Taxonomic rank

• 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
  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
  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

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
  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.^[24] 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).

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.
  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.^[33]
• In virology, the formal endings for taxa of
  viriforms are similar to viruses, only -vir- is replaced by -viroid-, -satellit- and -viriform-.^[25] The extra levels of realm and subrealm end with -viria and -vira respectively.^[25]

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:^[34]

• Superdomain
  • Domain, Empire or Realm (in virology)^[25]
    • Subdomain or Subrealm (in virology)^[25]
• 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)
        • Parvphylum^[35] or Microphylum
• Superclass
  • Class
    • Subclass
      • Infraclass
        • Subterclass
          • Parvclass
• Superdivision (in zoology)^[36]
  • Division (in zoology)^[36]
    • Subdivision (in zoology)^[36]
      • Infradivision (in zoology)^[36]
• Superlegion (in zoology)
  • Legion (in zoology)
    • Sublegion (in zoology)
      • Infralegion (in zoology)
  • Megacohort (in zoology)
    • Supercohort (in zoology)^[37]
      • Cohort (in zoology)^[37]
        • Subcohort (in zoology)^[37]
          • Infracohort (in zoology)^[37]
• Gigaorder (in zoology)^[38]
  • Magnorder or megaorder (in zoology)^[38]
    • Grandorder or capaxorder (in zoology)[38]
      • Mirorder or hyperorder (in zoology)^[38]
        • Superorder
          • Series (in ichthyology)
            • Subseries (in ichthyology)
              • Order
                • Parvorder (position in some zoological classifications)^{[citation needed}
                  ]
                  • Nanorder (in zoology)
                    • Hypoorder (in zoology)
                      • Minorder (in zoology)
                        • Suborder
                          • Infraorder
                            • Parvorder (usual position), or microorder (in zoology)[38]
• 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)^[39])
      • morph" (in zoology) or "aberration" (in lepidopterology
        )
        • Subvariety or subvarietas (in botany)
          • Form or forma (in botany)
            • Subform or subforma (in botany)
              • "Race"
                • "Subrace"

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,^[40] and this position gathered support from Graham C. D. Griffiths only a few years later.^[41] 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."^[42] 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.^[43] Furthermore, the diversity in some major taxa (such as vertebrates and angiosperms) is better known than 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.^[44][45]

Of these many ranks, many systematists consider that the most basic (or important) is the species, but this opinion is not universally shared.^[46][47][48] 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,^[49][50] as implemented under the PhyloCode and supported by the International Society for Phylogenetic Nomenclature,^[51] or using circumscriptional names, avoid this problem.^[52][53] 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).

Enigmatic taxa are taxonomic groups whose broader relationships are unknown or undefined.[61] (See Incertae sedis.)

There are several acronyms intended to help memorise the taxonomic hierarchy, such as "King Phillip Came Over For Great Spaghetti".^[62] (Kingdom(s), Phylum/Phyla, Class(es), Order(s), Family/Families, Genus, Species) (See taxonomy mnemonic.)

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