In this Google search, the very first hit would have shown you how evolution can explain why we do act morally, even though it says nothing about why you ought to care about morallity.
I've read the hit link and it doesn't explain the problem to me.
Just exactly who is the "we". If there was advantage to an act which was generally considered immoral some might do it and then more would follow. If the "how" , a teleology anyway, doesn't suit the self interest of some and they have no "why they ought" to inhibit them they are going to do it. Once they have the advantage they can use it to justify their actions and if the advantage gave them control of media and law the immoral act would become accepted by everyone.
I think Thomas that you are defining the "we" as a collective of nice guys like yourself.
The "how" and the "why they ought" look to be in opposition on things like global warming or general inequality of energy use. Does your "we" include the millions of poor people living in low lying areas of the world. Or take drug use in athletics. Or the French revolution. Or the way media is now being used to skim millions of small amounts out of the population by preying on loneliness and alienation. Or the vast differences in infant mortality which exist in the world.
Your argument seems to me to be a justification of the status quo in which you are doing okay and if you have no "why they ought" you can then define as immoral anything which challenges it just as the robber barons embraced Darwin to justify their luxurious lifestyles with the rest of the population suffering under their rule effectively as slaves.
And it is impossible to find a "why they ought" without a revealed moral code which, in the ideal case, is interpreted by an organised class of people who live under vows of chastity and poverty and whose conclusions transcend their lifetimes.
From an evolutionary point of view the belief of the mass of people in the Christian ideal has certainly worked. To remove that belief is a leap in the dark. A faith even. If the elite have cynically engineered such a mass belief they have been justified by the outcome.
What I think your problem is, and many others, is that you hate the idea of having been tricked despite the value of it to you and you seek to distance yourself from this mass for your own self respect. But it is a feature of the elite that they maintain the trick and you refusing to do puts you outside that elite and thus a member of the mass.
You forget that the appearance of virtue is as good as virtue itself and you call people fools and idiots who may well have as many reservations as yourself but bow, with some humility, to the appearence.
Hypocrisy is a valuable tool.
We all know what a father gives away at a proper wedding. Would you rather it was made plain to everyone.