Taxon

{{Short description|Unit of biological populations}} {{hatnote group| {{distinguish|Taxon (journal){{!}}''Taxon'' (journal)}} {{redirect|Taxa}} }}

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

In biology, a '''taxon''' is a group of one or more populations of an organism, or organisms, as seen by taxonomists to form a biological unit; (''taxon'': back-formation from ''taxonomy''; {{plural form}}: '''taxa'''). Although neither is required, a taxon, once its description has become established, is usually known by a particular name and is given a particular ranking.

==Methods== Taxonomists consider: *which organisms belong to a given taxon *which criteria are to be used for deciding inclusion. This is especially the case in context of rank-based nomenclature (''Linnaean taxonomy'').<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>

Once a taxon is given a formal scientific name, its use is governed by one of the nomenclature codes that specify the correct scientific name for a particular grouping.

Initial attempts at preserving human knowledge of plants and animals were presumably made in prehistoric times by hunter-gatherers, as suggested by folk taxonomies interpreted from archeological and anthropological studies.<ref>{{Cite journal |last=Berlin |first=B |last2=Breedlove |first2=DE |last3=Raven |first3=PH |title=Folk Taxonomies and Biological Classification |url=https://www.science.org/doi/10.1126/science.154.3746.273 |journal=Science |volume=154 |issue=3746 |pages=273-275 |doi=10.1126/science.154.3746.273|url-access=subscription }}</ref> Much later, as of Aristotle's teachings, and later still{{mdash}}as of the published works of 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> 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, (his ''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> and as of the unpublished works of Bernard and Antoine Laurent de Jussieu{{mdash}}then did European naturalists and scientists begin documenting this new field of human knowledge.

The idea of a unit-based system to classify the characteristics of plants and animals (later known as biological classification) was first made widely available in 1805 via Augustin Pyramus de Candolle's ''Principes élémentaires de botanique'', published as the introduction to Jean-Baptiste Lamarck's ''Flore françoise'', 3rd ed. (1805), which treatise presented a system for the "natural classification" of plants. From that time forward systematists have competed, collaborated, and published{{mdash}}while providing for organizing and classifying human knowledge of the life forms on planet Earth.

In modern biology studies, a "good" or "useful" taxon is commonly taken to be one that reflects evolutionary relationships.{{NoteTag|This is not considered as mandatory, however, as indicated by terms for non-monophyletic groupings ("invertebrates", "conifers", "fish", etc).}} Many modern systematists are advocates of ''phylogenetic nomenclature''; they use cladistic methods that require taxa to be monophyletic (i.e., show all the descendants of a common ancestor). Their basic unit, the ''clade'', is equivalent to the taxon, and their using the clade implies that taxa should reflect evolutionary relationships. Similarly, among those contemporary taxonomists working with the traditional Linnean (binomial) nomenclature, only a few still propose taxa they know to be 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 paraphyletic{{mdash}}meaning ''not also'' a clade{{mdash}}is the class Reptilia: the reptiles. Birds and mammals are descendants of animals long classed as reptiles; but traditionally, neither was placed in class Reptilia. Instead, birds are found in the class Aves, and mammals in the class Mammalia.<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 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 {{Wikt-lang|grc|τάξις}} ({{grc-transl|τάξις}}), meaning "arrangement", and {{Wikt-lang|grc|νόμος}} ({{grc-transl|νόμος}}), meaning "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-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=Tata McGraw Hill |location=New Delhi |page=2}}</ref>

== Definition == The glossary of the ''International Code of Zoological Nomenclature'' (1999)<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> 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. [...]"

== Ranks == {{Biological classification}}

A taxon can be assigned a taxonomic rank, usually (but not necessarily) when it is given a formal description, though it is common to place a taxon as an unranked group called a clade.<ref>{{Cite book |last=Simpson |first=Michael G. |title=Plant systematics |date=2019 |publisher=Academic Press, an imprint of Elsevier |isbn=978-0-12-812628-8 |edition=Third |location=Burlington, MA}}</ref>

"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.<ref>{{Cite book |last=Turland |first=Nick J. |title=The code decoded: a user's guide to the International code of nomenclature for algae, fungi, and plants |date=2019 |publisher=Pensoft |isbn=978-954-642-964-3 |edition=Second |location=Sofia, Bulgaria}}</ref>

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.{{Citation needed|date=January 2024}}

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 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 clades. 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}}

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

== References == {{Reflist}}

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

Category:Organisms Category:Taxa Category:Biological classification Category:Botanical nomenclature Category:Zoological nomenclature