“No abstract moral principles exist outside the particular nature of individual species”
But Evolution does not work on species ! Evolution (including social evolution) works on groups within a species.
Some consider that all human societies could share many common/ similar moral values. Therefore, an hypothesis is that Evolution has nothing to do with moral values. But is it not simplistic ? Human survival history, over the last few million years, made life difficult for all the pre-human and human groups. And the social organizations which they « invented » were strengthen by the beliefs & values which they shared within their group / larger group. And, as on earth there were not thousands of ways to survive, some « common rules/ moral values » had to be similar for the purpose (ref. the incest issue I mention later). There are therefore some key « moral bridges » through which most humans groups which survived had in common a key purpose to be addressed through rules/ values. And we come from there.
Evolution AND our specific environment shaped us physically & mentally.
Very different species, living on different continents, often show a parallel evolution when the environment is similar. We are no different in principle.
When we say « incest », we should say « incest prohibition ».
And this prohibition is linked with the concept of relatives and marriage.
Australian Aboriginals have an extremely complex conception of relatives.
Therefore, incest for them is very different from our judeo-chistian modern definition.
Hundreds of societies have different definitions of incest.
What remains is that the incest prohibition is present in all societies.