Oops, forgot about this thread.
A ring species is one where different populations of the species co-exist together in the same geographical region, but does not breed with each other, effectively making them seem like two different species. In particular, this occurs when the species seemed to spread and change slightly over time when they reach their new region, but their path brings them back to their original site, whereby the original population is different enough due to the small changes that they don't recognize each other. However, any other neighboring population is in fact similar enough to breed with each other readily, creating a situation where they're still the same species, but some individuals do not actually recognize and breed with others.
Wiki entry: http://en.wikipedia.org/wiki/Ring_species
A pseudo-ring species is essentially a ring species, just that the two 'end' populations aren't together.
In this case, you might have just one species that have migrated (either north or south, I dunno, sorry I dunno much about angels), and changed genetically along the way. So each neighboring population can breed with each other, but because of the genetic changes, the northern-most and southern-most populations may be different enough from each other (visibly as well) to have been classified as two different species.
Thanks for posting that. I read the artical and it makes sense.
That could be the case. It could just be the natural evolution of a species as it moves through a chemocline.
