Thanks for your response. I understand what you are trying to convey. However, it seems that what you are describing would result in a loose definition of species (matter of fact, you used the word 'subpopulations'). X1 and X2 wouldn't be that much different from another, would they? It seems to me that it would take the success of a 'cataclysmic' mutation, if you will, to make the two so distinct as to be noticeably different. Going from a pool of chemicals and information to "us" would require it (many 'cataclysmic' mutations). You would also need a *lot* of time for each 'jump'. Based upon the DNA evidence, some of the "transitions" occurred in a very short amount of time. Not over vast amounts of time as Darwin believed.
One more question. How do we know they can't interbreed unless we artificially inseminate them and see what happens? (there's a fun job...NOT). They may just be picky, or it could be a passe' thing to do at that point in time/history.