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Atype species (species typica) is thespecies considered to be permanentlytaxonomically associated with the name of agenus orsubgenus. This species acts as a fixed reference point for the genus and usually will not be changed.[1][2] Its purpose is to anchor the genus' name such that it always stays linked to the same species. This system provides consistency and stability to the group's identity in face of futuretaxonomic revisions. In exceptional circumstances, taxonomists may choose toconserve the name of a genus or othertaxon, which will change that group's type species to one that better fits the taxon concept.[3] The type species does not need to represent the most characteristic or average member of the group, as long as it allows its genus to be distinguished from the others.[4] A similar concept is used for othertaxonomic ranks: thetype specimen is the sole representative of a species, while thetype genus represents afamily.[5]
A type species is both a concept and a practical system that is used in theclassification and nomenclature (naming) of animals. The "type species" represents the reference species and thus "definition" for a particular genus name. Whenever a taxon containing multiple species must be divided into more than one genus, the type species automatically assigns the name of the original taxon to one of the resulting new taxa, the one that includes the type species.
The term "type species" is regulated inzoological nomenclature by article 42.3 of theInternational Code of Zoological Nomenclature, which defines a type species as thename-bearing type of the name of a genus or subgenus (a "genus-group name"). In the Glossary, type species is defined as
The nominal species that is the name-bearing type of a nominal genus or subgenus.[6]
The type species permanently attaches a formal name (the generic name) to agenus by providing just onespecies within that genus to which the genus name is permanently linked (i.e. the genus must include that species if it is to bear the name). The species name in turn is fixed, in theory, to a type specimen.
For example, the type species for theland snail genusMonacha isHelix cartusiana, the name under which the species was first described, known asMonacha cartusiana when placed in the genusMonacha. That genus is currently placed within the familyHygromiidae. The type genus for that family is the genusHygromia.
The concept of the type species in zoology was introduced byPierre André Latreille.[7]
Inbotanical nomenclature, scientific names also must be associated with a type according to theInternational Code of Nomenclature for algae, fungi, and plants for the same referencial purpose.[8][9]
Inbacteriology, a type species is assigned for each genus.[10] Whether or not currently recognized asvalid, every named genus or subgenus in zoology is theoretically associated with a type species. In practice, however, there is a backlog of untypified names defined in older publications when it was not required to specify a type.
The rule of genus typification was also included in the nomenclature of viruses; since 2021 it is not applied.[11]
TheInternational Code of Zoological Nomenclature states that the original name (binomen) of the type species should always be cited. It gives an example in Article 67.1.Astacus marinusFabricius, 1775 was later designated as the type species of the genusHomarus, thus giving it the nameHomarus marinus(Fabricius, 1775). However, the type species ofHomarus should always be cited using its original name, i.e.Astacus marinusFabricius, 1775, even though that is ajunior synonym ofCancer gammarusLinnaeus, 1758.[1]
Although theInternational Code of Nomenclature for algae, fungi, and plants does not contain the same explicit statement, examples make it clear that the original name is used, so that the "type species" of a genus name need not have a name within that genus. Thus in Article 10, Ex. 3, the type of the genus nameElodes is quoted as the type of the species nameHypericum aegypticum, not as the type of the species nameElodes aegyptica.[8] (Elodes is not now considered distinct fromHypericum.)
