Imagine a world in which we were products of natural selection. That world would be much different than ours. All of our actions would be motivated by survival. There would be no room for self-sacrifice or self-limitation. There would be only violence, with the weak subject to the will of the strong.
Similar to the animal kingdom, the only virtue of our survival-of-the-fittest world would be strength. Yet, in the real world, compassion and sacrifice are universally recognized as good by human beings. Clearly, natural biological forces do not create morality.