Genus (; : genera ) is a taxonomic rank above species and below family as used in the biological classification of living and fossil organisms as well as viruses. In binomial nomenclature, the genus name forms the first part of the binomial species name for each species within the genus. including the idea that a newly defined genus should fulfill these three criteria to be descriptively useful:

  1. Monophyly – all descendants of an ancestral taxon are grouped together (i.e. phylogenetic analysis should clearly demonstrate both monophyly and validity as a separate lineage).
  2. Reasonable Compactness – a genus should not be expanded needlessly.
  3. Distinctness – with respect to evolutionarily relevant criteria, i.e. ecology, morphology, or biogeography; DNA sequences are a consequence rather than a condition of diverging evolutionary lineages except in cases where they directly inhibit gene flow (e.g. postzygotic barriers).

Moreover, genera should be composed of phylogenetic units of the same kind as other (analogous) genera.

==Etymology==<!-- Do not remove Template:Anchor, since its terms are linked to as section titles -->

The term "genus" comes from Latin , a noun form cognate with ' ('to bear; to give birth to'). The French botanist Joseph Pitton de Tournefort (1656–1708) is considered "the founder of the modern concept of genera".. The Swedish taxonomist Carl Linnaeus popularized its use in his 1753 Species Plantarum.

Use

The scientific name (or the scientific epithet) of a genus is also called the generic name; in modern style guides and science, it is always capitalized.

Use in nomenclature

The rules for the scientific names of organisms are laid down in the nomenclature codes, which allow each species a single unique name that, for eukaryotes (animals, plants, protists, algae, fungi), and prokaryotes (bacteria and archaea), is Latin and binomial in form; this contrasts with common or vernacular names, which are non-standardised, can be non-unique, and typically also vary by country and language of usage.

Consequently, there will be more available names than valid names at any point in time; which names are currently in use depending on the judgement of taxonomists in either combining taxa described under multiple names, or splitting taxa which may bring available names previously treated as synonyms back into use. "Unavailable" names in zoology comprise names that either were not published according to the provisions of the ICZN Code, e.g., incorrect original or subsequent spellings, names published only in a thesis, and generic names published after 1930 with no type species indicated. According to "Glossary" section of the zoological Code, suppressed names (per published "Opinions" of the International Commission of Zoological Nomenclature) remain available but cannot be used as the valid name for a taxon; however, the names published in suppressed works are made unavailable via the relevant Opinion dealing with the work in question.

In botany, similar concepts exist but with different labels. The botanical equivalent of zoology's "available name" is a validly published name. An invalidly published name is a or ; a rejected name is a or ; a later homonym of a validly published name is a or ; for a full list refer to the International Code of Nomenclature for algae, fungi, and plants and the work cited above by Hawksworth, 2010.

An available (zoological) or validly published (botanical) name that has been historically applied to a genus but is not regarded as the accepted (current/valid) name for the taxon is termed a synonym; some authors also include unavailable names in lists of synonyms as well as available names, such as misspellings, names previously published without fulfilling all of the requirements of the relevant nomenclatural code, and rejected or suppressed names.

A particular genus name may have zero to many synonyms, the latter case generally if the genus has been known for a long time and redescribed as new by a range of subsequent workers, or if a range of genera previously considered separate taxa have subsequently been consolidated into one. For example, the World Register of Marine Species presently lists 8 genus-level synonyms for the sperm whale genus Physeter Linnaeus, 1758, and 13 for the bivalve genus Pecten O.F. Müller, 1776.

Identical names (homonyms)

Within the same kingdom, one generic name can apply to one genus only. However, many names have been assigned (usually unintentionally) to two or more different genera. For example, the platypus belongs to the genus Ornithorhynchus although George Shaw named it Platypus in 1799 (these two names are thus synonyms). However, the name Platypus had already been given to a group of ambrosia beetles by Johann Friedrich Wilhelm Herbst in 1793. A name that means two different things is a homonym. Since beetles and platypuses are both members of the kingdom Animalia, the name could not be used for both. Johann Friedrich Blumenbach published the replacement name Ornithorhynchus in 1800.

However, a genus in one kingdom is allowed to bear a scientific name that is in use as a generic name (or the name of a taxon in another rank) in a kingdom that is governed by a different nomenclature code. Names with the same form but applying to different taxa are called "homonyms". Although this is discouraged by both the International Code of Zoological Nomenclature and the International Code of Nomenclature for algae, fungi, and plants, there are some five thousand such names in use in more than one kingdom. For instance,

  • Anura is the name of the order of frogs but also is the name of a non-current genus of plants;
  • Aotus is the generic name of both golden peas and night monkeys;
  • Oenanthe is the generic name of both wheatears and water dropworts;
  • Prunella is the generic name of both accentors and self-heal; and
  • Proboscidea is the order of elephants and the genus of devil's claws.
  • The name of the genus Paramecia (an extinct red alga) is also the plural of the name of the genus Paramecium (which is in the SAR supergroup), which can also lead to confusion.

A list of generic homonyms (with their authorities), including both available (validly published) and selected unavailable names, has been compiled by the Interim Register of Marine and Nonmarine Genera (IRMNG).

Use in higher classifications

The type genus forms the base of names for higher taxonomic ranks, such as the name of the family Poaceae (true grasses), as well as the order Poales, based on the genus Poa. "Official" registers of taxon names at all ranks, including genera, exist for a few groups only such as viruses while for others there are compendia with no "official" standing such as Index Fungorum for fungi, Index Nominum Algarum and AlgaeBase for algae, Index Nominum Genericorum and the International Plant Names Index for plants in general, and ferns through angiosperms, respectively, and Nomenclator Zoologicus and the Index to Organism Names for zoological names.

Totals for both "all names" and estimates for "accepted names" as held in the Interim Register of Marine and Nonmarine Genera (IRMNG) are broken down further in the publication by Rees et al., 2020 cited above. The accepted names estimates are as follows, broken down by kingdom:

thumb|Estimated accepted genus totals by kingdom – based on Rees et al., 2020

  • Animalia: 239,093 accepted genus names (± 55,350)
  • Plantae: 28,724 (± 7,721)
  • Fungi: 10,468 (± 182)
  • Chromista: 11,114 (± 1,268)
  • Protozoa: 3,109 (± 1,206)
  • Bacteria: 3,433 (± 115) (LPSN lists 5 913 names of prokaryotic genera (2025-10-08), Archaea and Bacteria together)
  • Archaea: 140 (± 0)
  • Viruses: 851 (± 0) (ICTV Taxonomy lists 3768 accepted names of genera (2025-03-03))

The cited ranges of uncertainty arise because IRMNG lists "uncertain" names (not researched therein) in addition to known "accepted" names; the values quoted are the mean of "accepted" names alone (all "uncertain" names treated as unaccepted) and "accepted + uncertain" names (all "uncertain" names treated as accepted), with the associated range of uncertainty indicating these two extremes.

Within Animalia, the largest phylum is Arthropoda, with 151,697 ± 33,160 accepted genus names, of which 114,387 ± 27,654 are insects (class Insecta). Within Plantae, Tracheophyta (vascular plants) make up the largest component, with 23,236 ± 5,379 accepted genus names, of which 20,845 ± 4,494 are angiosperms (superclass Angiospermae).

By comparison, the 2018 annual edition of the Catalogue of Life (estimated >90% complete, for extant species in the main) contains currently 175,363 "accepted" genus names for 1,744,204 living and 59,284 extinct species, also including genus names only (no species) for some groups.

Genus size

thumb|upright=1.35|Number of reptile genera with a given number of species. Most genera have only one or a few species but a few may have hundreds. Based on data from the [[Reptile Database (as of May 2015).]]

The number of species in genera varies considerably among taxonomic groups. For instance, among (non-avian) reptiles, which have about 1180 genera, the most (>300) have only 1 species, roughly 360 have between 2 and 4 species, 260 have 5–10 species, approximately 200 have 11–50 species, and only 27 genera have more than 50 species. However, some insect genera such as the bee genera Lasioglossum and Andrena have over 1000 species each. The largest flowering plant genus, Astragalus, contains over 3,000 species.

Which species are assigned to a genus is somewhat arbitrary. Although all species within a genus are supposed to be "similar", there are no objective criteria for grouping species into genera. There is much debate among zoologists whether enormous, species-rich genera should be maintained, as it is extremely difficult to come up with identification keys or even character sets that distinguish all species. Hence, many taxonomists argue in favor of breaking down large genera. For instance, the lizard genus Anolis has been suggested to be broken down into 8 or so different genera which would bring its approximately 400 species to smaller, more manageable subsets.

See also

  • List of the largest genera of flowering plants

References

  • Interim Register of Marine and Nonmarine Genera (IRMNG): includes an estimated 95% of published genus names (accepted and unaccepted) in all groups (semi-continuously updated)
  • Nomenclator Zoologicus : index of genus and subgenus names (accepted and unaccepted) in zoological nomenclature from 1758 to 2004
  • Index to Organism Names: includes zoological taxon names at all ranks (including genera) as continuously indexed for the Zoological Record
  • Index Nominum Genericorum (ING): a compilation of generic names (accepted and unaccepted) published for organisms covered by the ICN: International Code of Nomenclature for Algae, Fungi, and Plants (semi-continuously updated)
  • LPSN – List of Prokaryotic names with Standing in Nomenclature: includes all currently accepted Bacteria and Archaea genus names (continuously updated)
  • ICTV taxonomy releases: latest and historical lists of accepted virus names compiled by the International Committee on Taxonomy of Viruses (ICTV), including all currently accepted virus genus names (updated via regular releases)