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;. 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.
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. Much later, as of Aristotle's teachings, and later stillas of the published works of Magnol, Tournefort, and Carl Linnaeus,, and as of the unpublished works of Bernard and Antoine Laurent de Jussieuthen did European naturalists and scientists begin documenting this new field of human knowledge.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 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., which treatise presented a system for the "natural classification" of plants. From that time forward systematists have competed, collaborated, and publishedwhile 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. Many modern systematists are advocates of phylogenetic nomenclature; they use cladistic methods that require taxa to be monophyletic. 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 nomenclature, only a few still propose taxa they know to be paraphyletic.
An example of a long-established taxon that is paraphyleticmeaning not also a cladeis 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.
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 , meaning "arrangement", and , meaning "method". For plants, it was proposed by Herman Johannes Lam in 1948, and it was adopted at the VII International Botanical Congress, held in 1950.Definition
The glossary of the International Code of Zoological Nomenclature defines a- "taxon,, n.
Ranks
"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.
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 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. 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 and animal phyla 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, 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.