For the curious...

BerryPick6's summary of Walter-Sinnott Armstrong's moral views — "Most of our Moral judgments and intuitions are much better explained by natural selection, evolutionary psychology and other natural processes than by an appeal to some ontological beings/facts/laws" — concerns what mainstream philosophy calls "evolutionary debunking arguments" (the classic paper here is Street 2006, but the point had been made less thoroughly many times before by both scientists and philosophers).

I should clarify, though, that evolutionary debunking arguments aren't the focus of Jackson's metaethical work, though as a naturalist Jackson assumes the basics of evolutionary biology and psychology and their implications for the origins of our moral attitudes.

The purpose of Jackson's analytic descriptivism is, rather, to explain why something that feels kinda like moral realism can still be true despite the universe's lack of spooky intrinsic normativity. Analytic descriptivism is one of several approaches for grounding (moral) normative properties in natural, descriptive properties. (Other well-known approaches to this include Railton's and, less well-developed, Foot's.)

For more, see my April 2011 blog post on Jackson's theory. The best explanation of Jackson's theory is, still, Miller (2003). Luckily, the next edition of Miller's excellent book should be available early next year.