Populations of larus gulls around the north pole show an unusual pattern of reproductive isolation: each population is able to interbreed with its neighboring populations, but populations separated by larger geographic distances are not able to interbreed. this type of species complex is known as a:

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.