Remember that all complex adaptions must be universal so there must be a simple difference, something like single gene present or absent, which controls how much someone desires certainty for any of the variance to be genetic.
This doesn't appear to be the case for genetic variation in intelligence. (Also, I don't see how it follows in the first place.)
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.
With intelligence, it seems like many different factors can affect it on the margins, because the brain is a complex organ that can...
Today's post, Zut Allais! was originally published on 20 January 2008. A summary (taken from the LW wiki):
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