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 fungi, and plants|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
Botanical ranks categorize organisms based on their relationships. They start with Kingdom, then move to Division, Class, Order, Family, Genus, and Species. Taxa at each rank generally possess shared characteristics and evolutionary history. Understanding these ranks aids in taxonomy and studying biodiversity.| 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.
Names of botanical taxa
Taxa at the rank of genus and above have a botanical name in one part ; those at the rank of species and above have a botanical name in two parts ; all taxa below the rank of species have a botanical name in three parts. To indicate the rank of the infraspecific name, a "connecting term" is needed. Thus Poa secunda subsp. juncifolia, where "subsp". is an abbreviation for "subspecies", is the name of a subspecies of Poa secunda.Hybrids can be specified either by a "hybrid formula" that specifies the parentage, or may be given a name. For hybrids receiving a hybrid name, the same ranks apply, prefixed with notho, with nothogenus as the highest permitted rank.
Outdated names for botanical ranks
If a different term for the rank was used in an old publication, but the intention is clear, botanical nomenclature specifies certain substitutions:- If names were "intended as names of orders, but published with their rank denoted by a term such as": "cohors" , "nixus", "alliance", or "Reihe" instead of "order", they are treated as names of orders.
- "Family" is substituted for "order" or "natural order" under certain conditions where the modern meaning of "order" was not intended.
- "Subfamily" is substituted for "suborder" under certain conditions where the modern meaning of "suborder" was not intended.
- In a publication prior to 1 January 1890, if only one infraspecific rank is used, it is considered to be that of variety. This commonly applies to publications that labelled infraspecific taxa with Greek letters, α, β, γ,...
Examples
Classifications of five species follow: the fruit fly familiar in genetics laboratories, humans, the peas used by Gregor Mendel in his discovery of genetics, 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.;Table notes
- In order to keep the table compact and avoid disputed technicalities, some common and uncommon intermediate ranks are omitted. For example, the mammals of Europe, Africa, and upper North America are in class Mammalia, legion Cladotheria, sublegion Zatheria, infralegion Tribosphenida, subclass Theria, clade Eutheria, clade Placentalia – but only Mammalia and Theria are in the table. Legitimate arguments might arise if the commonly used clades Eutheria and Placentalia were both included, over which is the rank "infraclass" and what the other's rank should be, or whether the two names are synonyms.
- The ranks of higher taxa, especially intermediate ranks, are prone to revision as new information about relationships is discovered. For example, the flowering plants have been downgraded from a division to a subclass, and the superorder has become the rank that distinguishes the major groups of flowering plants. The traditional classification of primates has been modified by new classifications such as McKenna and Bell with Theria and Eutheria assigned lower ranks between infraclass and the order Primates. These differences arise because there are few available ranks and many branching points in the fossil record.
- Within species further units may be recognised. Animals may be classified into subspecies or morphs. Plants may be classified into subspecies or varieties, with cultivated plants getting a cultivar name. Bacteria may be classified by strains.
Terminations of names
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 as set out in the table below.Pronunciations given are the most Anglicized. More Latinate pronunciations are also common, particularly rather than for stressed a.
;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 also formed from a generic name, or are descriptive.
- For animals, there are standard suffixes for taxa only up to the rank of superfamily. Uniform suffix has been suggested in AAAS as -ida for orders, for example; protozoologists seem to adopt this system. Many metazoan orders also have such suffix, e.g. Hyolithida and Nectaspida (Naraoiida).
- Forming a name based on a generic name may be not straightforward. For example, the homo has the genitive hominis, thus the genus Homo is in the Hominidae, not "Homidae".
- The ranks of epifamily, infrafamily and infratribe 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.
- In virology, the formal endings for taxa of viroids, of satellite nucleic acids, and of viriforms are similar to viruses, only -vir- is replaced by -viroid-, -satellit- and -viriform-. The extra levels of realm and subrealm end with -viria and -vira respectively.
All ranks
There is an indeterminate number of ranks, as a taxonomist may invent a new rank at will, at any time, if they feel this is necessary. In doing so, there are some restrictions, which will vary with the nomenclature code that applies.The following is an artificial synthesis, solely for purposes of demonstration of absolute rank, from most general to most specific. Terms marked with a skull and crossbones symbol are specifically used in virology, those marked with a biohazard symbol are used in bacteriology, those marked with a florette are used in botany, those marked with an umbrella are used in mycology, those marked with an asterisk are used in zoology, those marked with a fisheye are used in ichthyology, those marked with a flower are used in lepidopterology:
| Rank | Domain Empire Realm☠ | Kingdom | Phylum Division✿ | Class | Division* | Legion* |
| Subrank |
|
| Rank | Cohort* | Order | Section* | Family | Tribe | Genus |
| Subrank |
|
| Rank | Section✿ | Series | Species | Variety/Varietas✿ "Form"/"Morph"* Aberration⚘ | Form/Forma✿ | "Race" |
| Subrank |
|
Significance and problems
Ranks are assigned based on subjective dissimilarity, and do not fully reflect the gradational nature of variation within nature. These problems were already identified by Willi Hennig, who advocated dropping them in 1969, and this position gathered support from Graham C. D. Griffiths only a few years later. In fact, these ranks were proposed in a fixist context and the advent of evolution sapped the foundations of this system, as was recognised long ago; the introduction of The Code of Nomenclature and Check-list of North American Birds Adopted by the American Ornithologists' Union published in 1886 states "No one appears to have suspected, in 1842 , that the Linnaean system was not the permanent heritage of science, or that in a few years a theory of evolution was to sap its very foundations, by radically changing men's conceptions of those things to which names were to be furnished." Such ranks are used simply because they are required by the rank-based codes; because of this, some systematists prefer to call them nomenclatural ranks. In most cases, higher taxonomic groupings arise further back in time, simply because the most inclusive taxa necessarily appeared first. Furthermore, the diversity in some major taxa is better known than that of others not because they are more diverse than other taxa, but because they are more easily sampled and studied than other taxa, or because they attract more interest and funding for research.Of these many ranks, many systematists consider that the most basic is the species, but this opinion is not universally shared. Thus, species are not necessarily more sharply defined than taxa at any other rank, and in fact, given the phenotypic gaps created by extinction, in practice, the reverse is often the case. Ideally, a taxon is intended to represent a clade, that is, the phylogeny of the organisms under discussion, but this is not a requirement of the zoological and botanical codes.
A classification in which all taxa have formal ranks cannot adequately reflect knowledge about phylogeny. Since taxon names are dependent on ranks in rank-based nomenclature, taxa without ranks cannot be given names. Alternative approaches, such as phylogenetic nomenclature, as implemented under the PhyloCode and supported by the International Society for Phylogenetic Nomenclature, or using circumscriptional names, avoid this problem. The theoretical difficulty with superimposing taxonomic ranks over evolutionary trees is manifested as the boundary paradox which may be illustrated by Darwinian evolutionary models.
There are no rules for how many species should make a genus, a family, or any other higher taxon. It should be a natural group, as judged by a biologist, using all the information available to them. Equally ranked higher taxa in different phyla are not necessarily equivalent in terms of time of origin, phenotypic distinctiveness or number of lower-ranking included taxa. Of all criteria that have been advocated to rank taxa, age of origin has been the most frequently advocated. Willi Hennig proposed it in 1966, but he concluded in 1969 that this system was unworkable and suggested dropping absolute ranks. However, the idea of ranking taxa using the age of origin persists under the name of time banding, and is still advocated by several authors. For animals, at least the phylum rank is usually associated with a certain body plan, which is also, however, an arbitrary criterion.