I agree to this: two populations of the same species, separated from each other by some barrier (i.e., mountain, water, etc.) will slowly diverge or drift away genetically, eventually to the point where they now can't or won't interbreed.
Then we humans call them separate "species" or "genera", etc.
This has been observed in both natural and manmade selection.
For natural selection consider Zebras -- (see my post #281) in over four million years of fossils: nine different species (3 living 6 extinct) with eight living subspecies, obviously all the same "kind" and similar common ancestors.
For human selection consider any domesticated animal or plant compared to their ancient ancestors.
In the case of teosinte (ancient wild corn) and modern varieties, teosinte can still fertilize corn but not always visa versa.
Agreed?