Taxon

The hierarchy of scientific classification's major eight taxonomic ranks. Intermediate minor rankings are not shown.

Taxon (plural taxa) is a name designating a taxonomic category or taxonomic grouping of extant (living) or extinct organisms, such as species, genus, order, or phylum. Another name for this is taxonomic unit.

In biological classification, extinct and living species of organisms are categorized into particular groups. Each taxonomic grouping, or taxon, is assigned a taxonomic rank and can be placed at a particular level in a systematic hierarchy, ideally reflecting evolutionary relationships. The major eight taxonomic ranks, starting from the individual organism, are species, genus, family, order, class, phylum, kingdom, and domain. There also are intermediate minor rankings between these, such as subclass, subspecies, and superfamily.

Overview

A distinction is to be made between taxa/taxonomy and classification/systematics. The former refers to biological names and the rules of naming. The latter refers to rank ordering of taxa according to presumptive evolutionary (phylogenetic) relationships.

Note: "Phylum" applies formally to any biological domain, but traditionally it was always used for animals, whereas "Division" was traditionally often used for plants, fungi, etc.

A simple mnemonic phrase to remember the sequence of taxonomic levels is: "Dignified Kings Play Chess On Fine Green Silk"; another, highly expedient example is "King Philip's Class Orders the Family Genius to Speak".

A prefix is used to indicate a ranking of lesser importance. The prefix super- indicates a rank above, the prefix sub- indicates a rank below. In zoology the prefix infra- indicates a rank below sub-. For instance:

Superclass

Class

Subclass

Infraclass

Rank is relative, and restricted to a particular systematic schema. For example, liverworts have been grouped, in various systems of classification, as a family, order, class, or division (phylum). The use of a narrow set of ranks is challenged by users of cladistics; for example, the mere 10 ranks traditionally used between animal families (governed by the ICZN) and animal phyla (usually the highest relevant rank in taxonomic work) often cannot adequately represent the evolutionary history as more about a lineage's phylogeny becomes known. In addition, the class rank is quite often not an evolutionary but a phenetical and paraphyletic group and as opposed to those ranks governed by the ICZN, can usually not be made monophyletic by exchanging the taxa contained therein. This has given rise to phylogenetic taxonomy and the ongoing development of the PhyloCode, which is to govern the application of taxa to clades.

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	phylum1
divisio	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 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.

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.

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