A circumpolar "ring species" of gulls in the Larus argentatus complex

"Letters denote species and subspecies; numbers identify distinct populations. The argentatus subspecies A, B, and C apparently evolved as Pleistocene isolates. D represents a separate species (L. glaucoides) that evolved in North America. After the Pleistocene, subspecies A spread from a north Pacific refuge across North America and into western Europe where it became sympatric with subspecies B, from which it remains distinct." The circumpolar distribution is a classical Rassenkreis or "race circle". (From Mayr 1963)