Here are two questions for moral theorists.
1) Could people do wrong if your moral theory was true?
2) Could people do right if your moral theory was true?
It is obvious that most moral theorists would answer both questions in the affirmative.
But if people could do wrong, then there could also be natural selection for a disposition to do wrong when doing wrong would increase the wrongdoer’s fitness. (There are obviously cases in which doing wrong could increase the wrongdoer’s fitness. For example, stealing food during a famine could make the thief more viable and committing rape in wartime could make it more probable that the rapist would have offspring. Biological fitness is a function of viability, fertility, and the trade-offs between them.)
If there were natural selection for a disposition to do wrong when doing wrong would increase the wrongdoer’s fitness, the disposition would tend to render people unable to do right.
So, in a world in which people are products of evolution by heritable variation and natural selection, if people could do wrong, then they would tend to be unable to do right.
Conversely, there would be selection for a disposition to do right when doing right increased fitness. In such cases, people would tend to be unable to do wrong.
We take it as obvious that the answers to the questions set out at the beginning are both affirmative. In an evolutionary world, however, the answers should both be negative. Hence, moral theorists have to explain how the answers could be affirmative in the actual world, which is an evolutionary world. Therefore, moral theorists have to engage with the theory of evolution and have to explain how their moral worldview can be compatible with evolution by heritable variation and natural selection.
As a matter of fact, most moral theories are not fit for purpose.