Taxon

Taxon

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A taxon 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 with the science of taxonomy
Taxonomy
Taxonomy is the science of identifying and naming species, and arranging them into a classification. The field of taxonomy, sometimes referred to as "biological taxonomy", revolves around the description and use of taxonomic units, known as taxa...

. 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 evolution
Evolution
Evolution is any change across successive generations in the heritable characteristics of biological populations. Evolutionary processes give rise to diversity at every level of biological organisation, including species, individual organisms and molecules such as DNA and proteins.Life on Earth...

ary (phylogenetic) relationships. But this is not mandatory.

A taxon may be given a formal name, or a scientific name. Such a scientific name is governed by one of the Nomenclature Codes
Nomenclature Codes
Nomenclature codes or codes of nomenclature are the various rulebooks that govern biological taxonomic nomenclature, each in their own broad field of organisms...

, which sets out rules to determine which scientific name is correct for that particular grouping.

Advocates of phylogenetic nomenclature
Phylogenetic nomenclature
Phylogenetic nomenclature or phylogenetic taxonomy is an alternative to rank-based nomenclature, applying definitions from cladistics . Its two defining features are the use of phylogenetic definitions of biological taxon names, and the lack of obligatory ranks...

, 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 "clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s", 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. A famous example of a widely accepted taxon that is not also a clade is the "Reptilia".

Definition


The Glossary of the International Code of Zoological Nomenclature
International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...

(1999) defines 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.

Ranks


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
Domain (biology)
In biological taxonomy, a domain is the highest taxonomic rank of organisms, higher than a kingdom. According to the three-domain system of Carl Woese, introduced in 1990, the Tree of Life consists of three domains: Archaea, Bacteria and Eukarya...

, but traditionally it was always used for animals, whereas "Division" was traditionally often used for plant
Plant
Plants are living organisms belonging to the kingdom Plantae. Precise definitions of the kingdom vary, but as the term is used here, plants include familiar organisms such as trees, flowers, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The group is also called green plants or...

s, fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

, 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
Class (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...

 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 (phylum). The use of a narrow set of ranks is challenged by users of cladistics
Cladistics
Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

; for example, the mere 10 ranks traditionally used between animal families (governed by the ICZN
International Code of Zoological Nomenclature
The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...

) 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
PhyloCode
The International Code of Phylogenetic Nomenclature, known as the PhyloCode for short, is a developing draft for a formal set of rules governing phylogenetic nomenclature...

, which is to govern the application of names to clade
Clade
A clade is a group consisting of a species and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological...

s.

See also

  • ABCD Schema
    ABCD Schema
    The Access to Biological Collections Data schema is a highly-structured data exchange and access model for taxon occurrence data , i.e...

  • Alpha taxonomy
    Alpha taxonomy
    Alpha taxonomy is the discipline concerned with finding, describing and naming species of living or fossil organisms. This field is supported by institutions holding collections of these organisms, with relevant data, carefully curated: such institutes include natural history museums, herbaria and...

  • Folk taxonomy
    Folk taxonomy
    A folk taxonomy is a vernacular naming system, and can be contrasted with scientific taxonomy. Folk biological classification is the way peoples describe and organize their natural surroundings/the world around them, typically making generous use of form taxa like "shrubs", "bugs", "ducks",...

  • Chresonym
    Chresonym
    In biodiversity informatics, a chresonym refers to the cited use of a taxon name, usually a species name, within a publication. The term is derived from the Greek "chresis" meaning "a use" and refers to published usage of a name....

  • Cladistics
    Cladistics
    Cladistics is a method of classifying species of organisms into groups called clades, which consist of an ancestor organism and all its descendants . For example, birds, dinosaurs, crocodiles, and all descendants of their most recent common ancestor form a clade...

  • International Code of Botanical Nomenclature (ICBN)
  • International Code of Zoological Nomenclature
    International Code of Zoological Nomenclature
    The International Code of Zoological Nomenclature is a widely accepted convention in zoology that rules the formal scientific naming of organisms treated as animals...

     (ICZN)
  • Rank (botany)
  • Rank (zoology)
  • Segregate (taxonomy)
    Segregate (taxonomy)
    In taxonomy, a segregate, or a segregate taxon is created when a taxon is split off, from another taxon. This other taxon will be better known, usually bigger, and will continue to exist, even after the segregate taxon has been split off...