Wait - Gulls are no ring species:
The Herring Gull complex, consisting of some 20 species of so-called large white-headed gulls, has long been considered as the classic example of a ring-species whereby the ring was finally closed by the Herring Gull Larus argentatus invading NW.Europe from America. In a study based on mitochondrial DNA, Dr.'s Dorit Liebers, Peter de Knijff and Andreas Helbig show that this is not the case. In fact the ring was never closed and they predict that should it ever happen then the most likely scenario is that the Lesser Black-backed Gull Larus fuscus will settle in the New World where it's numbers have increased dramatically in recent times. Instead of one they have found two ancient refugia, an Atlantic and an Aralo-Caspian one from where gulls spread eastwards into the Mediterranean and into the Americas respectively.
source: Dorit Liebers, Peter de Knijff and Andreas Helbig. 2004. The herring gull complex is not a ring species. Proc.Royal Soc.Lond. 10.1098/rspb-2004.2679 (2004).
Here's a bit more.
Recent genetic study sheds light on the evolution of the Herring Gull complex
As reported in the May 2004 issue of Birding World, a recent genetic study (Liebers, de Knijff, and Helbig 2004) has shed some fascinating light on the evolution of the Herring Gull complex, and incidentally therefore on the relationships of some of the larger larids of South Korea.
The authors studied the mitochondrial DNA variation of 21 large gull taxa in an attempt to reconstruct their evolutionary history. Contrary to the expected "ring species" model, members of this complex apparently differentiated largely in geographical separation, not through isolation-by-distance. The divergence started from two glacial refugia, one in the North Atlantic and one in the Aral-Caspian region. Great Black-backed Gull Larus marinus, Yellow-legged Gull L. michahellis, Armenian Gull L. armenicus, and Herring Gull L. argentatus all originated from the North Atlantic refugium. However, the Lesser Black-backed Gull group (including - according to the authors - heuglini and barabensis), Caspian Gull L. cachinanns and a Pacific-North American clade (consisting of vegae, mongolicus, schistisagus, glaucoides, thayeri, and smithsonianus) derive from the Aral-Caspian refugium: it has only been this latter group that has attained circumpolar distribution.
Pesky DNA.