Ring species.
When there are neighbouring populations that can interbreed with each other while only two of those populations cannot breed between them, we are standing before a ring species. In the case of Larus gulls what is observed is that several of these species can interbreed, but it comes to two of them that are incompatible - they are probably to distantly related to interbreed. It is considered, that this species may be connected to each other in a line of derivation and is both ends on that line that are incompatible.
