There is no legal difference between two states of affairs without a difference in the material facts.
There is no moral difference between two states of affairs without a difference in the material facts.
Both of these statements appear to be true. Philosophers allude to supervenience in the latter case but not in the former. Given the parallelism of the two statements, however, it would be odd if supervenience were necessary in the latter case but superfluous in the former.
We speak of acts being lawful or unlawful. Synonymously, we speak of acts being consistent or inconsistent with the law. Hence, it is also possible to speak of acts being consistent or inconsistent with morality.
Indeed, we can speak of acts being consistent or inconsistent with morality, when morality is based on the commands of God, Platonic moral principles, or the foundational attitudes I postulate. Each of these can function in ways analogous to the ways in which statutes function.
Now, although almost no beliefs are products of evolution, it is not impossible for there to be selection for beliefs. If the content of a naturally selected belief included normative concepts, then we could explain how normativity could originate in a world that started out lacking it. Explaining the origin or morality in this way is not a matter of deriving prescriptions from descriptions – natural selection is not logical inference. It is probably the only realistic way of explaining how normativity came into the actual world.
It would be Procrustean to require talk about consistency and evolution to be translated into the idiom of supervenience.