Taxonomic rank

In biological classification, rank is the level (the relative position) in a taxonomic hierarchy. Examples of taxonomic ranks are species, genus, family, and class. Each rank subsumes under it a number of less general categories. The rank of species, and specification of the genus to which the species belongs is basic, which means that it may not be necessary to specify ranks other than these[1]. 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)[2]

Contents

Main ranks

"2.1. Every individual plant is treated as belonging to an indefinite number of taxa of consecutively subordinate rank, among which the rank of species (species) is basic."

In his landmark publications, such as the Systema Naturae, Carl Linnaeus used a ranking scale limited to: 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 division (see table), 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 used in zoology
2 used in botany

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).

Names of zoological taxa

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.)

Out-dated names for botanical ranks

If a different term for the rank was used in an in 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 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

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 Plants Algae Fungi Animals Bacteria[6]
Division/Phylum -phyta /ˈfaɪtə/ -mycota /maɪˈkoʊtə/
Subdivision/Subphylum -phytina /fɨˈtaɪnə/ -mycotina /maɪkɵˈtaɪnə/
Class -opsida /ˈɒpsɨdə/ -phyceae /ˈfaɪʃiː/ -mycetes /maɪˈsiːtiːz/ -ia /iə/
Subclass -idae /ɨdiː/ -phycidae /ˈfɪsɨdiː/ -mycetidae /maɪˈsɛtɨdiː/ -idae /ɨdiː/
Superorder -anae /ˈeɪniː/
Order -ales /ˈeɪliːz/ -ales /ˈeɪliːz/
Suborder -ineae /ˈɪnɨ.iː/ -ineae /ˈɪnɨ.iː/
Infraorder -aria /ˈɛəri.ə/
Superfamily -acea /ˈeɪʃə/ -oidea /ˈɔɪdi.ə/
Epifamily -oidae /ˈɔɪdiː/
Family -aceae /ˈeɪʃiː/ -idae /ɨdiː/ -aceae /ˈeɪʃiː/
Subfamily -oideae /ˈɔɪdɨ.iː/ -inae /ˈaɪniː/ -oideae /ˈɔɪdɨ.iː/
Infrafamily -odd /ɒd/[7]
Tribe -eae /ɨ.iː/ -ini /ˈaɪnaɪ/ -eae /ɨ.iː/
Subtribe -inae /ˈaɪniː/ -ina /ˈaɪnə/ -inae /ˈaɪniː/
Infratribe -ad /æd/

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 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:[10]

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 Code of Botanical Nomenclature Online, Vienna Code, 2005, articles 2 and 3
  2. ↑ International Commission on Zoological Nomenclature (1999) International Code of Zoological Nomenclature. Fourth Edition. - International Trust for Zoological Nomenclature, XXIX + 306 pp.
  3. ↑ Stearn, W.T. 1992. Botanical Latin: History, grammar, syntax, terminology and vocabulary, Fourth edition. David and Charles.
  4. ↑ mnemonic-device.eu
  5. ↑ thefreedictionary.com.
  6. ↑ Bacteriologocal Code (1990 Revision)
  7. ↑ For example, the chelonian infrafamilies Chelodd (Gaffney & Meylan 1988: 169) and Baenodd (ibid., 176).
  8. ↑ ICZN article 27.2
  9. ↑ As supplied by Gaffney & Meylan (1988).
  10. ↑ 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).
  11. ↑ 11.0 11.1 11.2 11.3 11.4 11.5 11.6 11.7 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 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: See International Code of Botanical Nomenclature, Vienna Code 2006, Article 17.2. The cohort has also been used between infraorder and family in saurischian dinosaurs (Benton 2005).
  12. ↑ 12.0 12.1 12.2 12.3 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. ↑ 13.0 13.1 13.2 13.3 13.4 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.
  14. ↑ 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

See also