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In biological classification, rank is the level (the relative position) in a taxonomic hierarchy. The most basic rank is that of species, the next most important is genus, and then family. Sometimes (but only rarely) the term "taxonomic category" is used instead of "rank".

The International Code of Zoological Nomenclature defines rank, in the nomenclatural 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.[1]

Contents

Main ranks

In his landmark publications, such as the Systema Naturae, Carl Linnaeus used a ranking scale limited to: empire, kingdom, class, order, genus, species, and one lower rank, below species. Today, nomenclature is regulated by the Nomenclature Codes, which allow names divided into an indefinite number of ranks. There are seven main taxonomic ranks: kingdom, phylum or division1, class, order, family, genus, species. In addition, the domain (proposed by Carl Woese) is now widely used as one of the fundamental ranks, although it is not mentioned in any of the Nomenclature Codes.

Main taxonomic ranks
Latin English
regio domain
regnum kingdom
phylum divisio phylum1 division2
classis class
ordo order
familia family
genus genus
species species
Notes to table
1 Phylum is used in zoology. It is at the same level as division in botany.
2 Preferred to phylum in botany, that is accounted as identical.

A taxon is usually assigned a taxonomic rank (in a hierarchy), usually when it is given its formal name. The basic rank is that of species. The next most important rank is that of genus: if an organism is given a species name it will at the same time be assigned to a genus, as the genus name is part of the species name. The third-most important rank, although it was not used by Linnaeus, is that of family.

The species name is sometimes called a binomial (a two-term name). For example, the zoological name for the human species is Homo sapiens: this is usually italicized in print (and underlined when italics are not available). In this case, Homo is the generic name and refers to the genus; it is capitalized; sapiens indicates the species: it is written in lower case.

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

Family

Subfamily
Tribe
Subtribe

Genus

Subgenus

Species

Subspecies

The rules in the Code apply to the ranks of 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 terms 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).

At higher ranks (family and above) a lower level may be denoted by adding the prefix "infra," meaning lower, to the rank. For example infraorder (below suborder) or infrafamily (below subfamily).

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Names of zoological 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, the lion).
  • 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 Canis lupus familiaris, the house dog). As there is only one rank below that of species, no connecting term to indicate rank is used.

Ranks in botany

There are definitions of the following taxonomic ranks in the International Code of Botanical Nomenclature: 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:


primary ranks

secondary ranks
further ranks

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

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. Also see descriptive botanical names.

Names of botanical taxa

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 hybrid's nothotaxon is an alias for a list of all of the taxa which are ancestral to the hybrid.)

Examples

Classifications of five 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
Domain Eukarya Eukarya Eukarya Eukarya Bacteria
Kingdom Animalia Animalia Plantae Fungi Monera
Phylum or Division Arthropoda Chordata Magnoliophyta Basidiomycota Proteobacteria
Subphylum or subdivision Hexapoda Vertebrata Magnoliophytina Agaricomycotina
Class Insecta Mammalia Magnoliopsida Agaricomycetes Gammaproteobacteria
Subclass Pterygota Theria Rosidae Agaricomycetidae
Order Diptera Primates Fabales Agaricales Enterobacteriales
Suborder Brachycera Haplorrhini Fabineae Agaricineae
Family Drosophilidae Hominidae Fabaceae Amanitaceae Enterobacteriaceae
Subfamily Drosophilinae Homininae Faboideae Amanitoideae
Genus Drosophila Homo Pisum Amanita Escherichia
Species D. melanogaster H. sapiens P. sativum A. muscaria E. coli

Table Notes:

  • 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 (class Mammalia — subclass Theriformes — infraclass Holotheria) with Theria and Eutheria assigned lower ranks between infraclass and the order Primates. See mammal classification for a discussion. 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 recognised. 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).
  • Mnemonics are available at[2] and.[3]

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. Pronunciations given are the most Anglicized; more Latinate pronunciations are also common, particularly /ɑː/ rather than /eɪ/ for stressed a.

Rank Plants Algae Fungi Animals Bacteria[4]
Division/Phylum -phyta /ˈfaɪtə/ -mycota /maɪˈkoʊtə/
Subdivision/Subphylum -phytina /fɨˈtaɪnə/ -mycotina /maɪkɵˈtaɪnə/
Class -opsida /ˈɒpsɨdə/ -phyceae /ˈfaɪʃiː/ -mycetes /maɪˈsiːtiːz/ -ia
Subclass -idae /ɨdiː/ -phycidae /ˈfɪsɨdiː/ -mycetidae /maɪˈsɛtɨdiː/ -idae
Superorder -anae /ˈeɪniː/
Order -ales /ˈeɪliːz/ -ales
Suborder -ineae /ˈɪnɨ.iː/ -ineae
Infraorder -aria /ˈɛəri.ə/
Superfamily -acea /ˈeɪʃə/ -oidea/ˈɔɪdi.ə/
Epifamily -oidae /ˈɔɪdiː/
Family -aceae /ˈeɪʃiː/ -idae /ɨdiː/ -aceae
Subfamily -oideae /ˈɔɪdɨ.iː/ -inae /ˈaɪniː/ -oideae
Infrafamily -odd /ɒd/[5]
Tribe -eae -ini /ˈaɪnaɪ/ -eae
Subtribe -inae /ˈaɪniː/ -ina /ˈaɪnə/ -inae
Infratribe -ad /æd/

Table notes:

  • 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.[6]
  • 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.[7]

All ranks

There is an indeterminate number of ranks, as a taxonomist may invent a new rank at will, at any time, if he or she feels this is necessary. In doing so, there are some restrictions, which will vary with the Nomenclature Code which applies.

The following is an artificial synthesis, solely for purposes of demonstration of relative rank (but see notes), from most general to most specific:[8]

Of these many ranks, the most basic is species. However, this is not to say that a taxon at any other rank may not be sharply defined, or that any species is guaranteed to be sharply defined. It varies from case to case. Ideally, nowadays, a taxon is intended to represent the phylogeny of the organisms under discussion, but in itself this is not a requirement.

Notes

  1. ^ International Commission on Zoological Nomenclature (1999) International Code of Zoological Nomenclature. Fourth Edition. - International Trust for Zoological Nomenclature, XXIX + 306 pp.
  2. ^ mnemonic-device.eu
  3. ^ thefreedictionary.com.
  4. ^ Bacteriologocal Code (1990 Revision)
  5. ^ For example, the chelonian infrafamilies Chelodd (Gaffney & Meylan 1988: 169) and Baenodd (ibid., 176).
  6. ^ ICZN article 27.2
  7. ^ As supplied by Gaffney & Meylan (1988).
  8. ^ For the general usage and coordination of zoological ranks between the phylum and family levels, including many intercalary ranks, see Carroll (1988). For additional intercalary ranks in zoology, see especially Gaffney & Meylan (1988); McKenna & Bell (1997); Milner (1988); Novacek (1986, cit. in Carroll 1988: 499, 629); and Paul Sereno's 1986 classification of ornithischian dinosaurs as reported in Lambert (1990: 149, 159). For botanical ranks, including many intercalary ranks, see Willis & McElwain (2002).
  9. ^ a b c d e f g h In zoological classification, the cohort and its associated group of ranks are inserted between the class group and the ordinal group. In botanical classification, the cohort group is inserted between the division (phylum) group and the class group: see Willis & McElwain (2002: 100–101). The cohort has also been used between infraorder and family in saurischian dinosaurs (Benton 2005).
  10. ^ a b c d These are movable ranks, most often inserted between the class and the legion or cohort. Nevertheless, their positioning in the zoological hierarchy may be subject to wide variation. For examples, see the Benton classification of vertebrates (2005).
  11. ^ a b c d e The supra-ordinal sequence gigaorder-megaorder-capaxorder-hyperorder (and the microorder, in roughly the position most often assigned to the parvorder) has been employed in turtles at least (Gaffney & Meylan 1988), while the parallel sequence magnorder-grandorder-mirorder figures in recently influential classifications of mammals. It is unclear from the sources how these two sequences are to be coordinated (or interwoven) within a unitary zoological hierarchy of ranks. Previously, Novacek (1986) and McKenna-Bell (1997) had inserted mirorders and grandorders between the order and superorder, but Benton (2005) now positions both of these ranks above the superorder.
  12. ^ Additionally, the terms biovar, morphovar and serovar designate bacterial strains (genetic variants) that are physiologically or biochemically distinctive. These are not taxonomic ranks, but are groupings of various sorts which may define a bacterial subspecies.

Bibliography

  • Benton, Michael J. 2005. Vertebrate Palaeontology, 3rd ed. Oxford: Blackwell Publishing. ISBN 0-632-05637-1. ISBN 978-0-632-05637-8
  • Brummitt, R.K., and C.E. Powell. 1992. Authors of Plant Names. Royal Botanic Gardens, Kew. ISBN 0947643443
  • Carroll, Robert L. 1988. Vertebrate Paleontology and Evolution. New York: W.H. Freeman & Co. ISBN 0-716-7-1822-7
  • Gaffney, Eugene S., and Peter A. Meylan. 1988. "A phylogeny of turtles." In M.J. Benton (ed.), The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds, 157–219. Oxford: Clarendon Press.
  • International Association for Plant Taxonomy. 2000. International Code of Botanical Nomenclature (Saint Louis Code), Electronic version. Retrieved on 2007-07-21.
  • Haris Abba Kabara. Karmos hand book for botanical names.
  • Lambert, David. 1990. Dinosaur Data Book. Oxford: Facts On File & British Museum (Natural History). ISBN 0-8160-2431-6
  • McKenna, Malcolm C., and Susan K. Bell (editors). 1997. Classification of Mammals Above the Species Level. New York: Columbia University Press. ISBN 0-231-11013-8
  • Milner, Andrew. 1988. "The relationships and origin of living amphibians." In M.J. Benton (ed.), The Phylogeny and Classification of the Tetrapods, Volume 1: Amphibians, Reptiles, Birds, 59–102. Oxford: Clarendon Press.
  • Novacek, Michael J. 1986. "The skull of leptictid insectivorans and the higher-level classification of eutherian mammals." Bulletin of the American Museum of Natural History 183: 1–112.
  • Sereno, Paul C. 1986. "Phylogeny of the bird-hipped dinosaurs (Order Ornithischia)." National Geographic Research 2: 234–56.
  • Willis, K.J., and J.C. McElwain. 2002. The Evolution of Plants. Oxford University Press. ISBN 0-19-850065-3

See also


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