Bobby A has Bobby B, Bobby B is bigger and more complex requiring more nutrients. Bobby A survives while Bobby B dies due to a lack of sufficient nutrients to support itself.

That's a common problem.

Or..

Bobby A eats Bobby B since Bobby B is too small.

Or...

Bobby A has a more efficient way of reproducing and so Bobby B mostly dies off.

Or...

Bobby A is more suited for the environment than Bobby B


All in all you must realize that "natural selection" would murderize Bobby B anytime he popped in. The environment would have to be factored just right to allow for a new species to exist as well as continue to create brand spanking new types.

And isn't it interesting how wide spread such things as plants and animals have become? Why is it that trees are mostly the same the world over, as well as animals, when they've primarily come from different environments as well as ..uh..that thing, which..changes people..not factors..that..thing...words evil.

DIE WORDS DIE

But pretty much: Animals have almost no real difference genetically speaking to their buddies. Tigers and Lions and Jaguars are mostly the same in genetics, save for about a .01 percent change.
As has been stated before on these dark forums and by scientists 95-98% of the DNA is "buffer" DNA. The rest of the DNA is all about characteristics and crap. That means that 2-5% of your DNA is where lies everything that makes you different, and yet somehow it hasn't really changed in comparison to animals not even related to you. Such huge differences in such a minor amount. Ah..I started rambling and going way off topic didn't I?