Need Help - Evolution Question Perhaps?

The bit of skin that makes asians eyes look narrower is called the epicanthic fold .

Doing a quick google search for Epicanthic fold evolution turns up a few links which seem to agree that it's a reasonable theory, although I'm not entirely sure I buy it myself.

[edited by - Sandman on April 15, 2004 6:26:48 AM]

try talkorigins.org, it has a lot of stuff on it. but your theory seems plausible id say.

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Original post by -cougar-
...it is because of exposure to sun light and it helps the person not have to squint as much and it protected the eye from wind-swept sand.

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Not an unreasonable theory except for two problems. Firstly, Asia isn''t the only place on earth exposed to a lot of sunlight. Furthermore, it cannot be considered a particularly arid geographic location; there are other parts of the world where protection from wind-swept sand would have been even more important.

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Original post by Graylien

quote:

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Original post by -cougar-
...it is because of exposure to sun light and it helps the person not have to squint as much and it protected the eye from wind-swept sand.

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Not an unreasonable theory except for two problems. Firstly, Asia isn''t the only place on earth exposed to a lot of sunlight. Furthermore, it cannot be considered a particularly arid geographic location; there are other parts of the world where protection from wind-swept sand would have been even more important.

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Actually my main problem with the theory is that I don''t see ''not having to squint as much'' as being something that impacts survival or sexual proliferation significantly.

Of course, it''s possible that the squinty people couldn''t see as well as the people with more developed folds, and thus were less able to hunt and avoid being eaten, but it seems a little far fetched. Sexual selection leading to an ability to survive more comfortably in these conditions (and thus allowing them to migrate there) seems more likely.

It''s hard to argue that a reduced need of squinting was somehow preferred by natural selection.

However, if the epicanthic fold helped to prevent infections due to sand entering the eyes, or other related injuries that might affect eyesight -and hence survival- then indeed narrow eyes could be seen as an evolutionary advantage in certain geographic regions.

Hey everyone,

Thank you for the wonderful replies so far. Keep them up as I''m very interested in what you guys have to say.

The only problem I have with my theory is Arabs come from a sandy environment yet they do not have the epicanthic fold like Asians do. This is what has me stumped. The only thing I can think of when this evolution step took place, things were alot different.

Thanks,

Cougar

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Original post by -cougar-
The only problem I have with my theory is Arabs come from a sandy environment yet they do not have the epicanthic fold like Asians do. This is what has me stumped.

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Without researching this particular question and without caring one way or another, I can throw out a few general-case hypotheses to reconcile your perceived Arab/Asian discrepancy. Assuming that an epicanthic fold is mostly designed to protect against wind-blown particulate matter, and that such protection is beneficial to survival in the Asian and Arabic environments:

1) A beneficial mutation is only beneficial to a population which has the mutation. If the mutation does not occur in the Arab genome, then the Arab population never had a chance to exploit it.

2) Linkage. If the mutation in question was linked to the allele of another gene which is detrimental to survival in the Arabic environment, then this gene group could be selected against.

3) It''s possible that the mutation has other, negative effects, in the Arabic environment which are not an issue in the environment it developed in for Asians.

4) Alternative strategies. Perhaps Arabs developed tougher eyes.
With a little thought, you could likely come up with something more plausible, but I think you get the idea here.

Don''t forget that Asia is a large geographic area with a diverse populuation partitioned both culturally and genetically. It''s not entirely impossible that the epicanthic fold developed by chance and not via selective pressure. There are lots of islands in Asia and it stands to reason that there were also many small yet isolated gene pools wherein mutations that were neither harmful nor beneficial could thrive. Populations eventually do mix, and a dominant trait can spread once firmly entrenched in even a small group.

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Following BerwynIrish, I''ll note everything I''m saying is off the top of my head. I make no claims as to the scientific veracity of these quick ideas.
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I''m not an expert on evolution but one thing that always strikes me about discussions like this is that there doesn''t necessarily have to be a reason.

It seems logical to me that in addition to traits being selected because they enhance survival, traits that do not hurt survival will also be passed on and eventually spread thoughout the population. In this case, maybe there was a good reason for the fold to develop and maybe there wasn''t - it doesn''t matter. As long as it doesn''t hurt your chances for survival or reproduction it will spread. Maybe slower than an actively beneficial trait but it will spread nonetheless.

Aye. It''s called "genetic drift".