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Phenotypically distinct but only modestly reproductively isolated populations.
Subspecies can be viewed as populations that are partway on their way towards speciation (see, e.g., parapatric speciation). They can also be seen as examples of peripheral isolates, particularly relatively successful peripheral isolates (see, e.g., peripatric speciation if subspecies become geographically isolated from other members of the same species).
In any case, subspecies as found within a single, larger species population are both phenotypically and genotypically distinct from one another.
Part of the difficulty in distinguishing organisms into separate species, that is, in terms of species concepts, is addressing how much difference is necessary before supspecies are instead considered to be separate species. In terms of the biological species concept, this is a matter strictly of reproductive isolation.
The key issue therefore is how to distinguish organisms into separate species under circumstances where reproductive isolation cannot be easily established. The existence of subspecies serves to exacerbate this problem since subspecies represent what essentially is an intermediate form that lies somewhere between same species and different species.
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