In biology , a taxon ( back-formation from taxonomy ; pl. : 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 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 ") nomenclature (much less so under phylogenetic nomenclature ). 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.
13-504: An Arctic–alpine taxon is one whose natural distribution includes the Arctic and more southerly mountain ranges, particularly the Alps . The presence of identical or similar taxa in both the tundra of the far north, and high mountain ranges much further south is testament to the similar environmental conditions found in the two locations. Arctic–alpine plants, for instance, must be adapted to
26-545: Is included in the Reptilia (birds are traditionally placed in the class Aves , and mammals in the class Mammalia ). 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 τάξις ( táxis ), meaning "arrangement", and νόμος ( nómos ), meaning " method ". For plants, it
39-500: 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
52-463: 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 paraphyletic . An example of a long-established taxon that is not also a clade is the class Reptilia , the reptiles; birds and mammals are the descendants of animals traditionally classed as reptiles, but neither
65-536: The area between the Arctic and the Alps. This is known from pollen records to be true for Dryas octopetala , for instance. In other cases, the disjunct distribution may be the result of long-distance dispersal . Examples of Arctic–alpine plants include: Taxon Initial attempts at classifying and ordering organisms (plants and animals) were presumably set forth in prehistoric times by hunter-gatherers, as suggested by
78-476: The fairly sophisticated folk taxonomies. Much later, Aristotle, and later still, European scientists, like Magnol , Tournefort and Carl Linnaeus 's system in Systema Naturae , 10th edition (1758), , as well as an unpublished work by 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
91-466: 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 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
104-682: 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, 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 evolutionary relationships . Many modern systematists, such as advocates of phylogenetic nomenclature , use cladistic methods that require taxa to be monophyletic (all descendants of some ancestor). Therefore, their basic unit,
117-520: The low temperatures, extremes of temperature, strong winds and short growing season; they are therefore typically low-growing and often form mats or cushions to reduce water loss through evapotranspiration . It is often assumed that an organism which currently has an Arctic–alpine distribution was, during colder periods of the Earth's history (such as during the Pleistocene glaciations ), widespread across
130-686: 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. Herman Johannes Lam Herman Johannes Lam (3 January 1892 in Veendam – 15 February 1977 in Leiden )
143-522: Was a Dutch botanist . Lam studied at Utrecht University , where he was awarded a Doctor of Biology in 1919. Lam was the appointed the director of the Rijksherbarium in 1933. He retired from academic work in 1962. He was member of the Royal Netherlands Academy of Arts and Sciences since 1960. In 1957, botanist Steenis published a genus of plants from New Guinea (belonging to
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#1732848471627156-420: 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
169-543: Was proposed by Herman Johannes Lam in 1948, and it was adopted at the VII International Botanical Congress , held in 1950. The glossary of the International Code of Zoological Nomenclature (1999) defines a A taxon can be assigned a taxonomic rank , usually (but not necessarily) when it is given a formal name. " Phylum " applies formally to any biological domain , but traditionally it
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