I agree with the gist of your post but I see some errors or points of disagreement which I've outlined below.
I agree with this statement, There isn't any such thing as objective morality, just subjective morality that's been accepted by more people
Morality in general reflects a society
rather than an individual.
When we are discussing morality, we are really discussing culture.
There isn't any such thing as objective morality, just subjective morality that's been accepted by more people.
Most moral systems are functional and the question is begged why
its accepted by many people.
And that includes religious morality systems. One of the axioms of morality is that you have to be able to recognize the difference between moral and immoral behavior, so I don't believe it can ultimately come from something like natural selection, because natural selection is utterly indifferent to moral matters.
The statement in bold is circular reasoning and for your information, a form of "natural" selection of memes ("ideas") has been apparently formalized into a theory by Blackwell et al. The theory is, memes like religions, value systems, design ideas, languages, evolve in a way that is analogous to Darwinian biological evolution ... Ironically, some of the humourous creationist examples of "evolution" are in fact possibly true in some cases in the context of memes rather than genes.
There are aspects of human moral behavior that probably
in my opinion are related in some way to natural selection:
1) empathy - an instinct for empathy exists. I can imagine this serves a purpose in making individuals in a group closer together which protects the group.
2) conformity - this is also group behavior which protects each of the members of the group. Those who don't conform risk being rejected by the group which can lead to problems
3) language - humans have a linguistic instinct which allows them to codify rules.
Enforcing the rule of not killing has an obvious benefit to those who support that idea. That is, each of them, is less likely to be killed and therefore more likely to breed.
On the otherhand, most human moral systems justify killing in war or self defense and that
is also condusive to surviving.
All that matters in natural selection is whether a behavior allows a species to successfully pass on its genes to the next generation, so (for example), you could have a species which killed and ate members which would not survive to puberty or which were too old to reproduce, and it might do quite well. Yet, such a behavior would be subjective in nature, not objective.
One point of correction, "all that matters in natural selection is whether behavior allows an individual
to pass on its genes or not".
If a given species has the same strategy, that
would indeed be "objective".Why I agree that human morality is subjective
The proof is in the pudding. Several well known moral systems exist and each of them values different things.
There may be a general "common core"; e.g., murder is usually immoral. There are functional reasons for this which is why, most likely its usually immoral.
I think human morality is best understood by understanding:
1) The human social instincts such as the 3 I mentioned above
2) The theory of memes viewed in the context of the history of a culture