The definition of species is and always has been full of arbitrary definitions. The only thing necessary to define a species is whether, in fact, groups interbreed. Not whether forced matings produce offspring, but whether unforced mating occurs.
Populations can be separated by geography or by something as simple as a variation of some signal, like a mating dance, a call, or a pheromone. The change might be trivial, but result in two, non-interbreeding populations, capable of additional differentiation.
Right, thanks. I am not going to try to figure out problems such as; the large dog/small dog situation. It doesn't mean they aren't both dogs, it is just a barrier. When "men" interfere, they usually find out why it didn't occur naturally.