which criteria are to be used for deciding inclusion. This is especially the case in context of rank-based nomenclature (Linnaean taxonomy).[1]
Once a taxon is given a formalscientific name, its use is governed by one of thenomenclature codes that specify the correct scientific name for a particular grouping.
The idea of a unit-based system to classify the characteristics of plants and animals (later known asbiological classification) was first made widely available in 1805 viaAugustin Pyramus de Candolle'sPrincipes élémentaires de botanique, published as the introduction toJean-Baptiste Lamarck'sFlore françoise, 3rd ed. (1805), which treatise presented a system for the "natural classification" of plants. From that time forwardsystematists have competed, collaborated, and published—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 reflectsevolutionary relationships.[note 1] Many modern systematists are advocates ofphylogenetic nomenclature; they usecladistic methods that require taxa to bemonophyletic (i.e., show all the descendants of a common ancestor). Their basic unit, theclade, 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 beparaphyletic.[6]
An example of a long-established taxon that is paraphyletic—meaningnot also a clade—is theclassReptilia: 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 classAves, and mammals in the classMammalia.[7]
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. [...]"
A taxon can be assigned ataxonomic rank, usually (but not necessarily) when it is given a formal name.[citation needed]
"Phylum" applies formally to any biologicaldomain, but traditionally it was always used for animals, whereas "division" was traditionally often used forplants,fungi, etc.[citation needed]
A prefix is used to indicate a ranking of lesser importance. The prefixsuper- indicates a rank above, the prefixsub- indicates a rank below. Inzoology, the prefixinfra- indicates a rank belowsub-. For instance, among the additional ranks ofclass are superclass, subclass and infraclass.[citation needed]
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 ofcladistics; for example, the mere 10 ranks traditionally used between animal families (governed by theInternational 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'sphylogeny becomes known.[citation needed]
In addition, the class rank is quite often not an evolutionary but aphenetic orparaphyletic group and as opposed to those ranks governed by the ICZN (family-level, genus-level andspecies-level taxa), can usually not be made monophyletic by exchanging the taxa contained therein. This has given rise tophylogenetic taxonomy and the ongoing development of thePhyloCode, which has been proposed as a new alternative to replace Linnean classification and govern the application of names toclades. 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]
^Romer, A. S. (1970) [1949].The Vertebrate Body (4th ed.). W.B. Saunders.
^Sylvain Adnet;Brigitte Senut; Thierry Tortosa; Romain Amiot, Julien Claude, Sébastien Clausen, Anne-Laure Decombeix, Vincent Fernandez, Grégoire Métais, Brigitte Meyer-Berthaud, Serge Muller (25 September 2013).Principes de paléontologie. Dunod. p. 122.ISBN978-2-10-070313-5.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).]{{cite book}}: CS1 maint: multiple names: authors list (link)
