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Subspecies
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Subspecies

In taxonomy, a subspecies is the taxon immediately subordinate to a species. Members of one subspecies differ morphologically from members of other subspecies of the species.

Table of contents
1 Conventions
2 Criteria
3 See also

Conventions

Conventions regarding infra-specific categories vary between biological disciplines as follows:

Zoology

In zoology, the scientific name of a subspecies is the binomen followed immediately by a subspecific epithet, e.g. Homo sapiens sapiens. The International Code of Zoological Nomenclature (4th edition, 2000) does not attempt to codify any "infrasubspecific entities".

Bacteriology

In bacteriology, the terms subspecies and variety are usually interchangeable.

Botany

In botany, different variations within a species are identified explicitly as subspecies ('subsp.'), varieties ('var.') or forms ('f.'); a species may be divided into one or more subspecies, with the subspecies further subdivided into one or more varieties, e.g. European black pine (Pinus nigra): Note that one of the taxa always repeats the same name as for the species as a whole; this is referred to as the type or nominate subspecies & variety, and includes the specimen the species was originally described from.

Variety has often been used as a synonym for subspecies, but is also used to distinguish groups of populations with ecological differences. A form is usually used to designate a minor variation within a population or region. For instance, white-flowered forms of species that usually have coloured flowers are often designated as "f. alba".

A cultivated variant is identified by quoting the cultivar epithet. For example:

Criteria

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 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:

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 the upstream and downstream 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 forms. 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.

Note that the distinction between a species and a subspecies depends only on the likelihood that (absent 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 Water Pipit was formerly classed as a subspecies of Rock Pipit, but is now recognised to be a full species. For an example of a subspecies, see Pied Wagtail.

It should be noted that if a subspecies is indicated by the repetition of the specific name, it is known as the nominate subspecies. Thus Motacilla alba alba is the nominate subspecies of White Wagtail, Motacilla alba.

See also