Re: Brain size, IQ

Len Piotrowski (
Mon, 26 Aug 1996 21:44:30 GMT

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.

>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.

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."

>>The evolutionary significance of altered developmental ratios for humans,
>>seems to affect the sexes differently. Females appear to experience prolonged
>>developmental growth with respect to males, thus apparently increasing the
>>time and development of the neural net. A byproduct of this developmental
>>retardation may just be something measurable by the human IQ test. Who knows?

>I don't follow this. Sorry. Could you reword this?

Developmental retardation leads to greater head to body ratio through the
early growth years, contributing to relatively larger brains, and accentuated
cognitive development into the adult. This developmental process appears to be
extended among female humans.