Taxonomic rank

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, nomenclature is regulated by the nomenclature codes. There are seven main taxonomic ranks: kingdom, phylum or division, class, order, family, genus, 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 (lat. dominium), introduced by Moore in 1974.[3][4]

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.

There are definitions of 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:

Superfamily

Family

Subfamily

Tribe

Subtribe

Genus

Subgenus

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 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.)

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).

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:[5]

primary ranks

secondary ranks

further ranks

kingdom (regnum)

subregnum

division or phylum (divisio, phylum)

subdivisio or subphylum

class (classis)

subclassis

order (ordo)

subordo

family (familia)

subfamilia

tribe (tribus)

subtribus

genus (genus)

subgenus

section (sectio)

subsection

series (series)

subseries

species (species)

subspecies

variety (varietas)

subvarietas

form (forma)

subforma

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. Also see descriptive botanical names.

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[lower-alpha 1] 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.[9] 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 a discussion. 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.[14]
• 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.[15]
• Many animal orders have the informal suffix -ida /ɪdə/, e.g. Hyolithida and Nectaspida (Naraoiida)
• In virology, the formal endings for taxa of viroids and of satellite nucleic acids are similar to viruses, only -vir- is replaced by -viroid-, -satellit-.[11]

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.

The following is an artificial synthesis, solely for purposes of demonstration of relative rank (but see notes), from most general to most specific:[16]

• Domain or Empire
  • Subdomain (biology)
• Realm (in virology)[11]
  • Subrealm (in virology)[11]
• 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)[17]
  • Division (in zoology)[17]
    • Subdivision (in zoology)[17]
      • Infradivision (in zoology)[17]
• Superlegion (in zoology)
  • Legion (in zoology)
    • Sublegion (in zoology)
      • Infralegion (in zoology)
• Supercohort (in zoology)[18]
  • Cohort (in zoology)[18]
    • Subcohort (in zoology)[18]
      • Infracohort (in zoology)[18]
• Gigaorder (in zoology)[19]
  • Magnorder or megaorder (in zoology)[19]
    • Grandorder or capaxorder (in zoology)[19]
      • Mirorder or hyperorder (in zoology)[19]
        • Superorder
          • Series (for fish)
            • 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)[19]
• 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)
• Superspecies or Species-group
  • Species
    • Subspecies, or forma specialis (for fungi), or pathovar (for bacteria)[20])
      • Variety or varietas (in botany); or form or morph (in zoology) or aberration (in lepidopterology)
        • Subvariety (in botany)
          • Form or forma (in botany)
            • Subform (in botany)

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.[21] 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 group.

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.

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.[22][23] 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).[24][25] 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 (e.g., it is incorrect to assume that families of insects are in some way evolutionarily comparable to families of mollusks).[25] For animals, at least the phylum rank is usually associated with a certain body plan, which is also, however, an arbitrary criterion.

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

• Breed
• Catalogue of Life
• Cladistics
• Landrace