OK--so explain right and wrong without God. Explain whyright is right and wrong is wrong.
Explain, for example, why killing the woman who is sleeping with your husband is wrong, when (according to Darwin), it is merely preserving your genetic line.
Why is theft wrong if it means you "get ahead" in life? You're just adapting to circumstances and ensuring your family does better--thus continuing your line.
I use the law as my guideline. It works well enough for me.