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3 See also
Conventions regarding infra-specific categories vary between biological disciplines as follows:
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".
In bacteriology, the terms subspecies and variety are usually interchangeable.
In botany, different variations within a species are identified
explicitly as subspecies ('subsp.'), varieties ('var.') or forms ('f.'), e.g.
Chimaphila umbellata subsp. cisatlantica:
A cultivated variant is identified by quoting the cultivar epithet. For example:
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 organisims like trees, flowers, birds, fish and humans, a species can be defined as a distinct and recognisable group that satisfies two conditions:
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.