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

    The three sub-species L. argentatus ( A, B, and C) evolved as Pleistocene isolates in North America, Central Asia, and southern Europe. L. glaucoides (D) evolved as a separate species complex in arctic North America and Greenland. 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 (A2 vs B4). Similarly, A2 remains distinct from the Central Asian C2. Continuity through a circumpolar distribution with reproductive isolation between the endpoints is a classical Rassenkreis or "race circle". (From Mayr 1963)