In biology, a type is that which fixes a name to a taxon. Depending on the nomenclature code which is applied to the organism in question, a type may be a specimen, culture, illustration, description or taxon.
For example, the Natural History Museum in London has a specimen numbered 1886.6.24.20 of the Spotted Harrier (Circus assimilis), which is the holotype for that species; the name Circus assimilis refers, by definition, to the species of that particular specimen.
Note that at least for type specimens there is no requirement for a "typical" individual to be used. When describing new species, this is often impossible to tell anyway, until more research has been done. Genera and families, particularly those established by early taxonomists, tend to be named after species that are more "typical" for them, but here too this is not always the case and due to changes in systematics cannot be. Hence, the term name-bearing type or onomatophore is sometimes used, to denote the fact that biological types do not define "typical" individuals or (in zoology) taxa, but rather fix a scientific name to a specific operational taxonomic unit. Type specimens are theoretically even allowed to be aberrant or deformed individuals or color variations, though this is rarely the case as it makes it hard to determine to which population the individual belonged.
The usage of the term type is somewhat complicated by slightly different uses in botany and zoology. In the PhyloCode, type specimens are replaced by clade delimitations.
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In botanical nomenclature, a type (typus, nomenclatural type), "is that element to which the name of a taxon is permanently attached."[1]
A botanical name, by itself, is only a phrase (of one to three words). For a name to be meaningful it is necessary to be sure what it applies to. A type fixes a botanical name to a taxon. In botany a type is either a specimen or an illustration. A specimen is a real plant (or one or more parts of a plant or a lot of small plants), dead and kept safe, "curated", in a herbarium (or the equivalent for fungi). Notable cases of where an illustration may serve as a type are (this is not an exclusive listing):
Note that a type only fixes a name to a single representative of the taxon. A type does not determine the circumscription of the taxon. For example, the common dandelion is a controversial taxon: some botanists consider it to consist of over a hundred species, although most botanists regard it to be a single species. The type of the name Taraxacum officinale is the same whether the circumscription of the species includes all those small species (Taraxacum officinale is a 'big' species) or whether the circumscription is limited to only one small species among the other hundred (Taraxacum officinale is a 'small' species). In this case the name Taraxacum officinale is the same and the type of the name is the same, but the extent of what the name actually applies to varies strongly. Setting the circumscription of a taxon is done by a taxonomist in a publication.
Miscellaneous notes:
In zoological nomenclature, a type is a specimen or a taxon. A "name-bearing type" "provides the objective standard of reference whereby the application of the name of a nominal taxon can be determined."
Although in reality biologists may examine many specimens (when available) of a new taxon before writing an official published species description, nonetheless, under the formal rules for naming species (the International Code of Zoological Nomenclature), a single type must be designated, as part of the published description.
When a single specimen is clearly designated in the original description, this specimen is known as the holotype of that species. The holotype needs to be placed in a major museum, or similar well-known public collection, so that it is freely available for later examination by other biologists.
Included in the type description should be a discussion of similarities to and differences from closely related species, and an indication of where the type specimen or specimens are deposited for examination. The geographical location where a type specimen was originally found is known as its type locality.
Zoological collections are maintained by universities and museums. Ensuring that types are kept in good condition and made available for examination by taxonomists are two important functions of such collections. And, while there is only one holotype designated, there can be other "type" specimens, the following of which are formally defined:
The various types listed above are necessary because many species were described one or two centuries ago, when a single type specimen, a holotype, was often not designated. Also, types were not always carefully preserved, and intervening events such as wars and fires have resulted in destruction of original type material. The validity of a species name often rests upon the availability of original type specimens; or, if the type cannot be found, or one has never existed, upon the clarity of the description.
The ICZN has only existed since 1961 when the first edition of the Code was published. The ICZN does not always demand a type specimen for the historical validity of a species, and many "type-less" species do exist, perhaps the most notable being Homo sapiens. This example is instructive: the current edition of the Code, Article 75.3, prohibits the designation of a neotype unless there is "an exceptional need" for "clarifying the taxonomic status" of a species; as the status and identity of H. sapiens is not questioned, there is no exceptional need for clarification, and "any such neotype designation is invalid" (Article 75.2).
Recently some species have been described where the type specimen was released alive back into the wild, such as the Bulo Burti Boubou (a bushshrike), described as Laniarius liberatus, in which the species description included DNA sequences from blood and feather samples. Assuming there is no future question as to the status of such a species, the absence of a type specimen does not invalidate the name, but it may be necessary in the future to designate a neotype for such a taxon, should any questions arise. However, in the case of the bushshrike, ornithologists have argued that the specimen was a rare and hitherto unknown color morph of a long-known species, using only the available blood and feather samples. While there is still some debate on the need to deposit actual killed individuals as type specimens, it can be observed that given proper vouchering and storage, tissue samples can be just as valuable even in case disputes about the validity of a species arise.
There are many other permutations and variations on terms using the suffix "-type" (e.g., allotype, cotype, topotype, generitype, isoneotype, etc.) but these are not formally regulated by the Code, and a great many are obsolete and/or idiosyncratic.
The term fixation is used by the Code for the declaration of a name-bearing type, whether by original or subsequent designation.
Each genus must have a designated type species (the term "genotype" was once used for this but has been abandoned because the word has been co-opted for use in genetics, and is much better known in that context). The description of a genus is usually based primarily on its type species, modified and expanded by the features of other included species. The generic name is permanently associated with the name-bearing type of its type species.
Ideally, a type species best exemplifies the essential characteristics of the genus to which it belongs, but this is subjective and, ultimately, technically irrelevant, as it is not a requirement of the Code. If the type species proves, upon closer examination, to belong to a pre-existing genus (a common occurrence), then all of the constituent species must be either moved into the pre-existing genus, or disassociated from the original type species and given a new generic name; the old generic name passes into synonymy, and is abandoned, unless there is a pressing need to make an exception (decided case-by-case, via petition to the International Commission on Zoological Nomenclature).
A type genus is that genus from which the name of a family or subfamily is formed. As with type species, the type genus is not necessarily the most representative, but is usually the earliest described, largest or best known genus. It is not uncommon for the name of a family to be based upon the name of a type genus which has passed into synonymy; the family name does not need to be changed in such a situation.