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

A taxon (plural: taxa) is a group of (one or more) organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement. Defining what belongs or does not belong to such a taxonomic group is done by a taxonomist. It is not uncommon for one taxonomist to disagree with another on what exactly belongs to a taxon, or on what exact criteria should be used for inclusion.

Taxonomists sometimes make a distinction between "good" (or natural) taxa and others that are "not good" (or artificial). Today it is common to define a good taxon as one that reflects presumptive evolutionary (phylogenetic) relationships. But this is not mandatory.

A taxon may be given a formal name, a scientific name. Such a scientific name is governed by one of the Nomenclature Codes, which sets out rules to determine which scientific name is correct for that particular grouping.

Advocates of phylogenetic nomenclature, using cladistic methods, do require taxa to be monophyletic, consisting of all descendants of some ancestor. They generally do not refer to taxa as their basic unit, but to "clades", a clade being a special form of taxon. However, even in traditional nomenclature, few taxonomists of our time would establish new taxa that they know to be paraphyletic.[1] A famous example of a widely accepted taxon that is not also a clade is the "Reptilia".



The Glossary of the International Code of Zoological Nomenclature (1999) defines[2] 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. [...]"

But there are other definitions.


A taxon can be assigned a rank, usually (but not necessarily) when it is given a formal name. The rank of a given taxon is not necessarily fixed, but can be altered later by another (or the same) taxonomist.

"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.

Life Domain Kingdom Phylum Class Order Family Genus Species

The hierarchy of biological classification's eight major taxonomic ranks. Intermediate minor rankings are not shown.

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:


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 (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 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 phenetical and 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 is to govern the application of names to clades.

See also


  1. ^ de Queiroz, K & J Gauthier (1990). "Phylogeny as a Central Principle in Taxonomy: Phylogenetic Definitions of Taxon Names" (PDF). Systematic Zoology 39 (4): 307–322.  
  2. ^ ICZN (1999) International Code of Zoological Nomenclature. Glossary. International Commission on Zoological Nomenclature.


Simple English

A taxon (plural taxa), or taxonomic unit, is a name that means an organism or group of organisms.

One can assign a rank to a taxon and can place at a particular level in a systematic hierarchy depending on their evolutionary relationships.

One has to distinct between taxa/taxonomy and classification/systematics. Taxonomy refers to biological names and the rules of naming. Classification refers to rank ordering of taxa according to presumptive evolutionary (phylogenetic) relationships.

A broad scheme of ranks in hierarchical order:

Phylum or Division

Note: "Phylum" applies formally to any biologial domain but traditionally it was always used of animals whereas "Division" was traditionally often used of plants, fungi etc.

A simple phrase to remember the order is "Dignified Kings Play Chess On Fine Green Silk".


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