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.
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.