Moral naturalism is the theory of evolution applied to ethics. It is a subset of Social Darwinism. Rather than defining morality as a set of divine commands, moral naturalism sees morality as merely an unintended side-effect of the social interaction of free agents, in the same manner that natural selection is said to adjust the characteristics of organisms to changing environmental conditions.
Flew, Anthony. A Dictionary of Philosophy, Revised Second Edition, St. Martin's Press, N.Y., 1979