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:
- 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 DNA sequences, and so on. There is no set minimum 'amount of difference': the only criterion is that the difference be reliably discernable. In practice, however, very small differences tend to be ignored.
- The flow of genetic material between the group and other groups is small and 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.
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. Motacilla alba alba is the nominate subspecies of White Wagtail, Motacilla alba.