Taxonomic rank

This graph shows the main taxonomic ranks: domain, kingdom, phylum, class, order, family, genus, and species. Here it demonstrates how taxonomic ranking is used to classify animals and earlier life forms related to the red fox, Vulpes vulpes.[1]
Life Domain Kingdom Phylum Class Order Family Genus Species
The hierarchy of biological classification's eight major taxonomic ranks. Intermediate minor rankings are not shown.

In biological classification, rank is the relative level of a group of organisms (a taxon) in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, order, class, phylum, kingdom, domain, etc.

A given rank subsumes under it less general categories, that is, more specific descriptions of life forms. Above it, each rank is classified within more general categories of organisms and groups of organisms related to each other through inheritance of traits or features from common ancestors. The rank of any species and the description of its genus is basic; which means that to identify a particular organism, it is usually not necessary to specify ranks other than these first two.[2]

Consider a particular species, the red fox Vulpes vulpes: its next rank, the genus Vulpes, comprises all the 'true foxes'. Their closest relatives are in the immediately higher rank, the family Canidae, which includes dogs, wolves, jackals, all foxes, and other caniforms; the next higher rank, the order Carnivora, includes feliforms and caniforms (lions, tigers, bears, hyenas, wolverines, and all those mentioned above), plus other carnivorous mammals. As one group of the class Mammalia, all of the above are classified among those with backbones in the Chordata phylum rank, and with them among all the animals in the Animalia kingdom rank. Finally, all of the above will find their earliest relatives somewhere in their domain rank Eukarya.

The International Code of Zoological Nomenclature defines rank 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)[3]

Main ranks

In his landmark publications, such as the Systema Naturae, Carolus Linnaeus used a ranking scale limited to: kingdom, class, order, genus, species, and one rank below species. Today, nomenclature is regulated by the nomenclature codes. There are seven main taxonomic ranks: kingdom, phylum or division, 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
phylumdivisio phylum (in zoology)division (in botany)
classis class
ordo order
familia family
genus genus
species species

A taxon is usually assigned a rank when it is given its formal name. The basic ranks are species and genus. When an organism is given a species name it is assigned to a genus, and the genus name is part of the species name.

The species name is also called a binomial, that is, 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 it is capitalized; sapiens indicates the species and it is not capitalized.

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).

Names of zoological taxa

Ranks in botany

According to Art 3.1 of the International Code of Nomenclature for algae, fungi, and plants (ICN) 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 ICN explicitly mentions:[4]

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)
subsection
series (series)
subseries

species (species)

subspecies
variety (varietas)
subvarietas
form (forma)
subforma

There are definitions of the following taxonomic categories in the International Code of Nomenclature for Cultivated Plants: cultivar group, cultivar, grex.

The rules in the ICN 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 ranks used by Linnaeus only names of genera, species and varieties are still used.

Taxa at the rank of genus and above have a botanical name in one part (unitary name); those at the rank of species and above (but below genus) have a botanical name in two parts (binary name); all taxa below the rank of species have a botanical name in three parts (an infraspecific name). To indicate the rank of the infraspecific name, a "connecting term" is needed. Thus Poa secunda subsp. juncifolia, where "subsp." is an abbreviation for "subspecies", is the name of a subspecies of Poa secunda.

Hybrids can be specified either by a "hybrid formula" that specifies the parentage, or may be given a name. For hybrids getting a hybrid name, the same ranks apply, prefixed with notho (Greek: 'bastard'), with nothogenus as the highest permitted rank.

Outdated names for botanical ranks

If a different term for the rank was used in an old publication, but the intention is clear, botanical nomenclature specifies certain substitutions:

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 Bacteria
Phylum or Division Arthropoda Chordata Magnoliophyta (Tracheophyta) Basidiomycota Proteobacteria
Subphylum or subdivision Hexapoda Vertebrata Magnoliophytina (Euphyllophytina) Agaricomycotina
Class Insecta Mammalia Magnoliopsida (Equisetopsida) Agaricomycetes Gammaproteobacteria
Subclass Pterygota Theria Rosidae (Magnoliidae) Agaricomycetidae
Superorder Euarchontoglires Rosanae
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

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 Bacteria[7] Plants Algae Fungi Animals
Division/Phylum -phyta /ˈfaɪtə/ -phycota /ˈfaɪkoʊtə/ -mycota /maɪˈkoʊtə/
Subdivision/Subphylum -phytina /fᵻˈtaɪnə/ -phycotina /fᵻkoʊˈtaɪnə/ -mycotina /maɪkoʊˈtaɪnə/
Class -ia /iə/ -opsida /ˈɒpsᵻdə/ -phyceae /ˈfaɪʃiː/ -mycetes /maɪˈsiːtiːz/
Subclass -idae /ᵻdiː/ -phycidae /ˈfɪsᵻdiː/ -mycetidae /maɪˈsɛtᵻdiː/
Superorder -anae /ˈeɪniː/
Order -ales /ˈeɪliːz/
Suborder -ineae /ˈɪnᵻ.iː/
Infraorder -aria /ˈɛəriə/
Superfamily -acea /ˈeɪʃə/ -oidea /ˈɔɪdiə/
Epifamily -oidae /ˈɔɪdiː/
Family -aceae /ˈeɪʃiː/ -idae /ᵻdiː/
Subfamily -oideae /ˈɔɪdᵻiː/ -inae /ˈaɪniː/
Infrafamily -odd /ɒd/[8]
Tribe -eae /ᵻiː/ -ini /ˈaɪnaɪ/
Subtribe -inae /ˈaɪniː/ -ina /ˈaɪnə/
Infratribe -ad /æd/ or -iti /ˈaɪti/
Table notes

All ranks

There is an indeterminate number of ranks, as a taxonomist may invent a new rank at will, at any time, if they feel 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:[11]

Empire
Subdomain(biology)

Significance and problems

Ranks are assigned based on subjective dissimilarity, and do not fully reflect the gradational nature of variation within nature. In most cases, higher taxonomic groupings arise further back in time: not because the rate of diversification was higher in the past, but because each subsequent diversification event results in an increase of diversity and thus increases the taxonomic rank assigned by present-day taxonomists.[16]

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, a taxon is intended to represent a clade, that is, the phylogeny of the organisms under discussion, but this is not a requirement.

Classification, in which all taxa have formal ranks, cannot adequately reflect our knowledge about phylogeny; at the same time, if taxon names are dependent on ranks, rank-free taxa can't be supplied with names. This problem is dissolved in cladoendesis, where the specially elaborated rank-free nomenclatures are used.[17][18]

See also

References

  1. ↑ http://www.123rf.com /clipart-vector/vulpes_vulpes.html
  2. ↑ International Code of Nomenclature for algae, fungi, and plants, Melbourne Code, 2012, articles 2 and 3
  3. ↑ International Commission on Zoological Nomenclature (1999), International Code of Zoological Nomenclature. Fourth Edition, International Trust for Zoological Nomenclature
  4. ↑ International Code of Nomenclature for algae, fungi, and plants, Melbourne Code, 2012, articles 3 and 4
  5. ↑ Stearn, W.T. 1992. Botanical Latin: History, grammar, syntax, terminology and vocabulary, Fourth edition. David and Charles.
  6. ↑ Chase, M.W.; Reveal, J.L. (2009), "A phylogenetic classification of the land plants to accompany APG III", Botanical Journal of the Linnean Society 161 (2): 122–127, doi:10.1111/j.1095-8339.2009.01002.x
  7. ↑ Bacteriologocal Code (1990 Revision)
  8. ↑ For example, the chelonian infrafamilies Chelodd (Gaffney & Meylan 1988: 169) and Baenodd (ibid., 176).
  9. ↑ ICZN article 29.2
  10. ↑ As supplied by Gaffney & Meylan (1988).
  11. ↑ 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).
  12. 1 2 3 4 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).
  13. 1 2 3 4 In zoological classification, the cohort and its associated group of ranks are inserted between the class group and the ordinal group. The cohort has also been used between infraorder and family in saurischian dinosaurs (Benton 2005). In botanical classification, the cohort group has sometimes been inserted between the division (phylum) group and the class group: see Willis & McElwain (2002: 100–101), or has sometimes been used at the rank of order, and is now considered to be an obsolete name for order: See International Code of Nomenclature for algae, fungi, and plants, Melbourne Code 2012, Article 17.2.
  14. 1 2 3 4 5 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.
  15. ↑ 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.
  16. ↑ Gingerich, P. D. (1987). "Evolution and the fossil record: patterns, rates, and processes". Canadian Journal of Zoology 65 (5): 1053–1060. doi:10.1139/z87-169.
  17. ↑ Kluge N.J. 1999. A system of alternative nomenclatures of supra-species taxa. Linnaean and post-Linnaean principles of systematics. // Entomological Review 79(2): 133-147
  18. ↑ Kluge N.J. 2010. Circumscriptional names of higher taxa in Hexapoda. // Bionomina 1: 15–55

Bibliography

Taxonomic ranks
Magnorder
Domain/Superkingdom Superphylum/Superdivision Superclass Superorder Superfamily Supertribe Superspecies
Kingdom Phylum/Division Class Legion Order Family Tribe Genus Species
Subkingdom Subphylum Subclass Cohort Suborder Subfamily Subtribe Subgenus Subspecies
Infrakingdom/Branch Infraphylum Infraclass Infraorder Section Infraspecies
Microphylum Parvclass Parvorder Series Variety
Form
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