Why 'should' atheists be moral?

[Mudhammam]

B-b-but Chad, evolution is not bound by human morals.

Sure it is. Once you have rational beings capable of acting contrary to evolved instincts then morality has already started to play a role in how that species evolves. Mankind has now reached the point where we can effectively shape the direction of our own evolution by eugenics, genetic manipulation, selective abortion, etc. In so doing, people must make moral choices based on something other than their evolutionary history.

The assumption here being that intellects aren't equally influenced by evolutionary development as any other process that allows a particular organism to thrive or fail in its environment.