What Makes Evolution a Defeater?

Evolutionary Debunking Arguments purport to show that our moral beliefs do not amount to knowledge because these beliefs are “debunked” by the fact that our moral beliefs are, in some way, the product of evolutionary forces. But there is a substantial gap in this argument between its main evolutionary premise and the skeptical conclusion. What is it, exactly, about the evolutionary origins of moral beliefs that would create problems for realist views in metaethics? I argue that evolutionary debunking arguments are best understood as offering up defeaters for our moral beliefs. Moreover, the defeater in question is a paradigmatic instance of undercutting defeat. If anything is an undercutting defeater, then learning about the evolutionary origins of our moral beliefs is a defeater for those beliefs.