What IS good, and how do we determine it?

[Pyrrho]

... Something else to add - if morality comes from trying to do what is best for society so that our species survives, what will it mean when/if overpopulation of humans becomes a major problem? ...

I don't think that morality is a question of what is best for society so that our species survives.  But if it were, it would mean that you Catholics should stop breeding like rabbits because overpopulation is a serious problem.  It is YOUR morality that is a problem.  Atheism is not a source for such immorality.

You might want to look back at what I stated earlier that you previously seemed to like.

It may also be worth mentioning that atheism is not a doctrine, it is the lack of a belief in a god (or, for strong atheism, it is the belief that there is no god).  There is no universal agreement among atheists about morality, and I certainly would not endorse the idea that morality must further the survival of the species (which must, in any case, be a futile thing, as humanity will eventually perish regardless of what anyone does).

Now, it may be that evolution gave rise to empathy, but it does not follow from that that empathy has a specific goal.

If god is added to the equation, it changes nothing as far as morality is concerned.  People who believe that morality is merely following what god says are grossly mistaken.