Can evolution explain the Moral Law?

A listener wrote in, asking about Lewis' argument in Book I of Mere Christianity. He asked:

What does Lewis have to say about evolution? I understand his objection to the instinct argument and herd mentality, but what if evolution has written into our nature that helping others is an evolutionary necessity? So, what we consider the moral law is simply a result of evolution… We had to learn to help one another in order to survive and now selflessness is engrained as more valuable. Our higher order thinking skills allow us to use context and choose between “instincts”. Not sure if I’m thinking about this correctly, but I would sure like to be pointed in the right direction.

Regarding evolution, Lewis seems to have broadly accepted the idea that different animal forms arose and were weeded out by natural selection.

Firstly, even if it could be shown conclusively that evolution is the material cause of the Moral Law, it wouldn't disprove the existence of God. The Christian can simply say that Evolution was the means means through which God taught us some basic moral truths. To steal Lewis' analogy, a mathematics teacher might teach you Calculus or arithmetic, but that doesn't mean that it's purely subjective - he's simply the means through which you come to understand objective truth.

Secondly, citing evolution doesn't explain what our conscience is. As Lewis explains, it can't be an instinct. If so, what actually is it?

Thirdly, the Moral Law seems to cut against an awful lot of what we would typically think of as Natural Selection's "Survival of the Fittest".

Fourthly, assuming that what we call "The Moral Law" was simply an accidental product of evolution, why should I obey it?! It has no authority over me! Why should I follow arbitrary rules of right-conduct just because evolution simply happens to have resulted in this? I would argue that I would be a chump if I did! Following the Moral Law is often extremely costly! I should really only focus on my own flourishing and to hell with everyone else. Simply saying that following the Moral Law helps society doesn't tell me why I should obey it. If we live in a mindless universe of pitiless indifference, why should I care about benefits to society unless it affects me directly?