In biology, a genus // (plural: genera) is a taxonomic rank used in the biological classification of living and fossil organisms. In the hierarchy of biological classification, genus comes above species and below family. In binomial nomenclature, the genus name forms the first part of the binomial species name for each species within the genus.
- E.G., Pongo pygmaeus and Pongo abelii are two species within the genus Pongo. Pongo is a genus within the family Hominidae.
The composition of a genus is determined by a taxonomist. 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, including the idea that a newly defined genus should fulfill these three criteria to be descriptively useful:
- monophyly – all descendants of an ancestral taxon are grouped together;
- reasonable compactness – a genus should not be expanded needlessly; and
- distinctness – with respect to evolutionarily relevant criteria, i.e. ecology, morphology, or biogeography; note that 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).
The rules for scientific names are laid down in the Nomenclature Codes; depending on the kind of organism and the Kingdom it belongs to, a different Code may apply, with different rules, laid down in a different terminology. The advantages of scientific over common names are that they are accepted by speakers of all languages, and that each species has only one name. This reduces the confusion that may arise from the use of a common name to designate different things in different places (example elk), or from the existence of several common names for a single species.
It is possible for a genus to be assigned to a kingdom governed by one particular Nomenclature Code by one taxonomist, while other taxonomists assign it to a kingdom governed by a different Code, but this is the exception, not the rule.
Pivotal in binomial nomenclature
The generic name is a component of the names of taxa of lower rank. For example, Canis lupus is the binomial name of the Gray wolf, a species, with Canis the generic name for the dog and its close relatives, and with lupus particular (specific) for the wolf (lupus is written in lower case).
- a third name is sometimes added (in zoology) to indicate a sub-species, Canis lupus is a gray wolf, but Canis lupus familiaris is the scientific name for the domestic dog which remains closely related enough to be considered part of the same species which has enough differences from other members of the species to justify the use of a subspecies name.
- Hibiscus brackenridgei subsp. mokuleianus is a subspecies of Hibiscus brackenridgei, a species in the genus Hibiscus.
- Crataegus azarolus var. aronia is a variety within the plant species Crataegus azarolus.
Taxonomic units in higher ranks frequently have a name that is based on a generic name, such as the family name Canidae, which is based on Canis. However, not all names in higher ranks are necessarily based on the name of a genus: for example, Carnivora is the name for the order to which the dog belongs.
Identical names used for different genera
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 genus of plants (although not current: it is a synonym); Aotus is the genus of golden peas and night monkeys; Oenanthe is the genus of wheatears and water dropworts, Prunella is the genus of accentors and self-heal, and Proboscidea is the order of elephants and the genus of devil's claws.
Within the same kingdom one generic name can apply to only one genus. This explains why the platypus genus is named Ornithorhynchus—George Shaw named it Platypus in 1799, but the name Platypus had already been given to a group of ambrosia beetles by Johann Friedrich Wilhelm Herbst in 1793. Since beetles and platypuses are both members of the kingdom Animalia, the name Platypus could not be used for both. Johann Friedrich Blumenbach published the replacement name Ornithorhynchus in 1800.
Types and genera
Because of the rules of scientific naming, or "binomial nomenclature", each genus should have a designated type, although in practice there is a backlog of older names that may not yet have a type. In zoology this is the type species; the generic name is permanently associated with the type specimen of its type species. Should this specimen turn out to be assignable to another genus, the generic name linked to it becomes a junior synonym, and the remaining taxa in the former genus need to be reassessed.
None of the nomenclature codes require such criteria for defining a genus, because these are concerned with the nomenclature rules, not with taxonomy. These regulate formal nomenclature, aiming for universal and stable scientific names.
The size of genera
The number of species in genera varies considerably among taxonomic groups. For instance, among (non-avian) reptiles, which have about 1180 genera, 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 (see figure). However, some insect genera such as the bee genera Lasioglossum and Andrena have over 1000 species each.
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 large, 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 ~400 species to smaller, more manageable subsets.
- Gill, F. B.; Slikas, B.; Sheldon, F. H. (2005). "Phylogeny of titmice (Paridae): II. Species relationships based on sequences of the mitochondrial cytochrome-b gene". Auk 122 (1): 121–143. doi:10.1642/0004-8038(2005)122[0121:POTPIS]2.0.CO;2.
- Merriam Webster Dictionary
- Genos, Henry George Liddell, Robert Scott, A Greek-English Lexicon, at Perseus
- Stuessy, T. F. (2009). Plant Taxonomy: The Systematic Evaluation of Comparative Data (2nd ed.). New York: Columbia University Press. p. 42. ISBN 9780231147125.
- Rafinesque, Prof. C. S. (1836). "Generic Rules". Flora telluriana Pars Prima First Part of the Synoptical Flora Telluriana, Centuries I, II, III, IV. With new Natural Classes, Orders and families: containing the 2000 New or revised Genera and Species of Trees, Palms, Shrubs, Vines, Plants, Lilies, Grasses, Ferns, Algas, Fungi, & c. from North and South America, Polynesia, Australia, Asia Europe and Africa, omitted or mistaken by the authors, that were observed or ascertained, described or revised, collected or figured, between 1796 and 1836. 1. Philadelphia: H. Probasco. Retrieved 2009-04-02.
...difficulties occurring in generic nomenclature: similar cases abound, and become complicated by the different views taken of the matter by the various botanists.
- The Reptile Database
- Nicholson, K. E.; B. I. Crother, C. Guyer & J.M. Savage (2012) It is time for a new classification of anoles (Squamata: Dactyloidae). Zootaxa 3477: 1–108
- Nomenclator Zoologicus: Index of all genus and subgenus names in zoological nomenclature from 1758 to 2004.
- Fauna Europaea Database for Taxonomy