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.
18-509: The inverted repeat-lacking clade ( IRLC ) is a monophyletic clade of the flowering plant subfamily Faboideae (or Papilionaceae). Faboideae includes the majority of agriculturally-cultivated legumes . The name of this clade is informal and is not assumed to have any particular taxonomic rank like the names authorized by the ICBN or the ICPN . The clade is characterized by the loss of one of
36-461: A polyphyletic grouping are not inherited from a common ancestor, but evolved independently. Monophyletic groups are typically characterised by shared derived characteristics ( synapomorphies ), which distinguish organisms in the clade from other organisms. An equivalent term is holophyly . The word "mono-phyly" means "one-tribe" in Greek. These definitions have taken some time to be accepted. When
54-421: Is composed of five traditional tribes ( Cicereae , Fabeae , Galegeae , Hedysareae , and Trifolieae ) and several genera that were traditionally placed in the tribe Millettieae : Afgekia , Callerya , Endosamara , Sarcodum , Wisteria , and possibly Antheroporum . The first five of these genera have been transferred to the tribe Wisterieae , so that as revised, the tribe Millettieae falls outside
72-502: 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
90-508: 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
108-463: 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
126-640: The IRLC clade. The clade is defined as: "The most inclusive crown clade exhibiting the structural mutation in the plastid genome (loss of one copy of the ~25-kb inverted repeat region) homologous with that found in Galega officinalis , Glycyrrhiza lepidota , and Vicia faba , where these taxa are extant species included in the crown clade defined by this name." This clade includes edible plants such as garden peas, lentils, chickpeas, licorice, alfafa, among others. Monophyly In biological cladistics for
144-404: The cladistics school of thought became mainstream in the 1960s, several alternative definitions were in use. Indeed, taxonomists sometimes used terms without defining them, leading to confusion in the early literature, a confusion which persists. The first diagram shows a phylogenetic tree with two monophyletic groups. The several groups and subgroups are particularly situated as branches of
162-689: The classification of organisms , monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of taxa which meets these criteria: Monophyly is contrasted with paraphyly and polyphyly as shown in the second diagram. A paraphyletic grouping meets 1. but not 2., thus consisting of the descendants of a common ancestor, excepting one or more monophyletic subgroups. A polyphyletic grouping meets neither criterion, and instead serves to characterize convergent relationships of biological features rather than genetic relationships – for example, night-active primates, fruit trees, or aquatic insects. As such, these characteristic features of
180-413: The fact that a monophyletic group includes organisms (e.g., genera, species) consisting of all the descendants of a unique common ancestor. Conversely, the term polyphyly , or polyphyletic , builds on the ancient Greek prefix πολύς ( polús ), meaning "many, a lot of", and refers to the fact that a polyphyletic group includes organisms arising from multiple ancestral sources. By comparison,
198-483: 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
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#1732794120505216-470: 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
234-687: 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,
252-729: The term paraphyly , or paraphyletic , uses the ancient Greek prefix παρά ( pará ), meaning "beside, near", and refers to the situation in which one or several monophyletic subgroups are left apart from all other descendants of a unique common ancestor. That is, a paraphyletic group is nearly monophyletic, hence the prefix pará . On the broadest scale, definitions fall into two groups. The concepts of monophyly, paraphyly , and polyphyly have been used in deducing key genes for barcoding of diverse group of species. Taxon Initial attempts at classifying and ordering organisms (plants and animals) were presumably set forth in prehistoric times by hunter-gatherers, as suggested by
270-493: The tree to indicate ordered lineal relationships between all the organisms shown. Further, any group may (or may not) be considered a taxon by modern systematics , depending upon the selection of its members in relation to their common ancestor(s); see second and third diagrams. The term monophyly , or monophyletic , derives from the two Ancient Greek words μόνος ( mónos ), meaning "alone, only, unique", and φῦλον ( phûlon ), meaning "genus, species", and refers to
288-473: The two 25-kb inverted repeats in the plastid genome that are found in most land plants. It is consistently resolved in molecular phylogenies . The clade is predicted to have diverged from the other legume lineages 39.0±2.4 million years ago (in the Eocene ). It includes several large, temperate genera such as Astragalus , Hedysarum , Medicago , Oxytropis , Swainsona , and Trifolium . This clade
306-420: 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
324-565: 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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