If our perception that some deeds are good and others are not is but a quirk of natural selection, none of us need feel any commitment to morality or thics.
Believers, on the other hand, accept a divine morality that is objective, in that we know the essence of what is right or wrong without appealing to our own subjective intuitions. An atheist might feel that murder is wrong, but what makes his opinion anymore valid than the serial killer on death row?
I believe this argument is flawed for a number of reasons. In theory I could see a value to objective morality, but reality negates that possibility.
But I want to focus on one specific point, which I think mitigates the whole argument for objective morality. While I accept that G-d gave us ethical guidelines that we must obey (and am willing to agree that those guidelines prescribe "good" conduct), those same guidelines are subject to human wants and desires.
Let me us an analogy. Very few serious legal scholars believe that all legal questions have correct answers. The easy questions certainly do. A person who drives through a red light is certainly subject to a fine. The difficult questions, however, usually have large grey areas and are subject to intuition and moral judgment.
The same applies to ethical guidelines. Let's assume that murder is the unnecessary killing of innocents. I think most people would agree that killing a random schoolchild for the fun of it is immoral. However, what about killing a bus full of Israeli civilians in retaliation for the IDF's targeted killing of a big Hamas leader? The suicide bomber might reason that the killing is necessary in order to prevent further attacks on Hamas. Moreover, he might argue that most Israeli civilians are soldiers or that they support the immoral regime politically, economically, and psychologically and are not innocents.
Now I'd assume most of us reading this post would disagree. But even if we agreed with the terrorists definition, we'd still disagree with his application of the principle to material facts.
I'm not even making the obvious point that if there is an objective morality, we still have no way of being sure what it is. Even if the whole world would agree about certain principles and their definitions, we'd still disagree about how to apply them. In what way are those ethical beliefs more objective than the atheists who disagree on the principles and the application? Sure, we're one step ahead because we believe we have the principles down pat, but as every good lawyer know, the law is in the details. The application is the key.
Land for peace is a classic example. Much of the Religious Zionist world believes that ceding land is wrong. Others argue that saving lives is paramount. Which group is right? I think both make compelling arguments from a halachic and moral standpoint. But if both positions are reasonable, then in what way are Jewish ethics anymore objective than Peter Singer's? Singer and other Utilitarians might disagree about animal rights (even if for the most part they agree about the general principle of the greatest good for the greatest number), just as we disagree about land for peace. Who's right? I don't know. But let's not pretend that believers have access to some divine morality that is clearly defined in every situation.