Watch this drama play out among three species of African animals and choose which of the animals is behaving morally and which immorally. But, you might object, morality is an inappropriate category when talking about animals. True enough, but if Darwinistic naturalism is correct then animals is all we are, so why do we think moral categories apply to humans?
If these were three groups of human beings involved in this episode which group would, on the Darwinian view, be behaving morally and which immorally?
If Darwinism is correct, if we are simply animals that have evolved bipedalism, then whence comes right and wrong? From where do we get the notion that we are duty-bound to refrain from cruelty and violence?
It will do no good to claim that humans have evolved a moral sense and other animals haven't. If our moral sense is a product of blind, impersonal forces then what obligates to pay it any heed? Just because we have it doesn't mean we need it or should value it. Evolution has equipped us with hair on our faces, but it doesn't follow that we should have it.
Moreover, just as we have evolved the sense that we shouldn't be selfish we have also evolved the desire to be selfish. Why should one of these be privileged over the other? By what standard do we decide which behavior is "right"?
Moral obligation can only exist if it is imposed by a personal, transcendent moral authority. Natural selection and random mutation can't obligate anyone to do anything.
Thus, the naturalist has to either give up his naturalism in order to keep his belief that moral duties exist, or he has to give up his belief in moral duties in order to hold on to his naturalism. What he can't do, and claim to be living rationally, is hold onto both of them.