Taxonomic rank (rank, category, taxonomic category) is an abstract term used in the scientific classification, or taxonomy, of organisms. Taxonomic rank indicates the level of a taxon in the taxonomic hierarchy. Taxa ranked at a particular taxonomic rank are groupings of organisms at the same classification level.
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) International Code of Zoological Nomenclature. Fourth Edition. - International Trust for Zoological Nomenclature, XXIX + 306 pp.
Main taxonomic ranks
Today, 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 for botany.
There are 8 main taxonomic ranks: domain, kingdom, phylum or division1, class, order, family, genus, species.
|Main taxonomic ranks|
- 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.
In zoology and in botanical nomenclature, a taxon is usually assigned to a taxonomic rank in a hierarchy. The basic rank is that of species, and if an organism is named it most often will receive a species name. 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.
A binomial is a two-word name which is used to describe a particular species. For example, the binomial name for a human is Homo sapiens. This is italicised 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 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 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.
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)
- 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 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:
- - - 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. 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 usual 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.
- 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).
- A mnemonic for remembering the order of the taxa is: Do Koalas Prefer Chocolate Or Fruit, Generally Speaking? Another easy one is Damn, Kinky People Can Often Find Great Sex. Other mnemonics are available at and.
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.
- 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.
All taxonomic ranks
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The following table lists all taxonomic ranks including those which are not in use today and those which are identical with other ranks.
- Notes to table
- 1 Level in plant taxonomy. 2 Level in animal taxonomy.
- R.K. Brummitt & C.E. Powell (1992). Authors of Plant Names. Royal Botanic Gardens, Kew. ISBN 0947643443.
- Eugene S. Gaffney & 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.
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