The reason why geographic separation is generally involved in subspecies
designations is that a subspecies, in order to be recognized as such, has to
have some distinct, reproducible, definable characteristics which separate it
from the parent stock, and which define it as a subspecies.  If both forms
coexist in the same locality, they will interbreed, since they are the same
species, and before long there will be a series of intergrade forms between
the original stock and what might otherwise have been a distinct subspecies or
form.  However, if the two forms are geographically isolated, then they can
each maintain and propagate the specific characteristics which define them as
distinct entities, without the formation of a range of hybrid forms to confuse
the issue.
Paul M.
Rhode Island