Taxonomic rank#All ranks

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.{{cite book |last1=Hennig |first1=Willi |title=Phylogenetic Systematics |date=1966 |publisher=University of Illinois Press |isbn=978-0-252-06814-0 |url=https://books.google.com/books?id=xsi6QcQPJGkC&dq=Phylogenetic+systematics&pg=PA8-IA9 |language=en}} 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{{cite journal |last1=Martinez-Gutierrez |first1=Carolina A |last2=Aylward |first2=Frank O |title=Phylogenetic Signal, Congruence, and Uncertainty across Bacteria and Archaea |journal=Molecular Biology and Evolution |date=26 August 2021 |volume=38 |issue=12 |pages=5514–5527 |doi=10.1093/molbev/msab254 |pmid=34436605 |url=https://doi.org/10.1093/molbev/msab254 |issn=1537-1719|pmc=8662615 }} 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.{{cite journal |last1=Tarver |first1=James E. |last2=dos Reis |first2=Mario |last3=Mirarab |first3=Siavash |last4=Moran |first4=Raymond J. |last5=Parker |first5=Sean |last6=O’Reilly |first6=Joseph E. |last7=King |first7=Benjamin L. |last8=O’Connell |first8=Mary J. |last9=Asher |first9=Robert J. |last10=Warnow |first10=Tandy |last11=Peterson |first11=Kevin J. |last12=Donoghue |first12=Philip C.J. |last13=Pisani |first13=Davide |title=The Interrelationships of Placental Mammals and the Limits of Phylogenetic Inference |journal=Genome Biology and Evolution |date=5 January 2016 |volume=8 |issue=2 |pages=330–344 |doi=10.1093/gbe/evv261 |pmid=26733575 |pmc=4779606 |url=https://doi.org/10.1093/gbe/evv261 |issn=1759-6653|hdl=1983/64d6e437-3320-480d-a16c-2e5b2e6b61d4 |hdl-access=free }}

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 ({{langx|la|dominium}}), introduced by Moore in 1974.{{Cite journal |last=Moore |first=R. T. |title=Proposal for the recognition of super ranks |journal=Taxon |date=1974 |volume=23 |issue=4 |pages=650–652 |url=http://www.iapt-taxon.org/historic/Congress/IBC_1975/Prop034bis-037.pdf |doi=10.2307/1218807 |jstor=1218807 |access-date=5 October 2016 |archive-date=6 October 2016 |archive-url=https://web.archive.org/web/20161006082804/http://www.iapt-taxon.org/historic/Congress/IBC_1975/Prop034bis-037.pdf |url-status=live }}{{Cite journal |last=Luketa |first=S. |title=New views on the megaclassification of life |journal=Protistology |date=2012 |volume=7 |issue=4 |pages=218–237 |url= http://protistology.ifmo.ru/num7_4/luketa_protistology_7-4.pdf |url-status=dead |archive-url= https://web.archive.org/web/20150402150257/http://protistology.ifmo.ru/num7_4/luketa_protistology_7-4.pdf |archive-date=2 April 2015}}

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 [https://talk.ictvonline.org/information/w/ictv-information/383/ictv-code Code for Viruses], the draft BioCode and the PhyloCode 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.{{cite book |author=International Commission on Zoological Nomenclature |author-link= |date=1999 |title=International Code of Zoological Nomenclature |url=https://www.iczn.org/the-code/the-code-online/ |location=London |publisher=The International Trust for Zoological Nomenclature |page= |isbn=0-85301-006-4 |chapter=Glossary |access-date=10 November 2023 |archive-date=3 September 2021 |archive-url=https://web.archive.org/web/20210903062842/https://www.iczn.org/the-code/the-code-online/ |url-status=live }}

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:{{rp|§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.{{cite journal |last1=Dubois |first1=Alain |title=Naming taxa from cladograms: A cautionary tale |journal=Molecular Phylogenetics and Evolution |date=1 February 2007 |volume=42 |issue=2 |pages=317–330 |doi=10.1016/j.ympev.2006.06.007 |pmid=16949307 |bibcode=2007MolPE..42..317D |url=https://doi.org/10.1016/j.ympev.2006.06.007 |issn=1055-7903}} 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.{{cite journal |last1=Dubois |first1=Alain |title=Synonymies and related lists in zoology: general proposals, with examples in herpetology |journal=Dumerilia |date=2000 |volume=4 |issue=2 |pages=33–98}})

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.
• 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),{{Cite web |last=jibran |first=jibran |title=Log In ‹ Information metBotanical Ranks: Understanding Taxonomic Classification" Meta Description: Explore the hierarchical structure of botanical classification, from Kingdom to Species, essential for understanding plant diversity and evolution. a description — WordPress |url=https://dev-information-meta-descrition.pantheonsite.io/wp-login.php?redirect_to=https%3A%2F%2Fdev-information-meta-descrition.pantheonsite.io%2Fwp-admin%2Fedit.php&reauth=1 |access-date=2024-05-07 |website=dev-information-meta-descrition.pantheonsite.io}} 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. {{crossref|(See also descriptive botanical name.)}}

===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.{{cite web |url=http://www.iapt-taxon.org/nomen/main.php?page=art4 |at=Articles 4.2 and 24.1 |title=International Code of Nomenclature for algae, fungi, and plants – Melbourne Code |date=2012 |website=IAPT-Taxon.org |access-date=3 August 2018 |archive-date=3 August 2018 |archive-url=https://web.archive.org/web/20180803133717/https://www.iapt-taxon.org/nomen/main.php?page=art4 |url-status=live }}

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.{{cite web |url=http://www.iapt-taxon.org/nomen/main.php?page=art3 |at=Article 3.2, and Appendix 1, Articles H.1–3 |title=International Code of Nomenclature for algae, fungi, and plants – Melbourne Code |date=2012 |website=IAPT-Taxon.org |access-date=28 April 2013 |archive-date=10 October 2020 |archive-url=https://web.archive.org/web/20201010230658/https://www.iapt-taxon.org/nomen/main.php?page=art3 |url-status=live }}

{{anchor|Cohort (taxonomy)}}

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|date=October 2017}}

• If names were "intended as names of orders, but published with their rank denoted by a term such as": "cohors" [Latin for "cohort";Stearn, W.T. 1992. Botanical Latin: History, grammar, syntax, terminology and vocabulary, Fourth edition. David and Charles. 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{{efn|The Virginia opossum is an exception.}} 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.{{citation |last1=Chase |first1=M. W. |last2=Reveal |first2=J. L. |date=2009 |title=A phylogenetic classification of the land plants to accompany APG III |journal=Botanical Journal of the Linnean Society |volume=161 |issue=2 |pages=122–127 |doi=10.1111/j.1095-8339.2009.01002.x|doi-access=free}} 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. {{crossref|(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 {{IPAc-en|ɑː}} rather than {{IPAc-en|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.ICZN article 29.2 Uniform suffix has been suggested (but not recommended) in AAASPearse, A.S. (1936) [https://www.biodiversitylibrary.org/page/6258024 Zoological names. A list of phyla, classes, and orders, prepared for section F, American Association for the Advancement of Science.] {{Webarchive|url=https://web.archive.org/web/20211115020601/https://www.biodiversitylibrary.org/page/6258024 |date=15 November 2021 }} American Association for the Advancement of Science, p. 4 as -ida {{IPAc-en|ᵻ|d|ə}} 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 {{lang|la|homo}} has the genitive {{lang|la|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.As supplied by Gaffney & Meylan (1988).
• 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-. The extra levels of realm and subrealm end with -viria and -vira respectively.

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.{{cn|date=October 2023}}

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 ornithischian dinosaurs as reported in Lambert (1990: 149, 159). For botanical ranks, including many intercalary ranks, see Willis & McElwain (2002).

• Superdomain
• Domain or Empire
• Subdomain (biology)
• Realm (in virology)
• Subrealm (in virology)
• 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)These are movable ranks, most often inserted between the class and the legion or cohort. Nevertheless, their positioning in the zoological hierarchy may be subject to wide variation. For examples, see the [http://palaeo.gly.bris.ac.uk/benton/vertclass.html Benton classification of vertebrates] {{Webarchive|url=https://web.archive.org/web/20190116071432/http://palaeo.gly.bris.ac.uk/benton/vertclass.html |date=16 January 2019 }} (2005).
• Division (in zoology)
• Subdivision (in zoology)
• Infradivision (in zoology)
• Superlegion (in zoology)
• Legion (in zoology)
• Sublegion (in zoology)
• Infralegion (in zoology)
• Megacohort (in zoology)
• Supercohort (in zoology)In zoological classification, the cohort and its associated group of ranks are inserted between the class group and the ordinal group. The cohort has also been used between infraorder and family in saurischian dinosaurs ([http://palaeo.gly.bris.ac.uk/benton/vertclass.html Benton] {{Webarchive|url=https://web.archive.org/web/20190116071432/http://palaeo.gly.bris.ac.uk/benton/vertclass.html|date=16 January 2019}} 2005). In botanical classification, the cohort group has sometimes been inserted between the division (phylum) group and the class group: see Willis & McElwain (2002: 100–101), or has sometimes been used at the rank of order, and is now considered to be an obsolete name for order: See International Code of Nomenclature for algae, fungi, and plants, Melbourne Code 2012, Article 17.2.
• Cohort (in zoology)
• Subcohort (in zoology)
• Infracohort (in zoology)
• Gigaorder (in zoology)The supra-ordinal sequence gigaorder–megaorder–capaxorder–hyperorder (and the microorder, in roughly the position most often assigned to the parvorder) has been employed in turtles at least (Gaffney & Meylan 1988), while the parallel sequence magnorder–grandorder–mirorder figures in recently influential classifications of mammals. It is unclear from the sources how these two sequences are to be coordinated (or interwoven) within a unitary zoological hierarchy of ranks. Previously, Novacek (1986) and McKenna-Bell (1997) had inserted mirorders and grandorders between the order and superorder, but Benton (2005) now positions both of these ranks above the superorder.
• Magnorder or megaorder (in zoology)
• Grandorder or capaxorder (in zoology)
• Mirorder or hyperorder (in zoology)
• Superorder
• Series (in ichthyology)
• Subseries (in ichthyology)
• Order
• Parvorder (position in some zoological classifications){{citation needed|reason=Parvorder is occasionally used without suborder or infraorder, but is it ever place above them? |date=February 2025}}
• Nanorder (in zoology)
• Hypoorder (in zoology)
• Minorder (in zoology)
• Suborder
• Infraorder
• Parvorder (usual position), or microorder (in zoology)
• 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)Additionally, the terms biovar, morphovar, phagovar, and serovar designate bacterial strains (genetic variants) that are physiologically or biochemically distinctive. These are not taxonomic ranks, but are groupings of various sorts which may define a bacterial subspecies.)
• Variety or varietas (in botany); or "form" or "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,{{cite book |last1=Hennig |first1=Willi |title=Die Stammesgeschichte der Insekten |date=1969 |publisher=Kramer |location=Frankfurt am Main |pages=436}} and this position gathered support from Graham C. D. Griffiths only a few years later.{{cite journal |last1=Griffiths |first1=Graham C. D. |title=Some Fundamental Problems in Biological Classification |journal=Systematic Zoology |date=December 1973 |volume=22 |issue=4 |pages=338–343 |doi=10.2307/2412942 |jstor=2412942 |url=https://doi.org/10.2307/2412942 |issn=0039-7989}} 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."{{cite book |last1=American Ornithologists' Union |title=The Code of Nomenclature and Check-list of North American Birds Adopted by the American Ornithologists' Union; Being the Report of the Committee of the Union on Classification and Nomenclature |date=1886 |pages=viii + 392 |url=https://books.google.com/books?id=GEsbAAAAIAAJ&dq=The+Code+of+Nomenclature+and+Check-list+of+North+American+Birds+Adopted+by+the+American+Ornithologists%27+Union&pg=PR1 |language=en}} 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.{{Cite journal |last1=Gingerich |first1=P. D. |doi=10.1139/z87-169 |title=Evolution and the fossil record: Patterns, rates, and processes |journal=Canadian Journal of Zoology |volume=65 |issue=5 |pages=1053–1060 |date=1987|bibcode=1987CaJZ...65.1053G }} 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.{{cite journal |last1=Blaxter |first1=Mark |last2=Mann |first2=Jenna |last3=Chapman |first3=Tom |last4=Thomas |first4=Fran |last5=Whitton |first5=Claire |last6=Floyd |first6=Robin |last7=Abebe |first7=Eyualem |title=Defining operational taxonomic units using DNA barcode data |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |date=29 October 2005 |volume=360 |issue=1462 |pages=1935–1943 |doi=10.1098/rstb.2005.1725 |pmid=16214751 |pmc=1609233 |url=https://doi.org/10.1098/rstb.2005.1725 |language=en |issn=0962-8436|hdl=1842/4732 |hdl-access=free }}{{cite journal |last1=Hibbett |first1=David S. |last2=Nilsson |first2=R. Henrik |last3=Snyder |first3=Marc |last4=Fonseca |first4=Mario |last5=Costanzo |first5=Janine |last6=Shonfeld |first6=Moran |title=Automated Phylogenetic Taxonomy: An Example in the Homobasidiomycetes (Mushroom-Forming Fungi) |journal=Systematic Biology |date=1 August 2005 |volume=54 |issue=4 |pages=660–668 |doi=10.1080/10635150590947104 |pmid=16126660 |url=https://doi.org/10.1080/10635150590947104 |issn=1063-5157}}

Of these many ranks, many systematists consider that the most basic (or important) is the species, but this opinion is not universally shared.{{cite journal |last1=Mishler |first1=Brent D. |last2=Donoghue |first2=Michael J. |title=Species Concepts: A Case for Pluralism |journal=Systematic Zoology |date=1982 |volume=31 |issue=4 |pages=491–503 |doi=10.2307/2413371 |jstor=2413371 |url=https://doi.org/10.2307/2413371 |issn=0039-7989}}{{cite journal |last1=Mishler |first1=Brent D. |title=Getting Rid of Species? |journal=Species |date=9 July 1999 |pages=307–316 |doi=10.7551/mitpress/6396.003.0020 |url=https://doi.org/10.7551/mitpress/6396.003.0020 |publisher=The MIT Press |isbn=978-0-262-28635-0 |language=en}}{{cite journal |last1=Ereshefsky |first1=Marc |title=Linnaean Ranks: Vestiges of a Bygone Era |journal=Philosophy of Science |date=September 2002 |volume=69 |issue=S3 |pages=S305–S315 |doi=10.1086/341854 |url=https://doi.org/10.1086/341854 |language=en |issn=0031-8248}} 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 phylogeny 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,{{cite journal |last1=de Queiroz |first1=Kevin |last2=Gauthier |first2=Jacques |title=Phylogeny as a Central Principle in Taxonomy: Phylogenetic Definitions of Taxon Names |journal=Systematic Zoology |date=December 1990 |volume=39 |issue=4 |pages=307 |doi=10.2307/2992353 |jstor=2992353 |url=https://doi.org/10.2307/2992353 |issn=0039-7989}}{{cite journal |last1=Sereno |first1=Paul C. |title=Definitions in Phylogenetic Taxonomy: Critique and Rationale |journal=Systematic Biology |date=1 June 1999 |volume=48 |issue=2 |pages=329–351 |doi=10.1080/106351599260328 |url=https://doi.org/10.1080/106351599260328 |issn=1076-836X}} as implemented under the PhyloCode and supported by the International Society for Phylogenetic Nomenclature,{{cite journal |last1=Laurin |first1=Michel |last2=Cantino |first2=Philip D. |title=First International Phylogenetic Nomenclature Meeting: a report |journal=Zoologica Scripta |date=September 2004 |volume=33 |issue=5 |pages=475–479 |doi=10.1111/j.0300-3256.2004.00176.x |url=https://doi.org/10.1111/j.0300-3256.2004.00176.x |language=en |issn=0300-3256}} or using circumscriptional names, avoid this problem.{{cite journal|last1=Kluge|first1= N.J.|date= 1999|title= A system of alternative nomenclatures of supra-species taxa. Linnaean and post-Linnaean principles of systematics|journal= Entomological Review|volume= 79|issue=2|pages= 133–147}}{{cite journal |last1=Kluge |first1=N.J. |title=Circumscriptional names of higher taxa in Hexapoda |journal=Bionomina |date=2010 |volume=1 |issue=1 |pages=15–55 |doi=10.11646/bionomina.1.1.3|doi-access=free}} 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).{{cite journal |last1=Dubois |first1=Alain |last2=Ohler |first2=Annemarie |last3=Pyron |first3=R. Alexander |title=New concepts and methods for phylogenetic taxonomy and nomenclature in zoology, exemplified by a new ranked cladonomy of recent amphibians (Lissamphibia) |journal=Megataxa |date=26 February 2021 |volume=5 |issue=1 |pages=1–738 |doi=10.11646/megataxa.5.1.1 |language=en |issn=2703-3090|doi-access=free }} 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.{{cite journal |last1=Avise |first1=John C. |last2=Johns |first2=Glenn C. |title=Proposal for a standardized temporal scheme of biological classification for extant species |journal=Proceedings of the National Academy of Sciences |date=22 June 1999 |volume=96 |issue=13 |pages=7358–7363 |doi=10.1073/pnas.96.13.7358 |doi-access=free |pmid=10377419 |language=en |issn=0027-8424|pmc=22090 |bibcode=1999PNAS...96.7358A }}{{cite journal |last1=Avise |first1=John C. |last2=Mitchell |first2=Dale |title=Time to Standardize Taxonomies |journal=Systematic Biology |date=1 February 2007 |volume=56 |issue=1 |pages=130–133 |doi=10.1080/10635150601145365 |pmid=17366143 |url=https://doi.org/10.1080/10635150601145365 |issn=1076-836X}}{{cite journal |last1=Zhao |first1=Rui-Lin |last2=Zhou |first2=Jun-Liang |last3=Chen |first3=Jie |last4=Margaritescu |first4=Simona |last5=Sánchez-Ramírez |first5=Santiago |last6=Hyde |first6=Kevin D. |last7=Callac |first7=Philippe |last8=Parra |first8=Luis A. |last9=Li |first9=Guo-Jie |last10=Moncalvo |first10=Jean-Marc |title=Towards standardizing taxonomic ranks using divergence times – a case study for reconstruction of the Agaricus taxonomic system |journal=Fungal Diversity |date=1 May 2016 |volume=78 |issue=1 |pages=239–292 |doi=10.1007/s13225-016-0357-x |url=https://doi.org/10.1007/s13225-016-0357-x |language=en |issn=1878-9129}}{{cite journal |last1=Lücking |first1=R. |title=Stop the Abuse of Time! Strict Temporal Banding is not the Future of Rank-Based Classifications in Fungi (Including Lichens) and Other Organisms |journal=Critical Reviews in Plant Sciences |date=4 May 2019 |volume=38 |issue=3 |pages=199–253 |doi=10.1080/07352689.2019.1650517 |bibcode=2019CRvPS..38..199L |url=https://doi.org/10.1080/07352689.2019.1650517 |language=en |issn=0735-2689}} For animals, at least the phylum rank is usually associated with a certain body plan, which is also, however, an arbitrary criterion.{{citation needed|date=August 2016}}

Enigmatic taxa are taxonomic groups whose broader relationships are unknown or undefined.{{cite web |url=http://plants.usda.gov/faq.html |title=Frequently Asked Questions |work=PLANTS database |publisher=United States Department of Agriculture |access-date=2011-06-12}} {{crossref|(See Incertae sedis.)}}

There are several acronyms intended to help memorise the taxonomic hierarchy, such as "King Phillip Came Over For Great Spaghetti".{{cite book|last=Evans|first=Rod L.|date=2007|title=Every Good Boy Deserves Fudge: The Book of Mnemonic Devices|url=https://books.google.com/books?id=_u10WSXeK4kC&dq=%22king+phillip+came+over+for+good+spaghetti%22&pg=PT26|publisher=Penguin|isbn=978-1-4406-2207-6|access-date=23 November 2023|archive-date=23 November 2023|archive-url=https://web.archive.org/web/20231123013844/https://books.google.com/books?id=_u10WSXeK4kC&dq=%22king+phillip+came+over+for+good+spaghetti%22&pg=PT26|url-status=live}} (Kingdom(s), Phylum/Phyla, Class(es), Order(s), Family/Families, Genus, Species) {{crossref|(See taxonomy mnemonic.)}}

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

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{{reflist|30em}}

{{refbegin|30em}}

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• {{cite journal |last=Novacek |first=Michael J. |date=1986 |title=The skull of leptictid insectivorans and the higher-level classification of eutherian mammals |journal=Bulletin of the American Museum of Natural History |issue=183 |pages=1–112}}
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• {{cite book |last1=Willis |first1=K. J. |last2=McElwain |first2=J. C. |date=2002 |title=The Evolution of Plants |publisher=Oxford University Press |isbn=0198500653}}

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{{Taxonomic ranks}}

Category:Botanical nomenclature

Category:Plant taxonomy

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Category:Biology terminology