When a new species is "discovered", more important than creating a new and unique name for the species is developing a reasonably detailed description. Although in reality a biologists may examine many specimens (if available) of the new species in coming up with a written species description, under the formal rules for naming species, a type specimen must be designated. The type description then describes the type specimen. The single specimen designated as the type for a species is called a holotype. The holotype is stored (a process called curation) in a collection (a place where biological specimens are kept and maintained) to be available for later examination by other biologists as needed. Included in the type description would be a discussion of similarities to and differences from closely related species and where a type specimen or specimens were sent for curation.
The importance of following this process relates to the short-comings of nearly all type descriptions: 1) they will include some subjective aspects and 2) they will not be complete with respect to all future question that might arise. Thus, a published type description becomes the official description of a new species, but the type specimen remains available to extend or modify that description in the future should it become necessary to do so.
Biological collections are maintained by universities and museums. Ensuring that biological 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:
- Isotype – A duplicate specimen of the holotype, collected from the same individual. An isotype can serve as a backup in the event that the holotype is lost or destroyed.
- Syntype – Any of two or more specimens listed in the species type description when a holotype was not designated. Isosyntypes are duplicates of a syntype.
- Paratype – A specimen not designated as a type, but listed as representative, in the original type description.
- Lectotype – A specimen later selected, usually from among the isotype, syntype(s), or paratype(s), to serve as if it were a holotype where a holotype was either never designated or was lost or destroyed. Isolectotypes are any duplicate specimens of the lectotype.
- Neotype – A specimen later selected to serve as if it were a holotype when all specimens available and listed by the original species author have been lost or destroyed, or where the original author never cited a specimen. Isoneotypes are duplicate specimens of the neotype.
- Epitype – "a specimen or illustration selected to serve as an interpretative type when the holotype, lectotype, or previously designated neotype, or all original material associated with a validly published name, is demonstrably ambiguous and cannot be critically identified for purposes of the precise application of the name of a taxon." (ICBN Ch. 2, Sec. 2, Art. 9.7)
The various types described above are needed because many species description go back one or two centuries and type designation was not always done, types were not always well kept or preserved, or intervening events have resulted in destruction of original type material. Some older botanical types are actually illustrations rather than specimens. The validity of a species name can rest upon the availability of original type specimens; or as important, upon the clarity of the description attached to the type material.
An isotype must be from the same individual as the type it duplicates; hence isotypes can only be designated to species with individuals that can be divided into more-or-less equal pieces, e.g. numerous shoots collected from one tree. Species that cannot be divided into several parts each carrying similar information content, e.g. birds, cannot have isotypes.
A generitype is the type specimen of a genus. It is designated by selecting a type for the name of a particular species within that genus. This selected species then becomes the type species for the genus; the description of a genus is based primarily on its type species, modified and expanded by the features of other included species. The generic name is permanently associated with its type species. For example, the type species for the genus Cupressus is Cupressus sempervirens; this species then defines the genus for the purpose of deciding whether other species belong to that genus or not.
Ideally, a type species best exemplifies the essential characteristics of the genus to which it belongs. However, this common definition is somewhat circular in as much as once a species type specimen is designated as a generitype that species largely defines the "essential characteristics" of the genus. Whether or not the species is the "best" example considering all characteristics of all species assigned to a particular genus is not so important. Species that are "poor" examples relative to the type species with respect to enough characteristics (or a few "important" characteristics) are candidates for removal from the genus. If this process seems somewhat subjective: it is. Extensive rules keep taxonomists reading from the same page. However, less subjective methods (see cladistics) are now being developed.
The term fixation is used by the ICZN for the determination of a name-bearing type whether by original or subsequent designation.
A type genus becomes 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.
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