His exact words were: "The fact that people naturally develop morals happens by means of natural selection."
First, I believe that the fundamental burden of proof rests on those who make the assertion that morals can develop and evolve without the existence of god. This is because the most basic understanding of morality logically assumes a single standard (or rule) of thought in order to exist. In other words, it is the assumption that a moral standard is according to something, and the charge that ethics develop without a single standard is difficult to imagine.
Second, there is the existence of a common authority, to which atheists cannot appeal, since the absence of a deity leaves the individual as autonomous and sovereign. And billions of autonomous and sovereigns do not make a common authority without the presence of a standard thought. C. S. Lewis wrote, "Ninety-nine per cent of the things you believe are believed on authority" (Mere Christianity). The question which the atheistic mind does not answer is which authority one is to believe.
Third, if such a statement were true then we would find that it applies to all morality. But it doesn't. Even though the history of civilization has always seen the rise and fall of various moral systems, and even our recent cultural history has seen trends of moral consciousness and apathy, there have always been consistent thoughts regarding life and death. That there is a certain wrongness to taking another's life is always scrutinized. Sacrifices to gods are given justification for the act, and infant exposure must be explained away (just like in modern abortion arguments). But taking life is a moment in time - perhaps the pivotal moment - where morality appears and we must confront the reality of the source and meaning of life.
Although others have made more complete attempts at this argument (and have undoubtedly articulated it with greater clarity), I wanted to post a brief response to some of the immediate thoughts that I had to the aforementioned statement. The notion that morals are a product of natural selection is absurd, for it would mean that someone was behind the selecting . . . and those who deny the existence of a god cannot rightly appeal to a universal conscience without a massive problem. But that is the basis for which I gave my response to this misguided gentleman, albeit in a much shorter statement.