Taxonomic rank
In biological taxonomy, taxonomic rank refers to either the relative level or the absolute level of a group of organisms as visualized in a hierarchy of biological classification that reflects evolutionary relationships. Some authors prefer to use the term nomenclatural rank, contending that according to some definitions, the ranking of organisms is more accurately described under nomenclature rather than that of taxonomy. Thus, the most inclusive taxons, or clades, are assigned the highest ranks of classification, where the least inclusive ones are given the lowest ranks.
Ranks can be either "absolute"in which several descriptive terms such as species, genus, tribe, family, order, class, phylum, kingdom, and domain designate rankor "relative", where instead ranks are designated by an indented taxonomy in which the level of indentation reflects the rank. This page emphasizes absolute ranks. And the rank-based codes all require absolute ranks.
Consider a particular species, the red fox, Vulpes vulpes, and its classification and ranking in context of the Zoological Code: the specific epithet vulpes identifies the particular species vulpes as found in the genus Vulpes ; which genus groups all species of the "true" foxes at a rank assigned one level above that of the specific epithet vulpes. The closest relatives of the foxes are grouped in the family taxon Canidae, which includes dogs, wolves, and jackals. The next higher taxon in hierarchy is the order Carnivora, which includes the suborder caniforms: all those mentioned above plus bears, seals, weasels, skunks, and raccoons; and the suborder feliforms: cats, civets, hyenas, mongooses. Carnivorans are but one group of the hairy, warm-blooded, nursing members of the class Mammalia, which all in turn are included among the animals with notochords in the phylum Chordata. And all those listed above are collected among all animals in the kingdom Animalia. Finally at the highest rank, all the above are grouped together with all other organisms possessing cell nuclei in the domain taxon Eukarya.
More generally, taxa describe and portray the hierarchical grouping of organisms resulting from the classifications process; and the assigned ranks of the taxa indicate the relative positions of such groups within the hierarchy. High-ranking taxa contain more sub-taxa groups than lower-ranking taxa, such as phyla or generaas illustrated by the inverted pyramid graphic of taxonomic rank. And species, or any subspecies, contain the least number of sub-taxa groups, or none at all. The ranking of a given taxon reflects the evolutionary inheritance of traits or molecular features from an ancestor common with other taxa.
The binomial name is basic; which means that to identify a given organism, it is usually not necessary to specify the names of ranks other than the first twogenus and species within a taxonomy comprising a rank-based code.
The International Code of Zoological Nomenclature defines rank as: "The level, for nomenclatural purposes, of a taxon in a taxonomic hierarchy." Note that discussions on this page generally assume that taxa are clades, but such is not required by either the Zoological Code or the Botanical Code, ; and some experts on biological nomenclature hold that it should not be required. And in that case, the hierarchy of taxahence, their rankswould not necessarily reflect the hierarchy of clades.
History
While older approaches to taxonomic classification were phenomenological, forming groups on the basis of similarities in appearance, organic structure and behavior, two important new methods developed in the second half of the 20th century changed drastically taxonomic practice. One is the advent of cladistics, which stemmed from the works of the German entomologist Willi Hennig. Cladistics is a method of classification of life forms according to the proportion of characteristics that they have in common. It is assumed that the higher the proportion of characteristics that two organisms share, the more recently they both came from a common ancestor. The second one is molecular systematics, based on genetic analysis, which can provide much additional data that prove especially useful when few phenotypic characters can resolve relationships, as, for instance, in many viruses, bacteria and archaea, or to resolve relationships between taxa that arose in a fast evolutionary radiation that occurred long ago, such as the main taxa of placental mammals.Main ranks
In his landmark publications, such as the Systema Naturae, Carl Linnaeus used a ranking scale limited to kingdom, class, order, genus, species, and one rank below species. Today, the nomenclature is regulated by the nomenclature codes. There are seven main taxonomic ranks: kingdom, phylum or division, class, order, family, genus, and species. In addition, domain is now widely used as a fundamental rank. Both regio and dominium have been proposed as the Latin form, and it was adopted into the International Code of Nomenclature of Prokaryotes in 2023 with the Latin form dominium.| Latin | English |
| dominium | domain |
| regnum | kingdom |
| phylum | phylum / division |
| 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 or 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. While not always used, some species include a subspecific epithet. For instance, modern humans are Homo sapiens sapiens, or H. sapiens sapiens.
In zoological nomenclature, higher taxon names are normally not italicized, but the Botanical Code, the Prokaryotic Code, the , the draft BioCode and the PhyloCode all recommend italicizing all taxon names.
Ranks in zoology
There are rules applying to 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 the following ranks for these categories:
- Family-groups
- *Superfamily
- *Family
- *Subfamily
- *Tribe
- *Subtribe
- Genus-groups
- *Genus
- *Subgenus
- Species-groups
- *Species
- *Subspecies
At higher ranks a lower level may be denoted by adding the prefix "infra", meaning lower, to the rank. For example, infraorder or infrafamily.
Names of zoological taxa
- A taxon above the rank of species has a scientific name in one part.
- A species has a name typically composed of two parts : generic name + specific name; for example Canis lupus. Sometimes the name of a subgenus can be intercalated between the genus name and the specific epithet, which yields a trinomial name that should not be confused with that of a subspecies. An example is Lithobates catesbeianus, which designates a species that belongs to the genus Lithobates and the subgenus Aquarana.
- A subspecies has a name composed of three parts : generic name + specific name + subspecific name; for example Canis lupus italicus. As there is only one possible rank below that of species, no connecting term to indicate rank is needed or used.
Ranks in botany
| Rank | Type | Suffix |
| kingdom | primary | |
| subregnum | further | |
| division phylum | primary | ‑phyta -mycota |
| subdivision or subphylum | further | ‑phytina -mycotina |
| class | primary | ‑opsida ‑phyceae -mycetes |
| subclass | further | ‑idae ‑phycidae -mycetidae |
| order | primary | -ales |
| Suborder | further | -ineae |
| family | primary | -aceae |
| subfamily | further | ‑oideae |
| tribe | secondary | -eae |
| subtribe | further | ‑inae |
| genus | primary | |
| subgenus | further | |
| section | secondary | |
| subsection | further | |
| series | secondary | |
| subseries | further | |
| species | primary | |
| subspecies | further | |
| variety | secondary | |
| subvariety | further | |
| form | secondary | |
| subform | further |
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.