We are playing dictionary games. Call it what you want, but morality is a reflection of that.
You are limited by your premise to thouroughly naturalistic explanations based on material causes.
The causes are how we interact socially.
No, here I simply accepted your premise for the sake of argument, not because I think it is entirely factual. My point is that Darwin had no foundation for any of his ethical judgments.
The naturalistic view was his foundation. It is the same foundation that religious ethics and morality is built upon, although religions don't like to credit the source.
In that case it would be just as arbitrary and subjective as your utilitarian ethic.
It is arbitrary and subjective. Long ago some people founded a religion, including in it the societal mores of the time. They later wrote these mores as laws into the religion's founding documents. That is your arbitrary and subjective starting point.
Notice how the Bible doesn't condemn slavery? That's because slavery was normal and accepted in the society of the time. I don't think slavery is moral or ethical, but that's because I live in a post-slavery society. You do have the rare person who comes along and introduces new ideas on how society could function better than it did before, and they'll be accepted and perpetuated if they actually work in society. Jesus had some great ideas, and that's why Christianity lasted and prospered. Hitler had some ideas on how to make society better, but they didn't last very long. In fact, the more sympathetic societies smacked him down quite hard. Stalin and Lenin had some ideas too, and the USSR is gone while we're still here.
Morality, as I am using the term, means a system of rules and propositions correctly describing how one should and should not act. Morality is prescriptive, not merely descriptive. It is objective, and by objective I mean that moral principles are valid, binding, and true independently of whether any of us think, feel, or believe them to be so. Morality says what ought to be, not merely what is.
The naturalistic view was his foundation. It is the same foundation that religious ethics and morality is built upon, although religions don't like to credit the source.
If what you are calling 'morality' resulted from natural biological selection, and nothing else, the logical implication of your presupposition is that moral beliefs are products of a process that is entirely independent of their truth and while they might happen to be true, you have no reason at all for thinking that they are. As there is no fact about the world that can vindicate the inescapable authority that moral judgments purport to have, you have, in terms of your own worldview no justification for endorsing ANY moral proposition at all. To do so, as Darwin did, and as you have done, is irrational.
Cordially,