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.
Educate me a little. Ring species? Hadn't seen the term before.