, can usually not be made monophyletic
by exchanging the taxa contained therein.
This has given rise to phylogenetic taxonomy 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 clades.
Many cladists do not see any need to depart from traditional nomenclature as governed
by the ICZN, ICBN, etc.
See also ABCD Schema
Alpha taxonomy Chresonym
Cladistics Folk taxonomy
Ichnotaxon International Code of Nomenclature for algae,
fungi, and plants International Code of Zoological Nomenclature
Rank Rank
Segregate Virus classification
Notes
References
External links