I’ve been searching for evolutionary advantages of the epicanthic fold. As a regular reader I was surprised to find a link to one of the columns that I apparently missed (straightdope.com/classics/a2_305a.html) What puzzles me is why this trait tends to show up in Down’s Syndrome and Fetal Alcohol Syndrome. Is an otherwise inactive set of genes being activated, or could there possibly be an advantage to developmentally challenged people as well?
Maybe this link might work better.
As for the adaptative benefits in individuals with Down’s syndrome? I doubt it.
For selection to work, you’d need (a) heritable variation to act on and (b) differential selective pressures.
I’m not too sure about (a) above. Are there any individuals with Down’s syndrome without the epicanthic fold?
Furthermore, the most common cause of Down’s syndrome today is trisomy-21 – the non-disjunction of chromosome 21 in a parent that is normal. This means that Down’s syndrome itself is not neccessarily an inheritable condition (there is another cause of the syndrome caused by a chromosomal translocation, that is inheritable, but this much rarer).
In any case, with a mean life-expectancy of 17 even in today’s world, and males always infertile, historic (in an evolutionary biology sense) pressures against individuals with the syndrome must have been very intense, to the extent that any advantage that the epicanthic fold may have conferred would have been quite overwhelmed.
As for FAS … one presumes that our ancestors (again, on an evolutionary time-scale), while they may not have needed to put up with greenhouse gases, taxes, Jesse Helms, MTV and the Y2K problem, also did not have Wild Turkey to look forward to after a dangerous day scavanging out on the savanna.
But, as with all evolutionary “just-so” stories … there’s always room for argument.
One current-day “evolutionary advantage”(?) to the epicanthic fold is the fact that, at least among the reasonably large sample with which I am in regular contact, Chinese people seem to feel that ‘shwong lien pi’ [twin eye skins] are a greatly-desired trait - even to the point that attaining them is now one of the most popular reasons for cosmetic surgery among that group.
I have asked why this desirability is - is it considered a mark of beauty? is there some functional difference? - but have not received a satisfactory answer. I don’t know that those individuals lacking would indeed find it harder to [find a] mate, but it is possible, ah s’pose. One speculation given me was that Orientals with the fatty eyelids but without the fold are more likely to have their eyelashes “turned inward” - a clear disadvantage, if true.
But since this seems to be one of the first things that Chinese observers look for (that and hair color and the “height” of the nose) after determining the gender/health of a newborn, personally I think it is mostly a matter of enforcing a certain group identity. At least, when it has happened in my presence it has always felt like I imagine it would feel to have a bunch of Occidental mothers sitting around commenting on whether or not the new baby is white and blonde enough - but maybe that’s just me.
The twin-eye fold is valued in Korean culture as well. It’s called sang-ga-pul (I apologize for the bad transliteration). I had the folds on both eyes but have lost one as I’ve grown older. I personally don’t care about them but my mother and sister (who both lack them) are jealous of me.
People do get cosmetic surgery for them though the results leave much to be desired (from the cases I have seen).
According to the female members of my familiy, the fold is desired for beauty reasons. It apparently makes the eye look bigger (I won’t comment on any sort of “occidental-envy” here).