Any complex adaptation, requiring many genes to work together, cannot all evolve at once, it would be too unlikely a mutation. Instead, pieces evolve one by one, each individually useful in the context they first appear. However, there is not enough selection pressure to evolve a new piece unless the old pieces are already universal, so you would not expect anything complicated to exist in some but not all members of a species.
I get that. I don't see how that could imply that quantitative variation must be controlled by a single gene.
I also don't see how the magnitude of variation in intelligence affects the argument ("particularly wide intelligence spread" is subjective).
It doesn't quite have to be controlled by a single gene, I was giving an example. Something like height, which is affected by many factors, could be affected by lots of single gene substitutions, but you would expect the over-all effect to look like an averaging out, not like some humans having one set of decision making machinery and others having a totally different set.
Today's post, Zut Allais! was originally published on 20 January 2008. A summary (taken from the LW wiki):
Discuss the post here (rather than in the comments to the original post).
This post is part of the Rerunning the Sequences series, where we'll be going through Eliezer Yudkowsky's old posts in order so that people who are interested can (re-)read and discuss them. The previous post was The Allais Paradox, and you can use the sequence_reruns tag or rss feed to follow the rest of the series.
Sequence reruns are a community-driven effort. You can participate by re-reading the sequence post, discussing it here, posting the next day's sequence reruns post, or summarizing forthcoming articles on the wiki. Go here for more details, or to have meta discussions about the Rerunning the Sequences series.