subspecies

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Subspecies (commonly abbreviated subsp. or ssp.) in biological classification, is 1) a taxonomic rank subordinate to species, or 2) a taxonomic unit, a taxon in that rank (plural: subspecies). A subspecies cannot be recognized in isolation: a species will either be recognized as having no subspecies at all or two or more, never just one.

The differences between subspecies are usually less distinct than the differences between species. The characteristics attributed to subspecies generally have evolved as a result of geographical distribution or isolation.

Nomenclature

The scientific name of a subspecies is a trinomen, that is a binomen followed immediately by a subspecific name, e.g. Panthera tigris sumatrae (Sumatran Tiger). The International Code of Zoological Nomenclature (4th edition, 1999) accepts only one rank below that of species, namely this rank of subspecies or race. Other groupings, "infrasubspecific entities" (e.g. pet breeds) do not have names regulated by the ICZN. Such forms have no official status, though they may be useful in describing altitudinal or geographical clines.

Nominate subspecies

A subspecies indicated by the repetition of the specific name is known as the nominate subspecies. Thus Motacilla alba alba (often abbreviated Motacilla a. alba) is the nominate subspecies of the White Wagtail (Motacilla alba).

(A somewhat similar system also applies in the case of genera: a genus may have a subgenus of the same name, and that is known as the nominate subgenus, for example the land slug, Geomalacus maculosus is also sometimes known as Geomalacus (Geomalacus) maculosus.)

Criteria

Members of one subspecies differ morphologically or by different coding sequences of a peptide from members of other subspecies of the species. Subspecies are defined in relation to species. It is not possible to understand the concept of a subspecies without first grasping what a species is. In the context of many large living organisms like trees, flowers, birds, fish and humans, a species can be defined as a distinct and recognisable group that satisfies two conditions:

1. Members of the group are reliably distinguishable from members of other groups. The distinction can be made in any of a wide number of ways, such as: differently shaped leaves, a different number of primary wing feathers, a particular ritual breeding behaviour, relative size of certain bones, different coding sequences of a peptide, and so on. There is no set minimum 'amount of difference': the only criterion is that the difference be reliably discernible. In practice, however, very small differences tend to be ignored.
2. The flow of genetic material between the group and other groups is small and sometimes can be expected to remain so because even if the two groups were to be placed together they would not interbreed to any great extent.

Note the key qualifier above: to be regarded as different groups rather than as a single varied group, the difference must be distinct, not simply a matter of continuously varying degree. If, for example, the population in question is a type of frog and the distinction between two groups is that individuals living upstream are generally white, while those found in the lowlands are black, then they are classified as different groups if the frogs in the intermediate area tend to be either black or white, but a single, varied group if the intermediate population becomes gradually darker as one moves downstream.

This is not an arbitrary condition. A gradual change, called a cline, is clear evidence of substantial gene flow between two populations. A sharp boundary between black and white, or a relatively small and stable hybrid zone, on the other hand, shows that the two populations do not interbreed to any great extent and are indeed separate species. Their classification as separate species or as subspecies, however, depends on why they do not interbreed.

If the two groups do not interbreed because of something intrinsic to their genetic make-up (perhaps black frogs do not find white frogs sexually attractive, or they breed at different times of year) then they are different species.

If, on the other hand, the two groups would interbreed freely provided only that some external barrier was removed (perhaps there is a waterfall too high for frogs to scale, or the populations are far distant from one another) then they are subspecies. Other factors include differences in mating behavior or time and ecological preferences such as soil content.

Note that the distinction between a species and a subspecies depends only on the likelihood that in the absence of external barriers the two populations would merge back into a single, genetically unified population. It has nothing to do with 'how different' the two groups appear to be to the human observer.

As knowledge of a particular group increases, its categorisation may need to be re-assessed. The Rock Pipit was formerly classed as a subspecies of Water Pipit, but is now recognised to be a full species. For an example of a subspecies, see Pied Wagtail.

Cryptic species are morphologically similar, but have differences in DNA or other factors.

Monotypic and polytypic species

A monotypic species has no races, or rather one race comprising the whole species. Monotypic species can occur in several ways:

• All members of the species are very similar and cannot be sensibly divided into biologically significant subcategories.
• The individuals vary considerably but the variation is essentially random and largely meaningless so far as genetic transmission of these variations is concerned.
• The variation among individuals is noticeable and follows a pattern, but there are no clear dividing lines among separate groups: they fade imperceptibly into one another. Such clinal variation always indicates substantial gene flow among the apparently separate groups that make up the population(s). Populations that have a steady, substantial gene flow among them are likely to represent a monotypic species even when a fair degree of genetic variation is obvious.

A polytypic species has two or more races or subspecies. These are separate groups that are clearly distinct from one another and do not generally interbreed (although there may be a relatively narrow hybridization zone), but which would interbreed freely if given the chance to do so. Note that groups which would not interbreed freely, even if brought together such that they had the opportunity to do so, are not races: they are separate species.

Miscellaneous notes

• In botany, subspecies is one of many ranks below that of species (variety, subvariety, form, subform, etc), and taxa in these ranks will get a ternary name.

• In bacteriology also, the only rank allowed below species is subspecies (names published before 1992 in the rank of variety are taken to be names of subspecies; see International Code of Nomenclature of Bacteria).

See also

Look up subspecies in Wiktionary, the free dictionary.

• color phase
• cultivar in botany
• strain in microbiology
• Landrace

References

• Ernst Mayr, Peter D. Ashlock: Principles of Systematic Zoology, Mcgraw-Hill College, 1991, ISBN 0070411441