More precisely it would be populations that do not interbreed naturally in the wild.
My post 286 goes into some of the problems in defining species.
To repeat and elaborate on some of it:
On the way to speciation is diminished fertility. They can breed but fewer offspring survive, or the offspring can't breed etc.
another route is distance: New York squirrels breed fine with Ohio, less well with Montana and not at all with California populations. How many species?
Or, picture a very large round lake. The population at 12:00 can breed with 1:00 which can breed with 3:00. which can breed with 5:00 etc, but here's the kicker: when you get back to 12:00 there are now TWO species: the original denizens and the ones which bred themselves all the way around the lake. They don't interbreed at all.
your round lake illo is identical to the paradigm of the arctic circle seagulls (I mentioned them some posts back)
yeah, there are hazes ruining the precision I would prefer.