LMFAO!!! Are you kidding me? Have you ever thought about a Pygmy standing next to a Basketball player? Or a Ballet dancer standing next to a Sumo wrestler? I think these are resonable examples of extreems in human beings and as far as I know, we are still, just, one species.
It's just a matter of Selective Breeding - it has been going on for thousands of years. Let's use dogs as a working model. I don't remember exactly how long it's been, but I think it's like 10 or 12 thousand years ago (no more than 40) - there were no dogs, just wolves! All of a sudden (in evolutionary time frames) things changed! People changed, societies changed, people started living in large permenant settlements (cities or towns). These cities and towns created a new problem - GARBAGE! Some of the local wolves weren't as skitish as others; some of them actually hung around the garbage heaps to eat and became accustomed to being around people. These few, brave, wolves eventually became "domesticated" in a very short period of time. All over the world, this happened as people banded together in different groups. These widely varied groupes had widely varied interests and thusly favored some of thier "wolf" puppies over others - that's how dog breeding got started. Some dogs became good at fetching game for hunters, some dogs became good at helping to herd livestock. Do you see what I'm getting at here? The whole point isn't just a cannine history lesson - HELLO! Do you realize how very different dogs are around the world? As far as I know, Great Danes and Chihuahuas are, still, just one species. They are just bred for different reasons - with very different results!
Hemp has been selectively bred (for thousands of years) to yield the best fiber and seeds. Marijuana has been selectively bred (for thousands of years) to yield the best "Bud". Yes, they are both varities of cannabis, it's just that simple. Go and figure!