Some people claim that there is no evidence that natural selection has caused differences in different human populations in the frequency of genetic variations that create differences in cognitive performance. However, a recent combination of reports about a single gene which affects cognitive function provides such evidence. Recent research reports on the effects that variations of the prion protein gene (PRNP) have on cognitive ability, other reports that PRNP variations affect the risk for getting prion diseases such as kuru and Creutzfeld Jacob Disease (CJD), and still other reports on the distribution of the PRNP gene variations in different human populations suggest that in different ecological niches natural selection has indeed operated to produce differences in human cognitive function.
For a specific example of a genetically caused difference in cognitive ability in different human populations that has been caused by natural selection first see my previous post Prion Gene Influences Cognitive Ability which reported on how the M129V variations in the prion protein gene (PRNP) may cause differences in cognitive ability between those who and those who do not have that variation. Here's an excerpt from the abstract of a research paper on PRNP genetic variations and cognitve function.
We have recently shown that methionine at codon 129 in the prion protein is associated with white matter reduction in a group of healthy volunteers and schizophrenic patients. The present study examines the influence of the same genetic variation on psychometric cognitive performance measurements in 335 community-based healthy volunteers. The polymorphism was associated with Full Scale IQ (genotype: F=4.38, df=2/317, P=0.013; allele: F=8.04, df=1/658, P=0.005), as measured by HAWIE-R (German version of the Wechsler Adult Intelligence Scale, Revised). Genotype accounted for 2.7% of the total variability in Full Scale IQ (partial eta2=0.027).
Why is this interesting in terms of natural selection for cognitive performance in different human environments? We know that the M129V variation occurs at different frequencies in different in populations. In fact, cannibalism have have selected for the heterozygous occurrence of M129V variations in cannibals in Papua New Guinea.
From approximately 1920 to 1950, a kuru epidemic devastated the Fore in the Highlands of Papua New Guinea. At mortuary feasts, kinship groups would consume deceased relatives, a practice that probably started around the end of the 19th Century, according to local oral history. The Australian authorities imposed a ban on cannibalism there in the mid-1950s.
The same genetic variation in the prion protein that helps protect against Creutzfeld Jacob disease turned out to do the same for kuru. Studying Fore women who had participated in mortuary feasts, Collinge's group found that 23 out of the 30 women were heterozygous for the prion protein gene, possessing one normal copy and one with the M129V mutation.
The researchers sequenced and analyzed the prion protein gene in more than 2000 chromosome samples from people selected to represent worldwide genetic diversity. They found either M129V or E219K in every population, with the prevalence decreasing in East Asia (except for the Fore, who have the highest frequency in the world).
Collinge's team also studied the diversity of sequence variations in a block of DNA containing the prion protein gene, in European, African, Japanese, and Fore populations. The prevalence of the M129V and E219K variations, even when the sequence at other spots was highly variable, indicated that the variations were ancient--more than 500,000 years old, according to authors' estimates.
Finally, the researchers identified a telltale signature of balancing selection in the gene: a greater than average number of highly variable sites, and a smaller than average number of low-frequency variations.
These findings are consistent with other lines of evidence indicating that prehistoric populations practiced cannibalism, such as cuts and burn marks on Neanderthal bones, and biochemical analysis of fossilized human feces.
M129V allele frequencies do not differ only among populations which were recently cannibals as compared to populations that were not recently cannibals. For instance, allele frequencies for M129V are different in Turkey than in most of Europe and East Asia.
Three known polymorphisms but no other gene variants were detected in the PRNP coding sequence of the Turkish individuals. Genotype frequencies at codon 129 were 57% Met/Met, 34% Met/Val and 9% Val/Val, with an allele frequency of 0.740:0.260 Met:Val. These distributions are considerably different from those reported for other normal populations residing in Western Europe and East Asia, except in Crete. The higher frequency of 129 Met-homozygotes in Turkey than in Western Europe suggests that the Turkish are at greater risk of developing CJD.
If the research work which shows that M129V reduces brain volume and reduces intelligence is confirmed upon further investigation then this will be an example of a difference in selective pressures in different environments causing differences in frequencies of alleles that affect cognitive function.
|Share |||Randall Parker, 2004 March 04 03:58 PM Brain Genetics|