June 06, 2008

ASPM, MCPH1, CDK5RAP and BRCA1 and general cognition, reading or language

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It seems that about three years after the much publicized discovery of ASPM and MCPH1 variants thought to be under selection, the reason for their selection has yet to be discovered. Is this because these genes influence a human property that we can't measure or haven't thought of measuring, or is it perhaps that the supposed selection signatures are in fact not evidence of selection as an early skeptic (*) suggested?

Intelligence doi:10.1016/j.intell.2008.04.001

Recently-derived variants of brain-size genes ASPM, MCPH1, CDK5RAP and BRCA1 not associated with general cognition, reading or language

Timothy C. Bates et al.


Derived changes in genes associated with primary microcephaly (MCPH) have been suggested to be “currently sweeping to fixation” i.e., increasing in frequency in most populations, with the likely outcome that the derived allele will completely displace the ancestral allele over time. Possible causes for this sweep include effects on human reasoning and language. Here we test the hypothesis that these derived alleles are associated with current variation in spoken or written language and related traits. The association of derived alleles of the ASPM, MCPH1, CDK5RAP2 and BRCA1 genes was tested against well-validated measures of dyslexia, specific language impairment, working memory, IQ, and head-size in a family-based association study of over 1776 subjects from 789 families of twins. No evidence for association was found for any gene to any trait. The results strongly did not support the hypothesis that derived alleles in MCPH-related genes are related to the evolution of human language or cognition. Results were compatible with the alternate hypothesis, suggesting that adaptations in these genes associated with a dramatic increase in brain size have long since reached fixation and are now maintained by stabilizing selection.



The Colonizing Ape said...

Wow, great post. Thanks. I've been following ASPM and MCPH1 closely since they came out in September, 2005. It's good to get an update.

Kosmo said...

Yeah, I second what Ape said, thanks for the update.

When I first heard about ASPM, I thought the gene wasn't likely to cause higher intelligence, since it is difficult to imagine high intelligence would have the kind of selective pressure behind it which this gene obviously has. If there is any reproductive advantage to having an IQ above 100, it is likely very small, and is also very probably offset by a balancing selection against extremely high IQs.

My guess is that all these genes are "domestication" alleles, selected for as we humans, as a species, become more and more dependent on culture to survive.

The Colonizing Ape said...

Well, I could argue interminably about these genes, but I would only be waiting our time. These new findings that disprove correlations to IQ, cooperation, and language skills are important, and just leave more questions swimming in my head. Above all: has ASPM experienced positive selection *at all?*, as Fuli et al put into question...

I'd be currious to hear more about a "domestication" theory. I have trouble comprehending what traits would be selected for that encourage domestication of other species, if it's not IQ or language... An increased empathy or other animals, perhaps? If so, wouldn't that go along with increased cooperation?

Well, either ASPM/MCHP1 can lead to more amazing conclusions, or it could be a complete dud. We'll have to find out.

Kosmo said...

Ape, I'm sorry, but I was typing quickly just as I was getting ready to leave earlier, and I obviously wasn't clear in what I wrote, and I confused you. The fault is mine. You wrote: "I have trouble comprehending what traits would be selected for that encourage domestication of other species"

And, of course, you're right! I would have trouble comprehending that, too.

What I meant by my "domestication" comment was that humans, as a species, for as long as we've been domesticating animals, have also been domesticating ourselves. We've actually gone through many of the same skeletal changes that other animals go through once they are domesticated by us. Reduction in cranial capacity, reduction of muscle mass-- as well as the developement of a long list of neotanous characteristics, which is truly the hallmark of domestication, when you get down to it.

We've been sliding more and more of ourselves into our culture, and relying less and less on our physical beings to survive. I think these brain genes that are shown to be under massive selective pressure are probably genes which predispose toward a certain temperament. This makes sense because, as any breeder will tell you, temperament is more important to a domestic animal's fitness than raw intelligence.