The two topics don't go together all that well. But, since a blog I found via RationalWiki posted this "Moral Argument Against Darwinism", I suppose I should let them hang together for now and start analyzing that post's logic.
There are a lot of random points I can pick on, but I think the critical failure point is in the definition of the human race. The post attempts to argue, in its first point, that the nature of humanity's evolutionary development means there is no single definition of the human race. Or rather, it argues that humanity has no essential nature, and then proceeds from there as if it is impossible to define the human race as a whole.
I think that's a serious definitional problem. I might actually agree with the idea that the human race has no essential nature. There are some traits in the here and now that most people would use to define the human race, but I wonder if they would all still be present in another couple of million years - or for that matter if there might not be new traits that we regard as a necessary part of being human by that time. With that said, those distant possibilities of a change in our nature do not imply that there are no traits which we can use to define humanity as a whole. That definition of humanity is not the same thing as our essential nature, since I think it might change over time, but that doesn't mean it's completely useless.
Naturally, one of the things I would use that definition for is to provide something which deserves objective intrinsic rights; which is exactly what his logic chain says I can't do at point 4. To be fair, he knows that it should be possible to give humanity those rights, since he's using an argument from absurdity to prove evolution false. The problem is that there are other things - like the definitional error I just pointed out - which might be the problem that led to the absurd conclusion.
In short? That particular logical argument is invalid - or at least incomplete. I'm sure there are ways to patch the holes if I were to actually engage this person in a debate, but I think they would ultimately founder on the naturalistic fallacy.
Specifically, even if we accept the first few points, 4 through 6 are all wrong, because how nature designed us has no bearing on what we believe to be right or wrong. We can have a philosophical basis to believe all humans are created equally in terms of rights even if nature grants us different physical traits.