Speciation
This was the classic example of ring species. The range of these gulls nearly forms a ring around the North Pole (which is not normally flown over by gulls). The lesser black-backed gulls and herring gulls are sufficiently different that they do not normally hybridize; so, it was said, the group of gulls forms a continuum except where the two lineages meet in Europe. However, it is generally agreed that this is not quite correct, though the details are extremely complicated.Naish, Darren 2910. Tetrapod zoology: book one, chapter 13. ISBN 978-1-905723-61-4