When you put it that way, then yes, genocide is a form of natural selection. It doesn't seem to be natural, but that is a remnant of old theistic thinking. That we are somehow not part of nature, so anything people do is not really natural. But the reality is, we are part of nature, so if we do something it is natural. If we create weapons of metal it is because we have the ability to create and use them. Because we have evolved the ability to do so and as such use those abilities to our fullest advantage.
I stand corrected, as much as we do not want it to be, genocide is a means of natural selection. Unless a more logical explanation of what natural selection is can suggest otherwise, I have not an argument to refute your assertions.
What you're talking about here is social Darwinism, and I suggest you refrain from it, lest you embarrass yourself as Syzygys has with his/her disgusting comment about the Nazi murders of the handicapped and mentally retarded as "natural selection". It's the misconception that murder (or genocide) based on race or ethnicity or any other qualifier is natural and therefore morally neutral.
But it isn't. The practice of culling the population only puts us at a disadvantage, because the best hope for humanity's survival is to promote the widest, most diverse population possible.