The long term definition was more or less 'A group of living organisms consisting of similar individuals capable of exchanging genes or interbreeding.' This brings up the arguably semantic idea that physical separation is enough even if they are still genetically compatible.

So, in theory you can have genetically compatible separate species as soon as they are no longer capable of interbreeding. However, as Humans and Brown bares are both biologically capable of interbreeding and live in such a manner that the do interbreed they don't fit that definition. Temporary barriers are not necessarily enough as they may exchange DNA in the future.

PS: Ring species are considered a single species because a mutation can start at one end and given time make it's way around the ring. Thus they are a single pool that can and will exchange DNA.