IIRC, there are two species of birds, A and B, in the North Atlantic that cannot interbreed. But A can breed with C which has a range to the east, and C can bre

IIRC, there are two species of birds, A and B, in the North Atlantic that cannot interbreed. But A can breed with C which has a range to the east, and C can breed with D again further east. And so on, around the arctic circle, until you get to Q which can breed with B.