Taxonomic rank

In biology, taxonomic rank (which some authors prefer to call nomenclatural rank^[1] because ranking is part of nomenclature rather than taxonomy proper, according to some definitions of these terms) is the relative or absolute level of a group of organisms (a taxon) in a hierarchy that reflects evolutionary relationships. Thus, the most inclusive clades (such as Eukarya and Opisthokonta) have the highest ranks, whereas the least inclusive ones (such as Homo sapiens or Bufo bufo) have the lowest ranks. Ranks can be either relative and be denoted by an indented taxonomy in which the level of indentation reflects the rank, or absolute, in which various terms, such as species, genus, family, order, class, phylum, kingdom, and domain designate rank. This page emphasizes absolute ranks and the rank-based codes (the Zoological Code, the Botanical Code, the Code for Cultivated Plants, the Prokaryotic Code, and the Code for Viruses) require them. However, absolute ranks are not required in all nomenclatural systems for taxonomists; for instance, the PhyloCode,^[2] the code of phylogenetic nomenclature, does not require absolute ranks.

Taxa are hierarchical groups of organisms, and their ranks describes their position in this hierarchy. High-ranking taxa (e.g. those considered to be domains or kingdoms, for instance) include more sub-taxa than low-ranking taxa (e.g. those considered genera, species or subspecies). The rank of these taxa reflects inheritance of traits or molecular features from common ancestors. The name of any species and genus are basic; which means that to identify a particular organism, it is usually not necessary to specify names at ranks other than these first two, within a set of taxa covered by a given rank-based code.^[3] However, this is not true globally because most rank-based codes are independent from each other, so there are many inter-code homonyms (the same name used for different organisms, often for an animal and for a taxon covered by the botanical code). For this reason, attempts were made at creating a BioCode that would regulate all taxon names,^[4] but this attempt has so far failed^[5] because of firmly entrenched traditions in each community.^[6]

Consider a particular species, the red fox, Vulpes vulpes: in the context of the Zoological Code, the specific epithet vulpes (small v) identifies a particular species in the genus Vulpes (capital V) which comprises all the "true" foxes. Their close relatives are all in the family Canidae, which includes dogs, wolves, jackals, and all foxes; the next higher major taxon, Carnivora (considered an order), includes caniforms (bears, seals, weasels, skunks, raccoons and all those mentioned above), and feliforms (cats, civets, hyenas, mongooses). Carnivorans are one group of the hairy, warm-blooded, nursing members of the class Mammalia, which are classified among animals with notochords in the phylum Chordata, and with them among all animals in the kingdom Animalia. Finally, at the highest rank all of these are grouped together with all other organisms possessing cell nuclei in the domain Eukarya.

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)."^[7] Note that the discussions on this page generally assume that taxa are clades (monophyletic groups of organisms), but this is required neither by the International Code of Zoological Nomenclature nor by the Botanical Code, and some experts on biological nomenclature do not think that this should be required,^[8] and in that case, the hierarchy of taxa (hence, their ranks) does not necessarily reflect the hierarchy of clades.

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.^[9] Cladistics is a method of classification of life forms according to the proportion of characteristics that they have in common (called synapomorphies). 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^[10] 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.^[11]

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 (proposed by Carl Woese) is now widely used as a fundamental rank, although it is not mentioned in any of the nomenclature codes, and is a synonym for dominion (Latin: dominium), introduced by Moore in 1974.^[12][13]

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^[4] and the PhyloCode^[2] all recommend italicizing all taxon names (of all ranks).

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.^[14]

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:^{[14]: §29–31}

• Family-groups
  • Superfamily (-oidea)
  • Family (-idae)
  • Subfamily (-inae)
  • Tribe (-ini)
  • Subtribe (-ina)
• Genus-groups
  • Genus
  • Subgenus
• Species-groups
  • 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. 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.^[15] 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.^[16][15])

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).

• A taxon above the rank of species has a scientific name in one part (a uninominal name).
• A species has a name typically composed of two parts (a binomial name or binomen): generic name + specific name; for example Canis lupus. Sometimes the name of a subgenus (in parentheses) 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 (Aquarana) catesbeianus, which designates a species that belongs to the genus Lithobates and the subgenus Aquarana.^[15]
• A subspecies has a name composed of three parts (a trinomial name or trinomen): 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.

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),^[17] 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.

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. (See also descriptive botanical name.)

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.^[19]

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.^[20]

If a different term for the rank was used in an old publication, but the intention is clear, botanical nomenclature specifies certain substitutions:^{[citation needed]}

• If names were "intended as names of orders, but published with their rank denoted by a term such as": "cohors" [Latin for "cohort";^[21] see also cohort study for the use of the term in ecology], "nixus", "alliance", or "Reihe" instead of "order" (Article 17.2), they are treated as names of orders.
• "Family" is substituted for "order" (ordo) or "natural order" (ordo naturalis) under certain conditions where the modern meaning of "order" was not intended. (Article 18.2)
• "Subfamily is substituted for "suborder" (subordo) under certain conditions where the modern meaning of "suborder" was not intended. (Article 19.2)
• In a publication prior to 1 January 1890, if only one infraspecific rank is used, it is considered to be that of variety. (Article 37.4) This commonly applies to publications that labelled infraspecific taxa with Greek letters, α, β, γ, ...

Classifications of five species follow: the fruit fly 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.

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^[a] 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 (Magnoliophyta) to a subclass (Magnoliidae), and the superorder has become the rank that distinguishes the major groups of flowering plants.^[23] 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 details.) 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 (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).

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.

Pronunciations given are the most Anglicized. More Latinate pronunciations are also common, particularly /ɑː/ rather than /eɪ/ 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 (like Gymnospermae or Fungi).
• For animals, there are standard suffixes for taxa only up to the rank of superfamily.^[30] Uniform suffix has been suggested (but not recommended) in AAAS^[31] as -ida /ɪdə/ for orders, for example; protozoologists seem to adopt this system. Many metazoan (higher animals) 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 (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.^[32]
• 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-.^[24] The extra levels of realm and subrealm end with -viria and -vira respectively.^[24]

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.^{[citation needed]}

The following is an artificial synthesis, solely for purposes of demonstration of absolute rank (but see notes), from most general to most specific:^[33]

• Superdomain
  • Domain or Empire
    • Subdomain (biology)
• Realm (in virology)^[24]
  • Subrealm (in virology)^[24]
• Hyperkingdom
  • Superkingdom
    • Kingdom
      • Subkingdom
        • Infrakingdom
          • Parvkingdom
• Superphylum, or superdivision (in botany)
  • Phylum, or division (in botany)
    • Subphylum, or subdivision (in botany)
      • Infraphylum, or infradivision (in botany)
        • Microphylum
• Superclass
  • Class
    • Subclass
      • Infraclass
        • Subterclass
          • Parvclass
• Superdivision (in zoology)^[34]
  • Division (in zoology)^[34]
    • Subdivision (in zoology)^[34]
      • Infradivision (in zoology)^[34]
• Superlegion (in zoology)
  • Legion (in zoology)
    • Sublegion (in zoology)
      • Infralegion (in zoology)
• Supercohort (in zoology)^[35]
  • Cohort (in zoology)^[35]
    • Subcohort (in zoology)^[35]
      • Infracohort (in zoology)^[35]
• Gigaorder (in zoology)^[36]
  • Magnorder or megaorder (in zoology)^[36]
    • Grandorder or capaxorder (in zoology)^[36]
      • Mirorder or hyperorder (in zoology)^[36]
        • Superorder
          • Series (in Ichthyology)
            • Order
              • Parvorder (position in some zoological classifications)
                • Nanorder (in zoology)
                  • Hypoorder (in zoology)
                    • Minorder (in zoology)
                      • Suborder
                        • Infraorder
                          • Parvorder (usual position), or microorder (in zoology)^[36]
• Section (in zoology)
  • Subsection (in zoology)
• Gigafamily (in zoology)
  • Megafamily (in zoology)
    • Grandfamily (in zoology)
      • Hyperfamily (in zoology)
        • Superfamily
          • Epifamily (in zoology)
            • Series (for Lepidoptera)
              • Group (for Lepidoptera)
                • Family
                  • Subfamily
                    • Infrafamily
• Supertribe
  • Tribe
    • Subtribe
      • Infratribe
• Supergenus
  • Genus
    • Subgenus
      • Section (in botany)
        • Subsection (in botany)
          • Series (in botany)
            • Subseries (in botany)
• Species complex
  • Species
    • Subspecies, or forma specialis (for fungi), or pathovar (for bacteria)^[37])
      • Variety or varietas (in botany); or form or morph (in zoology) or aberration (in lepidopterology)
        • Subvariety (in botany)
          • Form or forma (in botany)
            • Subform or subforma (in botany)

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,^[38] and this position gathered support from Graham C. D. Griffiths only a few years later.^[39] 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 the Strickland code was drafted], 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."^[40] 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.^[1][6] In most cases, higher taxonomic groupings arise further back in time, simply because the most inclusive taxa necessarily appeared first.^[41] Furthermore, the diversity in some major taxa (such as vertebrates and angiosperms) is better known that that of others (such as fungi, arthropods and nematodes) 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.^[42][43]

Of these many ranks, many systematists consider that the most basic (or important) is the species, but this opinion is not universally shared.^[44][45][46] 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.^[6] 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.^[6]

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,^[47][48] as implemented under the PhyloCode and supported by the International Society for Phylogenetic Nomenclature,^[49] or using circumscriptional names, avoid this problem.^[50][51] 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).^[52][53] 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).^[53][54][6] 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,^[9] but he concluded in 1969^[38] 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.^{[55][56][57][58]} For animals, at least the phylum rank is usually associated with a certain body plan, which is also, however, an arbitrary criterion.^{[citation needed]}

Enigmatic taxa are taxonomic groups whose broader relationships are unknown or undefined.^[59] (See Incertae sedis.)

There are several acronyms intended to help memorise the taxonomic hierarchy, such as "King Phillip came over for great spaghetti".^[60] (See taxonomy mnemonic.)

• Breed
• Catalogue of Life (a database)
• Cladistics
• Landrace
• Tree of life (biology)
• Alliance (taxonomy)
• Strain (biology)