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 be slowed down, sped up or damaged in many ways. However, I do not notice a particularly wide intelligence spread among humans, only in rare cases where something is genuinely broken do we find someone less intelligent than a chimpanzee, and we literally never find someone more intelligent by an equivalent amount.
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.
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 be slowed down, sped up or damaged in many ways. However, I do not notice a particularly wide intelligence spread among humans, only in rare cases where something is genuinely broken do we find someone less intelligent than a chimpanzee, and we literally never find someone more intelligent by an equivalent amount.
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.