Ring species in the making.
Ring species in the making.
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).