No, but let dohh see if he can answer.
I don't know if tests have been made to find out whether the two subspecies at the end of the ring could be fertilized with each other
If they have been they aren't widely known.
No, but let dohh see if he can answer.
Whose dohh?
Aside from those who specialize in them, who cares about any specific beastie? The evidence for evolution does not rest on specific beasties, it rests of the relations of the overall morphological patterns we see in successively dated geological layers--with particular regards to increases in general complexity.
I don't know if tests have been made to find out whether the two subspecies at the end of the ring could be fertilized with each other
If they have been they aren't widely known.
The Herring Gull is divided into breeds with a pattern of continuity that makes it obvious which direction around the earth it has been slowly migrating its nesting grounds. Every breed that's adjacent can interbreed--until you get to the end of the migration sequence, somewhere in Siberia, as I remember. As this point, the two adjacent breeds, taken, from the assumed direction of migration to be the oldest and newest, cannot interbreed.
I don't know if this is what y'all are talking about, but the Herring Gull's breeding pattern is kind of famous in biological science circles.