Taxon (plural taxa) is the name designating a taxonomic grouping, such as species, genus, order, or phylum (or division), of either living or extinct organisms. Another name for taxon is taxonomic unit. For example, the Lepus genus comprising the hares is a particular taxon of the kingdom Animalia (animals), and the division Pinophyta comprising the conifers is one of 13 or 14 division-level taxa within the kingdom Plantae (plants).
Taxonomy in the field of biology involves categorizing like organisms into particular groups. Each taxonomic grouping, or taxon, is assigned a taxonomic rank and can be placed at a particular level in a systematic hierarchy, traditionally reflecting shared physical characteristics but more recently aiming to reflect evolutionary relationships. The eight major taxonomic ranks, starting from the individual organism, are species, genus, family, order, class, phylum (or division), kingdom, and domain. There also are intermediate minor rankings between these, such as subclass, subspecies, and superfamily.
Classifying the members of the biological world into various taxa reflects the desire of human beings to group the great diversity of living and extinct organisms into natural categories—particularly identifying groupings according to their connectedness based on lineage or evolutionary relatedness.
Biologists group and categorize both extinct and living species of organisms by using the conceptual framework of scientific (or biological) classification. Scientific classifications, or taxonomies, are frequently hierarchical in structure. Taxon designates a particular taxonomic grouping of organisms. Mammals, for example, are a taxon of vertebrate animals. They comprise the class Mammalia.
Taxonomic rank (rank, category, taxonomic category) refers to the level of a taxon in the taxonomic hierarchy. Taxa placed at a particular taxonomic rank are groupings of organisms at the same classification level. The eight major categories used to rank organisms are species, genus, family, order, class, phylum or division, kingdom, and domain. (In biology, the terms "division" and "phylum" occupy the same taxonomic rank: "phylum" is applied traditionally to animals while "division" is more commonly applied to plants and fungi.) A simple mnemonic phrase to remember the sequence of taxonomic levels is "Dignified Kings Play Chess On Fine Green Silk." Others include "King Philip's Class Orders the Family Genius to Speak," or Do Koalas Prefer Chocolate Or Fruit, Generally Speaking?
Biologists use a prefix added to one of the eight major ranking categories to indicate finer distinctions of rank than are possible with the eight major categories. The prefix super- indicates a rank above, the prefix sub- indicates a rank below. In zoology, the prefix infra- indicates a further rank distinction below sub-. For instance:
The International Code of Zoological Nomenclature defines rank, in the taxonomic sense, as:
|“||The level, for nomenclatural purposes, of a taxon in a taxonomic hierarchy (e.g. all families are for nomenclatural purposes at the same rank, which lies between superfamily and subfamily). The ranks of the family group, the genus group, and the species group at which nominal taxa may be established are stated in Articles 10.3, 10.4, 35.1, 42.1 and 45.1.||”|
—International Commission on Zoological Nomenclature (1999)
Whereas modern classification has its roots in the system of Carolus Linnaeus, who grouped species according to shared physical characteristics, modern groupings have been revised since Linnaeus to reflect the Darwinian principle of common descent. In differentiating between the Linnaeus-based classification, which is used for biological naming, and modern classification, scientists make a distinction between taxa/taxonomy and classification/systematics. The former refers to biological names and the rules of naming. The latter refers to rank ordering of taxa according to presumptive evolutionary (phylogenetic) relationships.
An organism's 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). Crustaceans (Crustacea) are variously grouped as a phylum, subphylum, superclass, or class.
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 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. In addition, the class rank is quite often not an evolutionary but a phenetic and 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 is to govern the application of taxa to clades.
Main taxonomic ranks
Carolus Linnaeus devised Linnaean taxonomy using a six-level ranking scale: kingdom, class, order, genus, species, and variety. Today's nomenclature remains quite similar in its foundations to that established by Linnaeus, with the addition of the two major ranks of phylum and family and a de-emphasis on variety. The nomenclature is regulated by the Nomenclature Codes, which allow names divided into exactly defined ranks. Despite this there are slightly different ranks for zoology and botany.
In both zoology and botany, a taxon is usually assigned to a taxonomic rank in a hierarchy and organisms are identified by combining the two lowest major ranks in today's nomenclature, genus and species. The resulting binomial, a two-word name, is widely used to describe a particular species. For example, the binomial name for a human is Homo sapiens. This is italicized when typing, and underlined when writing. The first word refers to the genus, which is a broad grouping of closely related species, and is capitalized. The second word, in lower case, always indicates the species to which the organism is assigned within its genus.
Ranks in zoology
There are definitions of the following taxonomic ranks in the International Code of Zoological Nomenclature: superfamily, family, subfamily, tribe, subtribe, genus, subgenus, species, subspecies.
The International Code of Zoological Nomenclature divides names into "family-group names," "genus-group names," and "species-group names." The Code explicitly mentions:
- - - superfamily
- - - subfamily
- - - tribe
- - - subtribe
- - - subgenus
- - - subspecies
The rules in the Code apply to the ranks from superfamily to subspecies, and only to some extent to those above the rank of superfamily. In the "genus group" and "species group," no further ranks are allowed. Among zoologists, additional ranks such as species group, species subgroup, species complex, and superspecies are sometimes used for convenience as extra, but unofficial, ranks between the subgenus and species levels in taxa with many species (e.g. the genus Drosophila).
Ranks of taxa at lower levels may be denoted in their groups by adding the prefix "infra," meaning lower, to the rank. For example infraspecies or infrasubspecies. Infraspecific taxa then include all divisions of the species into subspecies or lower taxa.
- A taxon above the rank of species gets a scientific name in one part (a uninominal name).
- A species (a taxon at the rank of species) gets a name composed of two names (a binominal name or binomen : generic name + specific name; for example Panthera leo).
- A subspecies (a taxon at the rank of subspecies) gets a name composed of three names (a trinominal name or trinomen : generic name + specific name + subspecific name; for example Felis silvestris catus, the house cat). As there is only one rank, subspecies, below that of species, the subspecific name follows the specific name directly with no intermediate term to identify the subspecific rank.
Ranks in botany
There are definitions of the following taxonomic ranks in the International Code of Botanical Nomenclature (ICBN): kingdom (regnum), subregnum, division or phylum (divisio, phylum), subdivisio or subphylum, class (classis), subclassis, order (ordo), subordo, family (familia), subfamilia, tribe (tribus), subtribus, genus (genus), subgenus, section (sectio), subsectio, series (series), subseries, species (species), subspecies, variety (varietas), subvarietas, form (forma), subforma.
There are definitions of following taxonomic ranks in International Code of Nomenclature for Cultivated Plants: cultivar group, cultivar.
According to Art 3.1 of the ICBN the most important ranks of taxa are: kingdom, division or phylum, class, order, family, genus, and species. According to Art 4.1 the secondary ranks of taxa are tribe, section, series, variety and form. There is an indeterminate number of ranks. The ICBN explicitly mentions:
- - - secondary ranks
- - - - - - - further ranks
- - - - - - - subregnum
division or phylum (divisio, phylum)
- - - - - - - subdivisio or subphylum
- - - - - - - subclassis
- - - - - - - subordo
- - - - - - - subfamilia
- - - tribe (tribus)
- - - - - - - subtribus
- - - - - - - subgenus
- - - section (sectio)
- - - - - - - subsectio
- - - series (series)
- - - - - - - subseries
- - - - - - - subspecies
- - - variety (varietas)
- - - - - - - subvarietas
- - - form (forma)
- - - - - - - subforma
The rules in the ICBN apply primarily to the ranks of family and below, and only to some extent to those above the rank of family. Of the botanical names used by Linnaeus only names of genera, species and varieties are still used.
Taxa at the rank of genus and above get a botanical name in one part (unitary name); those at the rank of species and above (but below genus) get a botanical name in two parts (binary name); all taxa below the rank of species get a botanical name in three parts (ternary name).
For hybrids getting a hybrid name, the same ranks apply, preceded by "notho," with nothogenus as the highest permitted rank.
The usual classifications of five representative species follow: the fruit fly so familiar in genetics laboratories (Drosophila melanogaster), humans (Homo sapiens), the peas used by Gregor Mendel in his discovery of genetics (Pisum sativum), the "fly agaric" mushroom Amanita muscaria, and the bacterium Escherichia coli. The eight major ranks are given in bold; a selection of minor ranks are given as well.
|Rank||Fruit fly||Human||Pea||Fly Agaric||E. coli|
|Phylum or Division||Arthropoda||Chordata||Magnoliophyta||Basidiomycota||Proteobacteria|
|Subphylum or subdivision||Hexapoda||Vertebrata||Magnoliophytina||Agaricomycotina|
|Species||D. melanogaster||H. sapiens||P. sativum||A. muscaria||E. coli|
- The ranks of higher taxa, especially intermediate ranks, are prone to revision as new information about relationships is discovered. For example, the traditional classification of primates (class Mammalia—subclass Theria—infraclass Eutheria—order Primates) has been modified by new classifications such as McKenna and Bell (1997) (class Mammalia—subclass Theriformes—infraclass Holotheria, with Theria and Eutheria assigned lower ranks between infraclass and the order Primates). These differences arise because there are only a small number of ranks available and a large number of branching points in the fossil record.
- Within species further units may be recognized. Animals may be classified into subspecies (for example, Homo sapiens sapiens, modern humans) or morphs (for example Corvus corax varius morpha leucophaeus, the Pied Raven). Plants may be classified into subspecies (for example, Pisum sativum subsp. sativum, the garden pea) or varieties (for example, Pisum sativum var. macrocarpon, snow pea), with cultivated plants getting a cultivar name (for example, Pisum sativum var. macrocarpon 'Snowbird'). Bacteria may be classified by strains (for example Escherichia coli O157:H7, a strain that can cause food poisoning).
Terminations of names
Taxa above the genus level are often given names based on the type genus, with a standard termination. The terminations used in forming these names depend on the kingdom, and sometimes the phylum and class, as set out in the table below.
- In botany and mycology, names at the rank of family and below are based on the name of a genus, sometimes called the type genus of that taxon, with a standard ending. For example, the rose family Rosaceae is named after the genus Rosa, with the standard ending "-aceae" for a family. Names above the rank of family are formed from a family name, or are descriptive (like Gymnospermae or Fungi).
- For animals, there are standard suffixes for taxa only up to the rank of superfamily (ICZN 1999).
- Forming a name based on a generic name may be not straightforward. For example, the Latin "homo" has the genitive "hominis," thus the genus "Homo" (human) is in the Hominidae, not "Homidae."
- The ranks of epifamily, infrafamily, and infratribe (in animals) are used where the complexities of phyletic branching require finer-than-usual distinctions. Although they fall below the rank of superfamily, they are not regulated under the International Code of Zoological Nomenclature and hence do not have formal standard endings. The suffixes listed here are regular, but informal (Gaffney and Meylan 1988).
- Brummitt, R. K., and C. E. Powell. 1992. Authors of Plant Names. Royal Botanic Gardens, Kew. ISBN 0947643443.
- Gaffney, E. S., and P. A. Meylan. 1988. A phylogeny of turtles. Pages 157-219 in M.J. Benton (ed.), The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds. Oxford: Clarendon Press. ISBN 0198577052.
- International Botanical Congress (16th : 1999 : St. Louis, Mo.), W. Greuter, and J. McNeill. 2000. International Code of Botanical Nomenclature (Saint Louis Code) Adopted by the Sixteenth International Botanical Congress, St. Louis, Missouri, July-August 1999. Prepared and Edited by W. Greuter, chairman, J. McNeill, et al.. Konigstein, Germany: Koeltz Scientific Books. ISBN 3904144227.
- International Commission on Zoological Nomenclature (ICZN) and W. D. L. Ride. 1999. International Code of Zoological Nomenclature, 4th edition. London : International Trust for Zoological Nomenclature, c/o Natural History Museum. ISBN 0853010064.
- McKenna, M. C., and S. K. Bell. 1997. Classification of Mammals Above the Species Level. New York, NY: Columbia University Press. ISBN 023111012X.
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