Just Google on 'ring species'. There are quite a few.
I did, (You dodn't think I carry the scientific names of gull species around in my head, do you?)
The point is that the two ends of the ring (or more strictly "loop", as the ends meet but not not join) are, despite genetic transfer around the ring, two species - non-interbreeding, morphologically different.
Besides Google in sinful. (That's the only explanation I have for Creationist posts of the "I'm not sure where I heard it, but I helieve that Charles Darwin was Karl Marx's gay lover" standard.)