View source for Taxon

{{Short description|Grouping of biological populations}}
{{for-multi|the journal|Taxon (journal){{!}}''Taxon'' (journal)|other uses|Taxa (disambiguation)}}

[[File:Elephants in Kenya.jpg|thumb|270px|[[African elephant]]s form the [[genus]] ''Loxodonta'', a widely accepted taxon.]]

In [[biology]], a '''taxon''' ([[back-formation]] from ''[[Taxonomy (biology)|taxonomy]]''; {{plural form}}: '''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 [[Taxonomic rank|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, especially in the context of rank-based ("[[Linnaean taxonomy|Linnaean]]") nomenclature (much less so under [[phylogenetic nomenclature]]).<ref name="Cantino & de Queiroz 2020">{{cite book |last1=Cantino |first1=Philip D. |last2=de Queiroz |first2=Kevin |title=International Code of Phylogenetic Nomenclature (PhyloCode): A Phylogenetic Code of Biological Nomenclature |date=2000 |publisher=CRC Press |location=Boca Raton, Fl |isbn=0429821352 |pages=xl + 149 |url=https://www.routledge.com/International-Code-of-Phylogenetic-Nomenclature-PhyloCode/Queiroz-Cantino/p/book/9781138332829 |language=en}}</ref> 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.

Initial attempts at classifying and ordering organisms (plants and animals) were presumably set forth in prehistoric times by hunter-gatherers, as suggested by the fairly sophisticated folk taxonomies. Much later, Aristotle, and later still, European scientists, like [[Pierre Magnol|Magnol]],<ref name="Magnol 1689">{{cite book |last1=Magnol |first1=Petrus |title=Prodromus historiae generalis plantarum in quo familiae plantarum per tabulas disponuntur |date=1689 |publisher=Pech |location=Montpellier |pages=79 |url=https://books.google.com/books?id=RShVAAAAcAAJ&dq=Prodromus+historiae+generalis+plantarum+in+quo+familiae+plantarum+per+tabulas+disponuntur&pg=PA1 |language=la}}</ref> [[Joseph Pitton de Tournefort|Tournefort]]<ref name="Tournefort 1694">{{cite book |last1=Tournefort |first1=Joseph Pitton de |title=Elemens de botanique, ou Methode pour connoître les plantes. I. [Texte.] / . Par Mr Pitton Tournefort... [T. I-III] |date=1694 |publisher=L’Imprimerie Royale |location=Paris |pages=562 |url=https://gallica.bnf.fr/ark:/12148/btv1b8454361d/f9.item |language=EN}}</ref> and [[Carl Linnaeus]]'s [[Linnaean taxonomy|system]] in ''[[Systema Naturae]]'', 10th edition (1758),<!--
--><ref>{{cite web |last=Quammen |first=David |title=A Passion for Order |publisher=National Geographic Magazine |date=June 2007 |url=http://ngm.nationalgeographic.com/print/2007/06/linnaeus-name-giver/david-quammen-text |archive-url=https://web.archive.org/web/20080827105848/http://ngm.nationalgeographic.com/print/2007/06/linnaeus-name-giver/david-quammen-text |url-status=dead |archive-date=August 27, 2008 |access-date=27 April 2013}}</ref><!--ENDRef-->, as well as an unpublished work by [[Bernard de Jussieu|Bernard]] and [[Antoine Laurent de Jussieu]], contributed to this field. 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, [[Systematics|systematists]] continue to construct accurate classifications encompassing the diversity of life; today, a "good" or "useful" taxon is commonly taken to be one that reflects [[Phylogenetics|evolutionary relationships]].{{NoteTag|This is not considered as mandatory, however, as indicated by terms for non-[[Monophyly|monophyletic]] groupings ("invertebrates", "conifers", "fish", etc).}}

Many modern systematists, such as advocates of [[phylogenetic nomenclature]], use [[Cladistics|cladistic]] methods that require taxa to be [[Monophyly|monophyletic]] (all descendants of some ancestor). Therefore, their basic unit, the [[clade]], 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 [[Paraphyly|paraphyletic]].<ref>{{cite journal |title=Phylogeny as a Central Principle in Taxonomy: Phylogenetic Definitions of Taxon Names |author=de Queiroz, K & J Gauthier |journal=Systematic Zoology |volume=39 |issue=4 |year=1990 |pages=307–322 |url=http://vertebrates.si.edu/herps/herps_pdfs/deQueiroz_pdfs/1990deQ_GauSZ.pdf |doi=10.2307/2992353 |jstor=2992353}}</ref> An example of a long-established taxon that is not also a clade is the [[Class (biology)|class]] [[Reptile|Reptilia]], the reptiles; birds and mammals are the descendants of animals traditionally classed as reptiles, but neither is included in the Reptilia (birds are traditionally placed in the class [[Bird|Aves]], and mammals in the class [[Mammal]]ia).<ref name=Romer>{{cite book |last=Romer |first=A. S. |orig-year=1949 |title=The Vertebrate Body |publisher=W.B. Saunders |year=1970 |edition=4th}}</ref>

== History ==
The term ''taxon'' was first used in 1926 by [[:de:Adolf Meyer-Abich|Adolf Meyer-Abich]] for animal groups, as a [[back-formation]] from the word ''[[taxonomy (biology)|taxonomy]]''; the word ''taxonomy'' had been coined a century before from the Greek components {{Wikt-lang|grc|τάξις}} ([[wikt:taxo-|{{grc-transl|τάξις}}]]), meaning "arrangement", and {{Wikt-lang|grc|νόμος}} ([[wikt:-nomy|{{grc-transl|νόμος}}]]), meaning "[[scientific method|method]]".<ref name="AdnetSenut2013">{{cite book |author1=Sylvain Adnet |url=https://books.google.com/books?id=SUz8AAAAQBAJ&pg=PA122 |title=Principes de paléontologie |author2=[[Brigitte Senut]] |author3=Thierry Tortosa |author4=Romain Amiot, Julien Claude, Sébastien Clausen, Anne-Laure Decombeix, Vincent Fernandez, Grégoire Métais, Brigitte Meyer-Berthaud, Serge Muller |date=25 September 2013 |publisher=Dunod |isbn=978-2-10-070313-5 |pages=122 |quote=La taxinomie s'enrichit avec l'invenition du mot «taxon» par Adolf Meyer-Abich, naturaliste allemand, dans sa Logik der morphologie, im Rahmen einer Logik der gesamten Biologie (1926) [Translation: Taxonomy is enriched by the invention of the word "taxon" by Adolf Meyer-Abich, German naturalist, in his Logik der morphologie, im Rahmen einer Logik der gesamten Biologie (1926).]}}</ref><ref name="Meyer">{{cite book |last=Meyer-Abich|first=Adolf |title=Logik der Morphologie im Rahmen einer Logik der gesamten Biologie |url=https://books.google.com/books?id=C8miBgAAQBAJ&pg=PA127 |year=1926 |publisher=[[Springer Science+Business Media|Springer-Verlag]] |isbn=978-3-642-50733-5 |page=127}}</ref> For plants, it was proposed by [[Herman Johannes Lam]] in 1948, and it was adopted at the VII [[International Botanical Congress]], held in 1950.<ref>{{cite book |last=Naik|first=V. N. |year=1984 |title=Taxonomy of Angiosperms |publisher=[[McGraw Hill Education|Tata McGraw Hill]] |location=[[New Delhi]] |page=2}}</ref>

== Definition ==
The glossary of the ''[[International Code of Zoological Nomenclature]]'' (1999) defines<ref>ICZN (1999) [http://www.iczn.org/iczn/index.jsp?booksection=glossary&nfv=true&mF= International Code of Zoological Nomenclature. Glossary] {{Webarchive|url=https://web.archive.org/web/20050103202015/http://www.iczn.org/iczn/index.jsp?booksection=glossary&nfv=true&mF= |date=2005-01-03 }}. International Commission on Zoological Nomenclature.</ref> 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 ==
{{Unreferenced section | date = January 2024}}
{{Biological classification}}

A taxon can be assigned a [[taxonomic rank]], usually (but not necessarily) when it is given a formal name.{{Citation needed|date=January 2024}}

"[[Phylum]]" applies formally to any biological [[domain (biology)|domain]], but traditionally it was always used for animals, whereas "division" was traditionally often used for [[plant]]s, [[fungus|fungi]], etc.{{Citation needed|date=January 2024}}

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 (biology)|class]] are superclass, subclass and infraclass.{{Citation needed|date=January 2024}}

Rank is relative, and restricted to a particular systematic schema. For example, [[Marchantiophyta|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 [[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.{{Citation needed|date=January 2024}}

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 (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]] 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 [[clade]]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]], etc.{{Citation needed|date=January 2024}}

== See also ==
{{Portal|Biology}}
* [[ABCD Schema]]
* [[Alpha taxonomy]]
* [[Chresonym]]
* [[Cladistics]]
* [[Folk taxonomy]]
* [[Ichnotaxon]]
* [[International Code of Nomenclature for algae, fungi, and plants]]
* [[International Code of Nomenclature of Prokaryotes]]
* [[International Code of Phylogenetic Nomenclature]]
* [[International Code of Zoological Nomenclature|International Code of Zoological Nomenclature (ICZN)]]
* [[List of taxa named by anagrams]]
* [[Rank (botany)]]
* [[Rank (zoology)]]
* [[Segregate (taxonomy)]]
* [[Virus classification]]
* [[Wastebasket taxon]]

== Notes ==
{{NoteFoot}}
{{reflist|group=note}}

== References ==
{{Reflist}}

== External links ==
* {{Wiktionary-inline}}

{{Clear}}
{{Evolution}}

{{Authority control}}

[[Category:Organisms]]
[[Category:Taxa| ]]
[[Category:Biological classification]]
[[Category:Botanical nomenclature]]
[[Category:Zoological nomenclature]]