While I am on the subject of atheists debating the possibility of (moral) value without God, without a sufficient understanding of their subject matter, I should add a word about those who think that we have an evolved set of moral dispositions.
One thing I expect any atheist, who steps into the debate ring with a theist to discuss the issue of morality, to know is the Euthyphro dilemma. This was Plato's argument (attributed to Socrates) that is thought to destroy any possibility that morality comes from God,
In the dialogue called Euthyphro, Plato has Socrates talking to a fellow citizen named Euthyphro on the nature of the good. Euthyphro tells Socrates that what is good is that which is loved by the gods. Socrates then asks whether it is good because it is loved by the gods, or is it loved by the gods because it is good.
If it is good because it is loved by the gods, then anything loved by the gods would be good. The torturing of a young child for pleasure would be good, if only the gods named it so. Furthermore, there is no reason for the gods to choose goodness in something other than the torturing of a child for pleasure, because there is nothing outside of what the gods like for even the gods to appeal to.
On the other hand, if it is loved by the gods because it is good, then goodness exists independent of whatever it is that is loved by the gods. Even the gods have to ask, “What is good?” before they can determine what deserves their love and what does not. So, we have not answered the question of what goodness is by appeal to what is loved by the gods. We have only said that, whatever goodness is, the gods love it.
The evolutionary ethicist goes into his debate with the theist knowing this objection to divine command theories of ethics in most cases, and yet utterly ignores this same problem when he presents his own theory.
When asked, "What is good?" the evolutionary ethicist effectively answers, "What is good is that which is loved by the genes."
Against this, a 21st century Socrates can ask, "Is it loved by the genes because it is good, or is it good because it is loved by the genes?"
If it is good because it is loved by our genes, then anything that comes to be loved by the genes can become good. If humans, like lions, had a disposition to slaughter their step children, or to behead their mates and eat them, or to attack neighboring tribes and tear their members to bits (all of which occurs in the natural kingdom), then these things would be good. We could not brag that humans evolved a disposition to be moral because morality would be whatever humans evolved a disposition to do.
If, instead, it is loved by our genes because it is good, then we have not yet answered the question of what goodness is. We could not even defend or prove such a claim unless we had an independent criteria for determining what is good, and then looked at evolution to discover if it identified the right things. How can we demonstrate (or how can we attempt to falsify) the thesis that what is good is loved by our genes if we have no account of what goodness is that is independent of what is loved by our genes?
This, in itself, should be sufficient to destroy any evolutionary account of morality - just as the original argument should be sufficient to destroy any divine command account of morality.
Unfortuantely, in these debates, the evolutionary ethicist will go on to morally condemn the theist for their "willful ignorance" of such a fool-proof objection to divine command theories of ethics that the Euythyphro argument provides. In doing so, he asserts, at least implicitly, that he is too good of a person to do anything like that himself - that he would never simply ignore an argument that is fatal to his position because he loves the position to much to consider objections.
The evolutionary ethicist ends up being wrong here as well.
In fact, the evolutionary ethicist is exactly like the theist in this regard. So much so that when the two debate each other on questions of morality, we really are not given much of a choice.