You may have noticed how easy it is to slide from “there is such a thing as human nature” to “humans naturally behave in a certain way”. These statements seem semantically equivalent, but for many people, to say that something is “natural” is almost the same as saying that it is virtuous (see the Appeal to nature.) In this worldview, saying that “selfishness is natural” is a logical contradiction. It cannot be true.
Philosophers have long noted the difference between saying that this is the way things are (these are the facts) and this is what we should do (see the Is and Ought problem). But is it entirely unsound to move from is to ought? If we can't use what we know about the world, about ourselves, and about how we got this way, then we have no rational basis at all for moral argument.
Consider the observation that the human hand happens to be peculiarly well-formed to be used as a weapon (by making a fist, see Fighting Shaped Human Hands). This has the obvious evolutionary explanation that at some point in our history there was a significant advantage to having a powerful punch. People who could form a closed fist where the thumb buttressed the fist were more successful at surviving and reproducing than those who (like non-human primates) couldn't punch properly.
You might question the validity of this sort of reasoning in general (see Just-So Stories) or based on the details of this specific claim, but for the moment suppose that you accept this as true. Where does this leave the moral status of punching people? “Your honor, people of the jury, this man's hand was made for punching.”
This is ridiculous as a legal defense of aggressive behavior, and is equally suspect as a moral argument. Yet to deny the importance of violence in the formative environments for human nature is equally unreasonable. A capacity for violence and an inclination to be violent when our interests are threatened is part of our behavioral legacy. The appeal of taking what you want by force is also sufficiently obvious that it can be rediscovered by each generation of toddlers.
See Evolutionary Ethics.