Us humans seem to have a specialized mental mechanism for identifying faces. It’s pretty easy to see that could have been favored by selection for a long, long time.
In 2006, Ingo Kennerknecht and associates published a study indicating that about 2.5% of people in Germany had a hereditary, congenital form of prosopagnosia (face blindness). Looking at the pattern of occurrence in families, they concluded that it is inherited as an autosomal dominant.
That’s strange. Once in a while, a recessive allele with negative effects in homozygotes can get that common – for example, malaria defenses like sickle-cell. In a case like that, big disadvantage in homozygotes are balanced by smaller (still sizeable) advantages in heterozygotes. The frequency of the sickle-cell allele stabilizes at an intermediate frequency, say 10%.
But here, the negative effects are in heterozygotes. There’s no way that they could be balanced by advantages in homozygotes, which are much rarer.
So how can prosopagnosia exist at such a high frequency? Well, maybe it’s like red-green color-blindness. The mutation rate for errors in the genes coding red and green opsins
is unusually high, due to illegitimate recombination. If you assume that the high frequency we see today in Europeans is a product of mutation accumulation due to relaxed selection since the dawn of agriculture some ten thousand years ago, it’s the highest mutation rate known, around 2 x 10-4 per generation, higher even that of dystrophin (the longest gene). It seems that the fitness loss due to red-green color blindness isn’t too large, at least in some societies. It’s hard for me to believe that face blindness matters so little, but maybe.
The next thought is that the allele for prosopagnosia is linked to some driving gene, something like the t-haplotype in mice. The t-haplotype cheats, so that 90% of a male carrier’s offspring carry the t-haplotype, instead of the usual 50%. Homozygous males are sterile, which ( along with other factors) keeps the t-allele from going to fixation. People have looked for transmission distortion in the human genome, but there are no definitive results, as far as I know.
The third explanation is that this research is just wrong..
Interesting that the authors don’t seem to have noticed how strange their claim really is.