Definitions

Taxonomic_rank

Taxonomic rank

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

Carl Linnaeus devised Linnaean taxonomy using a ranking scale: kingdom, class, order, genus, species, and variety.

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
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.

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

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 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:


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.

Examples

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.

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 Magnoliidae 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 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.

Rank Plants Algae Fungi Animals Bacteria
Division/Phylum -phyta -mycota
Subdivision/Subphylum -phytina -mycotina
Class -opsida -phyceae -mycetes -ia
Subclass -idae -phycidae -mycetidae -idae
Superorder -anae
Order -ales -ales
Suborder -ineae -ineae
Infraorder -aria
Superfamily -acea -oidea
Epifamily -oidae
Family -aceae -idae -aceae
Subfamily -oideae -inae -oideae
Infrafamily -odd
Tribe -eae -ini -eae
Subtribe -inae -ina -inae
Infratribe -ad

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.
  • 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

The following table lists all taxonomic ranks including those which are not in use today and those which are identical with other ranks.

All taxonomic ranks
Latin English Notes
  superdomain  
regio domain  
  subdomain  
  infradomain  
imperium empire  
superregnum superkingdom  
  suprakingdom  
  midkingdom  
regnum kingdom  
subregnum subkingdom  
  interkingdom  
  branch  
  infrakingdom  
superphylum superphylum  
  supraphylum  
  midphylum  
phylum phylum  
subphylum subphylum  
infraphylum infraphylum  
divisio division1  
subdivisio subdivision1  
claudius claudius  
superclassis superclass  
  grade  
classis class  
subclassis subclass  
  infraclass  
parvclassis parvclass  
superdivisio superdivision2  
divisio division2  
subdivisio subdivision2  
sectio section2  
  subsection2  
  group  
  subgroup  
  superlegion  
legio legion  
  sublegion  
  infralegion  
supercohors supercohort  
cohors cohort  
subcohors subcohort  
  magnorder  
superordo superorder  
  series  
  subseries  
  grandorder  
  gigaorder  
  megaorder  
  mirorder  
ordo order  
hyperordo hyperorder  
subordo suborder  
infraordo infraorder  
  parvorder  
falanx    
superfamilia superfamily  
  family group  
familia family  
  subfamily group  
subfamilia subfamily  
  infrafamily  
supertribus supertribe  
tribus tribe  
subtribus subtribe  
  infratribe  
supergenus supergenus  
  genus group  
genus genus  
subgenus subgenus  
  infragenus  
supersectio supersection1  
sectio section1  
subsectio subsection1  
  infrasection1  
  superseries1  
series series1  
  subseries1  
  infraseries1  
superspecies superspecies  
  complex  
  species group  
  species subgroup  
  aggregate  
  synklepton  
species species  
  microspecies  
klepton klepton  
  aggregate  
subspecies subspecies  
  infraspecies  
  group  
hybrid hybrid  
convarietas convariety  
supervarietas supervariety  
varietas variety  
subvarietas subvariety  
  infravariety  
natio    
superforma superform  
forma, morpha form  
subforma subform  
  infraform  
  cultivar group  
  cultivar  
  group of breeds  
  section of breeds  
  breed, race  
  strain  
aberratio aberration  
  serogroup  
  serotype, serovar  
  biotype, biovar  
  pathotype, pathovar  
  population  
lusus lusus  
Notes to table: 1 Level in plant taxonomy. 2 Level in animal taxonomy.

A summary of this scheme, from most general to most specific, would be:

Of these many ranks, the only one that has an exact biological definition is species. The other levels are intended to represent the phylogeny of the organisms under discussion, and are to some extent a matter of judgement. For most groups of organisms, not all the ranks would actually be used; they have been defined to deal with the most complicated cases, such as insects and vertebrates.

Notes

Bibliography

  • Benton, Michael J. 2005. Vertebrate Palaeontology, 3rd ed. Oxford: Blackwell Publishing. ISBN 0-632-05637-1. ISBN-13: 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

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