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. The usage of the term type is somewhat complicated by slightly different uses in botany and zoology.
In botanical nomenclature, a type (typus, nomenclatural type), "is that element to which the name of a taxon is permanently attached." 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):
- A detailed drawing, painting, etc, depicting the plant, from the early days of plant taxonomy (as we now know it). In those days a dried plant was difficult to transport and hard to keep safe for the future: many specimens that famous botanists looked at have since been lost or damaged. However, there were devoted botanical artists who upon assignment by a botanist (or naturalist) could make a faithful and detailed work of botanical art, for inclusion in a costly book.
- A detailed picture of something that can be seen only through a microscope. A tiny 'plant' on a microscope slide makes for a poor type: the microscope slide may be lost or damaged, or it may be very difficult to find the 'plant' in question among whatever else is on the microscope slide. An illustration makes for a much more reliable type (Art 37.5 of the Vienna Code, 2006).
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.
- Usually, only a species or an infraspecific taxon can have a type of its own. For a new taxon (published on or after 1 January 1958) at these ranks a type should not be an illustration.
- A genus (almost always) has the same type as that of one of its species. For convenience this species may, unofficially, be called its type species, a phrase that has no standing under the ICBN. Only by conservation may a genus have its own type.
- A family has the same type as that of one of its genera (that is, almost always the type of a species). For convenience this genus may, unofficially, be called its type genus, a phrase that has no standing under the ICBN.
- The ICBN provides a listing of the various kinds of type in Art 9, the most important of which is the holotype. Note that the word "type" appears in botanical literature as a part of several terms that have no status under the ICBN: for example a clonotype.
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."
- A type specimen is a vernacular term (not a formally defined term) typically used for an individual or fossil that is any of the various name-bearing types for a species. For example, the type specimen for the species Homo neanderthalensis was the specimen "Neanderthal-1" discovered by Johann Karl Fuhlrott in 1856 at Feldhofer in the Neander Valley in Germany, consisting of a skullcap, thigh bones, part of a pelvis, some ribs, and some arm and shoulder bones. Historically, there may be more than one type specimen, but this practice has been largely discontinued.
- A type species is the nominal species that is the name-bearing type of a nominal genus or subgenus.
- A type genus is the nominal genus that is the name-bearing type of a nominal family-group taxon.
- The type series are all those specimens included by the author in a taxon's formal description, unless the author explicitly or implicitly excludes them as part of the series.
Use of types
Although in reality biologists may examine many specimens (if available) of a new taxon in coming up with a written species description, under the formal rules for naming species (the International Code of Zoological Nomenclature), a single type should be designated. When a single specimen is designated in the original description as the type for a species it is called a holotype. The holotype is typically stored in a museum or other collection to be available for later examination by other biologists as needed. Included in the type description should be a discussion of similarities to and differences from closely related species and where a type specimen or specimens are deposited for examination. The geographical location where a type specimen originated is known as its type locality or type location.
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:
- Paratype – Any additional specimen listed in the type series, where the original description designated a holotype. These are not name-bearing types.
- Neotype – A specimen later selected to serve as the single type specimen when an original holotype has been lost or destroyed, or where the original author never cited a specimen.
- Syntype – Any of two or more specimens listed in a species description where a holotype was not designated; historically, syntypes were often explicitly designated as such, and under the present Code this is a requirement, but modern attempts to publish species description based on syntypes are generally frowned upon by practicing taxonomists, and most are gradually being replaced by lectotypes. Those which still exist are still considered name-bearing types.
- Lectotype – A specimen later selected to serve as the single type specimen for species originally described from a set of syntypes.
- Paralectotype – Any additional specimen from among a set of syntypes, after a lectotype has been designated from among them. These are not name-bearing types.
- Hapantotype – A special case in Protistans where the type consists of two or more specimens of "directly related individuals representing distinct stages in the life cycle"; these are collectively treated as a single entity, and lectotypes cannot be designated from among them.
The various types listed above are needed because many species descriptions go back one or two centuries and holotype designation was not always done, types were not always well kept or preserved, or intervening events have resulted in destruction of original type material. The validity of a species name can rest 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 not always existed (the first edition of the Code was only in 1961), nor always required a type specimen, and many "type-less" species 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 Bush-shrike (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 the 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.
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.