Why is there any "ought" at all if evolution has nothing to say about it?
You might as well ask "why is there any 'gravity' at all if evolution has nothing to say about it?"
Well, morality, unlike gravity, seems to be an activity that is at least in some senses uniquely human, and so should be able to accounted for in naturalistic, evolutionary terms, since the theory says that humans are the product of undirected natural forces.
See for example, The Moral Animal — Why We Are the Way We Are: The New Science of Evolutionary Psychology, by Robert Wright.
or
The Origins of Virtue : Human Instincts and the Evolution of Cooperation by Matt Ridley
Cordially,