Neo-Darwinian evolutionary theory tries to explain everything that it tries to explain about the behavior of organisms in terms of "inclusive fitness": if a gene influences the behavior of its host organism in such a way as to increase the number of copies of itself in future generations of related organisms, then it, and the behaviors it encourages, will tend to spread.
Among the behaviors that neo-Darwinian evolutionary theory tries to explain in these terms are various human moral prescriptions & proscriptions.
For example, the aversion to and prohibition of incest between close family members - especially between parents and children, and between brothers and sisters: since the offspring of such unions suffer from elevated levels of genetic diseases, a gene that influenced its host to feel averse to sexual relations with parents, &/or children, &/or siblings would tend to spread.
In due course, feelings of aversion to such relations would become prevalent. The prevalence of such feelings would in turn lead to the adoption of moral rules prohibiting such conduct.
Personally, I find this, in outline, a fairly persuasive explanation for the ubiquity of traditional moral rules forbidding incest between close family members.
But is it - could it ever be - a justification for such rules?
Imagine a father and daughter (of the age of consent), in lust with one another, who argued as follows:
The prohibition of father-daughter incest is a product of primitive conditions that no longer obtain. We have no intention of producing any offspring. I've had a vasectomy, and she's on the pill. Moreover, in the unlikely event that she still managed to conceive a child, we could always have it checked before birth for genetic defects, and abort if necessary. So what's the problem?
Could a neo-Darwinian evolutionary theorist, qua neo-Darwinian evolutionary theorist, make any effective rebuttal to such an argument?
I don't think so.
More generally: while I think that neo-Darwinian evolutionary theorists can offer all sorts of interesting explanations for traditional moral judgments, I do not think that they can justify any moral judgments whatsoever.