The composition of a genus is determined bytaxonomists. The standards for genus classification are not strictly codified, so different authorities often produce different classifications for genera. There are some general practices used, however,[2][3] including the idea that a newly defined genus should fulfill these three criteria to be descriptively useful:
monophyly – all descendants of an ancestraltaxon are grouped together (i.e.phylogenetic analysis should clearly demonstrate both monophyly and validity as a separate lineage).
reasonable compactness – a genus should not be expanded needlessly.
The scientific name (or the scientific epithet) of a genus is also called thegeneric name; in modern style guides and science, it is always capitalised. It plays a fundamental role inbinomial nomenclature, the system of namingorganisms, where it is combined with the scientific name of aspecies: seeBotanical name andSpecific name (zoology).[6][7]
Except forviruses, the standard format for aspecies name comprises the generic name, indicating the genus to which the species belongs, followed by the specific epithet, which (within that genus) is unique to the species. For example, thegray wolf's scientific name isCanis lupus, withCanis (Latin for 'dog') being the generic name shared by the wolf's close relatives andlupus (Latin for 'wolf') being the specific name particular to the wolf. A botanical example would beHibiscus arnottianus, a particular species of the genusHibiscus native to Hawaii. The specific name is written in lower-case and may be followed bysubspecies names inzoology or a variety ofinfraspecific names inbotany.
When the generic name is already known from context, it may be shortened to its initial letter, for example,C. lupus in place ofCanis lupus. Where species are further subdivided, the generic name (or its abbreviated form) still forms the leading portion of the scientific name, for example,Canis lupus lupus for theEurasian wolf subspecies, or as a botanical example,Hibiscus arnottianus ssp.immaculatus. Also, as visible in the above examples, the Latinised portions of the scientific names of genera and their included species (and infraspecies, where applicable) are, by convention, written initalics.
As with scientific names at other ranks, in all groups other than viruses, names of genera may be cited with their authorities, typically in the form "author, year" in zoology, and "standard abbreviated author name" in botany. Thus in the examples above, the genusCanis would be cited in full as "Canis Linnaeus, 1758" (zoological usage), whileHibiscus, also first established byLinnaeus but in 1753, is simply "Hibiscus L." (botanical usage).
Each genus should have a designatedtype, although in practice there is a backlog of older names without one. In zoology, this is thetype species, and the generic name is permanently associated with thetype specimen of its type species. Should the specimen turn out to be assignable to another genus, the generic name linked to it becomes ajunior synonym and the remainingtaxa in the former genus need to be reassessed.
In zoological usage, taxonomic names, including those of genera, are classified as "available" or "unavailable". Available names are those published in accordance with theInternational Code of Zoological Nomenclature; the earliest such name for any taxon (for example, a genus) should then be selected as the "valid" (i.e., current or accepted) name for the taxon in question.
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.[8] According to "Glossary" section of the zoological Code, suppressednames (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 suppressedworks 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 avalidly published name. An invalidly published name is anomen invalidum ornom. inval.; a rejected name is anomen rejiciendum ornom. rej.; a later homonym of a validly published name is anomen illegitimum ornom. illeg.; for a full list refer to theInternational Code of Nomenclature for algae, fungi, and plants and the work cited above by Hawksworth, 2010.[8] In place of the "valid taxon" in zoology, the nearest equivalent in botany is "correct name" or "current name" which can, again, differ or change with alternative taxonomic treatments or new information that results in previously accepted genera being combined or split.
Prokaryote andvirus codes of nomenclature also exist which serve as a reference for designating currently[when?] accepted genus names as opposed to others which may be either reduced to synonymy, or, in the case of prokaryotes, relegated to a status of "names without standing in prokaryotic nomenclature".
Anavailable (zoological) orvalidly 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 asynonym; 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, theWorld Register of Marine Species presently lists 8 genus-level synonyms for the sperm whale genusPhyseter Linnaeus, 1758,[9] and 13 for the bivalve genusPecten O.F. Müller, 1776.[10]
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, theplatypus belongs to the genusOrnithorhynchus althoughGeorge Shaw named itPlatypus in 1799 (these two names are thussynonyms). However, the namePlatypus had already been given to a group ofambrosia beetles byJohann Friedrich Wilhelm Herbst in 1793. A name that means two different things is ahomonym. 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 nameOrnithorhynchus in 1800.
However, a genus in onekingdom 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 theInternational Code of Zoological Nomenclature and theInternational 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 theorder of frogs but also is the name of anon-current genus of plants;
The name of the genusParamecia (an extinct red alga) is also the plural of the name of the genusParamecium (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 theInterim Register of Marine and Nonmarine Genera (IRMNG).[11]
Thetype genus forms the base for highertaxonomic ranks, such as the family nameCanidae ("Canids") based onCanis. However, this does not typically ascend more than one or two levels: theorder to which dogs and wolves belong isCarnivora ("Carnivores").
The numbers of either accepted, or all published genus names is not known precisely; Rees et al., 2020 estimate that approximately 310,000 accepted names (valid taxa) may exist, out of a total of c. 520,000 published names (including synonyms) as at end 2019, increasing at some 2,500 published generic names per year.[12] "Official" registers of taxon names at all ranks, including genera, exist for a few groups only such as viruses[1] and prokaryotes,[13] while for others there are compendia with no "official" standing such asIndex Fungorum for fungi,[14]Index Nominum Algarum[15] andAlgaeBase[16] for algae,Index Nominum Genericorum[17] and theInternational Plant Names Index[18] for plants in general, and ferns through angiosperms, respectively, andNomenclator Zoologicus[19] and theIndex to Organism Names for zoological names.
Totals for both "all names" and estimates for "accepted names" as held in theInterim 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:
Estimated accepted genus totals by kingdom - based on Rees et al., 2020
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 isArthropoda, with 151,697 ± 33,160 accepted genus names, of which 114,387 ± 27,654 areinsects (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 areangiosperms (superclass Angiospermae).
By comparison, the 2018 annual edition of theCatalogue of Life (estimated >90% complete, for extant species in the main) contains currently[when?] 175,363 "accepted" genus names for 1,744,204 living and 59,284 extinct species,[20] also including genus names only (no species) for some groups.
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 theReptile 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, ~360 have between 2 and 4 species, 260 have 5–10 species, ~200 have 11–50 species, and only 27 genera have more than 50 species. However, some insect genera such as the bee generaLasioglossum andAndrena have over 1000 species each. The largest flowering plant genus,Astragalus, contains over 3,000 species.[21][22]
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 genusAnolis has been suggested to be broken down into 8 or so different genera which would bring its ~400 species to smaller, more manageable subsets.[23]
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)
ICTV taxonomy releases: latest and historical lists of accepted virus names compiled by the International Committee on Taxonomy of Viruses (ICTV), including all currently[when?] accepted virus genus names (updated via regular releases)
