Darwinism proposes a comprehensive explanation of human nature. There is no aspect of human nature, not the smallest part, that is not the product of evolution. There is nothing that human beings do, or can possibly do, that doesn't have its origin in evolution. Darwinism insists that no aspect of nature transcends its own mechanisms.
So the facts, whatever they are, can always be accounted for by saying "because X evolved that way." Here is John Derbyshire explaining attitudes about abortion:
The problem is, of course, that a fetus is a rather particular kind of person: one sharing the body resources of another human being. It is manifestly not the case that we all agree the killing of this particular kind of person should not be permitted. Stamping your foot and yelling “But it’s a person! It’s a person!” doesn’t advance the argument.
Supernaturalists can of course point to divine ordinances, scripture, the Tao, and so on. That’s great, except that we are not all supernaturalists, so these appeals fall on a lot of stony ground. Since a great majority of people claim to be supernaturalists, these appeals might none the less form the basis for a consensus; but there is no sign that that is happening. The supernaturalist case seems to be unconvincing even to a lot of supernaturalists.
I’d guess that all the noise and confusion has origins within the scope of cognitive science and evolutionary psychology. Our brains evolved to cope with commonplace features of the world: physical features (water flows downhill) and social features (every human group has gradations of status). Fetuses — and embryos much less — just were not a feature of everyday experience until recently. The brain has no developed categories for coping with them, either as physical or social objects.
Yes, all that noise and confusion has origins within the scope of cognitive science and evolution. That is because everything about human nature, and all its possible manifestations, has origins within the scope of cognitive science and evolution. Our brains evolved to cope with commonplace features of the world, yes. Yet we must also deal with uncommonplace features of the world - the occasional tsunami, for example. Do we deal with them through means that escape, or even possibly transcend, evolutionary mechanisms? Of course not; nothing is allowed to escape evolutionary mechanisms, so in whatever way we deal with uncommonplace features of the world, our brains evolved to do it that way.
Why do religious believers invoke sacred scripture, the Tao, etc. in condemnation of abortion? Because their brains evolved that way. Why are some people not supernaturalists? Because their brains evolved that way. Why do some people think that tracing moral and social issues to a remote evolutionary past adds anything to the discussion? Because their brains evolved that way. Why do I think that discussions about evolutionary history add nothing one way or the other to discussions of the morality of abortion? Because my brain evolved that way. Why does John Derbyshire conclude from the fact that evolution has not lead to a moral consensus on abortion, that no reasonable case can be made against abortion? Because his brain evolved that way.
The real trick in invoking Darwinism on moral issues is the sleight of hand that turns an apparently descriptive account of human nature into a prescriptive one. Anti-abortion advocates advance all sorts of rational arguments against abortion. The fact that these arguments are made, and found persuasive by many, is a natural fact as much as any other. The only way people could find these arguments persuasive is if their brains had evolved "the categories for coping with them." Yet Derbyshire tells us authoritatively that asserting that the unborn is a person doesn't advance the argument. Why not? Well, because we haven't evolved the "developed categories for coping with them." But the people who make the arguments clearly have developed the categories for coping with them, or they couldn't make the arguments. Everything is a result of evolution.
This is where the argument turns prescriptive. Some manifestations of human nature are more equal than others. The fact that many people find anti-abortion arguments reasonable and compelling is not a fact that must be accounted for by evolution; it is an inconvenient piece of data that must be undermined. Those anti-abortion folks aren't making real arguments, you see, because our brains have not evolved the categories for dealing with them. They are just fake arguments. In a few years, when enough people have fallen for this nonsense, nobody will make those arguments anymore - then our prescriptive argument will safely collapse into a descriptive one.
How do I know this will happen? Because my brain evolved to...