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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, (such as the Eukarya and ) are assigned the highest ranks of classification, where the least inclusive ones (such as Homo sapiens, Bufo bufo, Tyrannosaurus rex, Vulpes vulpes) 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 (such as the Zoological Code, the Botanical Code, the Code for Cultivated Plants, the Prokaryotic Code, and the Code for Viruses) all require absolute ranks. (However, absolute ranks are not required in all nomenclature, as for instance the PhyloCode, (the code of phylogenetic nomenclature), does not 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 (small v) identifies the particular species vulpes (red fox) as found in the genus Vulpes (capital V); 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 Caniformia: all those mentioned above plus bears, seals, weasels, skunks, and raccoons; and the suborder Feliformia: 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 (e.g., domains or kingdoms) 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 phenotypic trait 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 (i.e., genus and specific epithet)within a taxonomy comprising a rank-based code. (However, this is not true globally because most rank-based codes are independent of each other, and there are many inter-code homonyms (i.e., the same name used for different organisms among the several codes)typically for an animal or for a taxon covered by a given rank-based code. For this reason, attempts were made at creating a BioCode that would regulate all taxon names; but this mission has failed thus far due largely to firmly entrenched naming traditions among the communities.)
The International Code of Zoological Nomenclature defines rank 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)." Note that discussions on this page generally assume that taxa are (monophyletic groups of organisms), but such is not required by either the Zoological Code or the Botanical Code, (i.e., the International Code of Nomenclature for algae, fungi, and plants); and some experts on biological nomenclature hold that it should not be required. (2026). 9781003182535, CRC Press. ISBN 9781003182535 And in that case, the hierarchy of taxahence, their rankswould not necessarily reflect the hierarchy of clades.
| +Main taxonomic ranks |
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 Code for Viruses, the draft BioCode and the PhyloCode all recommend italicizing all taxon names (of all ranks).
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:
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. Among "genus-group names" and "species-group names" no further ranks are officially allowed, which creates problems when naming taxa in these groups in speciose clades, such as Rana. Zoologists sometimes use additional terms such as "species group", "species subgroup", "species complex" and "superspecies" for convenience as extra, but unofficial, ranks between the subgenus and species levels in taxa with many species, e.g. the genus Drosophila. (Note the potentially confusing use of "species group" as both a category of ranks as well as an unofficial rank itself. For this reason, Alain Dubois has been using the alternative expressions "nominal-series", "family-series", "genus-series" and "species-series" (among others) at least since 2000.)
At higher ranks (family and above) a lower level may be denoted by adding the prefix "infra", meaning lower, to the rank. For example, infraorder (below suborder) or infrafamily (below subfamily).
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.
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 (Greek: 'bastard'), with nothogenus as the highest permitted rank.
Pronunciations given are the most Anglicized. More Latinate pronunciations are also common, particularly rather than for stressed a.
The following is an artificial synthesis, solely for purposes of demonstration of absolute rank (but see notes), from most general to most specific.For the general usage of zoological ranks between the phylum and family levels, including many intercalary ranks, see Carroll (1988). For additional intercalary ranks in zoology, see especially Gaffney & Meylan (1988); McKenna & Bell (1997); Milner (1988); Novacek (1986, cit. in Carroll 1988: 499, 629); and Paul Sereno's 1986 classification of dinosaurs as reported in Lambert (1990: 149, 159). For botanical ranks, including many intercalary ranks, see Willis & McElwain (2002). 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:
Of these many ranks, many systematists consider that the most basic (or important) 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 phylogenetics 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 (Linnaean) 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 (that is, a taxon in a category above the species level).Stuessy, T.F. (2009). Plant Taxonomy: The Systematic Evaluation of Comparative Data. 2nd ed. Columbia University Press, p. 175.Brusca, R.C. & Brusca, G.J. (2003). Invertebrates. 2nd ed. Sunderland, Massachusetts: Sinauer Associates, pp. 26–27. It should be a natural group (that is, non-artificial, non-polyphyletic), 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 (e.g., it is incorrect to assume that families of insects are in some way evolutionarily comparable to families of mollusks). 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 (either as the sole criterion, or as one of the main ones) 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.
Names of zoological taxa
Ranks in botany
Botanical ranks categorize organisms based (often) on their relationships (monophyly is not required by that clade, which does not even mention this word, nor that of "clade"). They start with Kingdom, then move to Division (or Phylum), 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.
+Ranks in ICN ‑phytina
-mycotina (fungi)‑idae (plant)
‑phycidae (algae)
-mycetidae (fungi)-ineae ‑oideae ‑inae
Names of botanical taxa
Taxa at the rank of genus and above have a botanical name in one part (unitary name); those at the rank of species and above (but below genus) have a botanical name in two parts (binary name); all taxa below the rank of species have a botanical name in three parts (an infraspecific name). 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.
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:
Examples
Classifications of five species follow: the fruit fly familiar in genetics laboratories ( Drosophila melanogaster), ( Homo sapiens), the 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.
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.
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.
+ Upper Ranks +Lower Ranks
!Rank
!Section✿
!Series
! Species
!Variety/Varietas✿
"Form"/"Morph"*
Aberration⚘
!Form/Forma✿
!"Race"
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 when, 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 (such as and Flowering plant) is better known than that of others (such as Fungus, and ) 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.
Enigmatic taxa
Enigmatic taxa are taxonomic groups whose broader relationships are unknown or undefined.
Mnemonic
There are several mnemonics intended to help memorise the taxonomic hierarchy, such as " King Phillip Came Over For Great Spaghetti". ( Kingdom(s), Phylum/ Phyla, Class(es), Order(s), Family/ Families, Genus, Species)
See also
Footnotes
Bibliography
(1992). 9780947643447, Royal Botanic Gardens. ISBN 9780947643447
David Lambert (1990). 9780816024315, Facts on File / British Museum (Natural History. ISBN 9780816024315
(1997). 9780231110136, Columbia University Press. ISBN 9780231110136
(2026). 9780198500650, Oxford University Press. ISBN 9780198500650
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