Taxonomic rank

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.

Ranks in zoology

There are definitions of the following taxonomic ranks in the International Code of Zoological Nomenclature: superfamily, family, subfamily, tribe, subtribe, genus, subgenus, species, subspecies.^[5]

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:^{[5]: §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. 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

• A taxon above the rank of species has a scientific name in one part (a uninominal name).
• A species has a name composed of two parts (a binomial name or
  generic name + specific name
  ; for example Canis lupus.
• 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.

According to Art 3.1 of the International Code of Nomenclature for algae, fungi, and plants (ICN) the most important ranks of taxa are: kingdom, division or phylum, class, order, family, genus, and species. According to Art 4.1 the secondary ranks of taxa are tribe, section, series, variety and form. There is an indeterminate number of ranks. The ICN explicitly mentions:^{[6]: Articles 3 and 4}

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.)

Names of botanical taxa

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";^[9] 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, ...

Examples

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.^[10] 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).

Terminations of names

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.^[16] Uniform suffix has been suggested (but not recommended) in AAAS^[17] 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.^[18]
• In virology, the formal endings for taxa of
  viriforms are similar to viruses, only -vir- is replaced by -viroid-, -satellit- and -viriform-.^[12] The extra levels of realm and subrealm end with -viria and -vira respectively.^[12]

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 which applies.^{[citation needed]}

The following is an artificial synthesis, solely for purposes of demonstration of relative rank (but see notes), from most general to most specific:^[19]

• Superdomain
  • Domain or Empire
    • Subdomain (biology)
• Realm (in virology)^[12]
  • Subrealm (in virology)^[12]
• 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)^[20]
  • Division (in zoology)^[20]
    • Subdivision (in zoology)^[20]
      • Infradivision (in zoology)^[20]
• Superlegion (in zoology)
  • Legion (in zoology)
    • Sublegion (in zoology)
      • Infralegion (in zoology)
• Supercohort (in zoology)^[21]
  • Cohort (in zoology)^[21]
    • Subcohort (in zoology)^[21]
      • Infracohort (in zoology)^[21]
• Gigaorder (in zoology)^[22]
  • Magnorder or megaorder (in zoology)^[22]
    • Grandorder or capaxorder (in zoology)[22]
      • Mirorder or hyperorder (in zoology)^[22]
        • 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)^[22]
• 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)^[23])
      • morph (in zoology) or aberration
        (in lepidopterology)
        • Subvariety (in botany)
          • Form or forma (in botany)

Significance and problems

Ranks are assigned based on subjective dissimilarity, and do not fully reflect the gradational nature of variation within nature. In most cases, higher taxonomic groupings arise further back in time: not because the rate of diversification was higher in the past, but because each subsequent diversification event results in an increase of diversity and thus increases the taxonomic rank assigned by present-day taxonomists.^[24] Furthermore, some groups have many described species not because they are more diverse than other species, but because they are more easily sampled and studied than other groups.^{[citation needed]}

Of these many ranks, the most basic is species. However, this is not to say that a taxon at any other rank may not be sharply defined, or that any species is guaranteed to be sharply defined. It varies from case to 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.^{[citation needed]}

A classification in which all taxa have formal ranks cannot adequately reflect knowledge about phylogeny. Since taxon names are dependent on ranks in traditional Linnaean systems of classification, taxa without ranks cannot be given names. Alternative approaches, such as using circumscriptional names, avoid this problem.^[25][26] 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).

There are several acronyms intended to help memorise the taxonomic hierarchy, such as "King Phillip came over for great spaghetti".[29] (See taxonomy mnemonic.)

See also

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

Footnotes

References