Stunning new research has shown that almost all gull species to be hybrids of Great Black-backed Gulls and unbelievably, Little Gulls. Mitochondrial DNA or mtDNA taken from a host of former gull "species" show genes in all gull species that be traced back to both of these parent species. Backcrosses and breeding among the hybrids have resulted in the wide range of gulls we see today. Gulls with more Little Gull genes are more of the smaller gulls (like Ring-billed and Franklin's) whereas gulls with more Great Black-backed genes tend to be larger (like Glaucous and Western).
I'm sure of all of you have many questions, just as I did upon my first reading of this research. What about mantle color? It turns out that Little Gulls and Great Black-backed Gulls share a common ancestor, one that most likely had a paler back. In both the species, the ancestral genes are turned off, however when they hybridize, these ancestral genes can be expressed, leading to the wide variety of mantle colors we see today. What about their breeding ranges? It's true that both species breeding ranges don't overlap now, but it turns out that right after the last Ice Age, Little and Great Black-backed Gulls shared a common breeding ground where the hybridizing started. It also seems like there may in fact be no "pure" Great Black-backed Gulls or Little Gulls given the amount of cross breeding.
Personally, I am not looking forward to losing 18 species off my life list, but science is always moving ahead and we birders just need to catch up.
A link to the article detailing all the research can be found here: http://tinyurl.com/bsomrcr