Re: Brain size, IQ

Bryant (
28 Aug 1996 07:05:00 -0600

In article <>,
Len Piotrowski <> wrote:
>In article <4vt1qo$> (Bryant) writes:
>>What's controversial about the notion that cranial capacity was heritable
>>during human evolution? How else do you explain the dramatic changes in
>>head size through time, as evidenced in the fossil record of our ancestors?
>The controversy is over the simple notion that brain size was heritable and
>not something else of which head size (and thus brain size) was an effect.

Um, so you believe that Australopithicine brains and Homo sapiens brains
would be the same if subjected to the same set of environemntal effects?
Why stop there? Why not posit that plopping a canary brain in a mouse
skull will result in a perfectly mouse-ly brain in the developed adult?

>>Heritability means, evolutionarily, "not fixed." That is, genetic or
>>allelic variability is responsible for phenotypic variability for a given
>>trait. Once an allele is fixed in a population, heritability is zero,
>>because everybody has the same allele for that trait, and phenotypic
>>variation in that trait cannot be accounted for genetically.
>By this definition, the human brain size can be viewed as relatively "fixed"
>since Neanderthal times, and even relatively unvarying before that with Homo
>erectus, which by your definition could indicate that brain size was not
>"heritable." I'm sure this is not what you are driving at.

I don't recall the cubic centimeters, but do seem to recall some
substantial growth in cranial capacity since H. erectus. My argument was
that brain size was clearly heritable in the evolutionary past, as
demonstrated by the fossil record.

>At any rate, if developmental genes are heritable, their affect on phenotypic
>variability of other characteristics like the brain could be considerable,
>even if those other genetic characteristics were "fixed."

In which case, via pleiotropy, those are also genes "for" whatever brain
characteristics they affect, and brain development is therefore, by
definition, not fixed and species typical.