I'm not sure exactly what you are asking here, but I think I know. In
many cases I suspect that when two closely related allopatric species have
potential to intermix they probably do a little bit. Thus the lines are
usually a little blurry between many species. That is what would be called
a hybrid zone. These are probably far more common than people appreciate
as they can be difficult to demonstrate and document. My own work on
Australian rainbowfishes finds this to be the norm, not the exception.
This is normal, and does not suggest they are the same species, as once
you get a little distance away you see no evidence of introgression
(hybridization).
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