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#1
Old 12-15-2013, 03:17 PM
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Evolution question - asian eye skin flap

Just wondering about this....

Asian people have unique-looking eyes, and as I understand it, one of the reasons (and maybe the only reason, I don't know) that there is an extra fold of skin in the eye-lid, giving the eye its unique shape and appearance.

I assume this was due to an evolutionary need... For some reason, that eye feature was needed by people who lived in a certain region of the world, and it was so beneficial, those humans in that area continued to evolve with this eye feature intact. I guess it could be the other way around, in that maybe eyes on caucasians, for example evolved their round shape and lost that extra fold of skin, but I am assuming the asian eye features evolved from the eyes that most of the rest of the world seem to have.


I have heard or read somewhere that the extra skin permits a tighter seal, protecting the eye from harsh winds and cold temperatures, but I have no idea if this is true.

My questions:

1) is there an explanation for this eye development, as far as why it evolved and what the benefit of the extra skin flap provides?

2) did any animals in the same area of the globe develop a similarly-equiped eye lid? I'm guessing no, but I don't know anything about animal eye lids, so I thought i'd ask.

I would think that if something beneficial to a mammal's sight (or protection of its sight) evolved for one mammal, it may evolve for all mammals in a similar environment. Or, would this be a trait that would evolve too far down the lines of evolution to only be seen on the humans of a particular region?


My apologies for anything that may be offensive in this question. Nothing was meant to be offensive, and I hope anyone who might be offended would accept this pre-emptive apology.
#2
Old 12-15-2013, 03:34 PM
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The trait is called an epicanthic fold. It is most prevalent in populations of northeast and central Asia. It's also found in Inuits and Eskimos, and even some northern Europeans. A couple of isolated populations that show it are the San (Bushmen) of southern Africa and the Andamese of islands off South Asia. The trait in these groups may have separate origins.

Like many such traits, the adaptive value of the fold is speculative. It is usually postulated to be an adaptation to shield the eyes in a cold, dry climate (or in the case of the San, just dry.) However, this would be difficult to prove.

Animals in cold dry environments may show adaptations to protect the eyes such as thick eyelashes, but I am not aware of any that have a similar kind of fold, even cold-weather primates.

Last edited by Colibri; 12-15-2013 at 03:35 PM.
#3
Old 12-15-2013, 03:34 PM
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Quote:
Originally Posted by Stink Fish Pot View Post
is there an explanation for this eye development, as far as why it evolved and what the benefit of the extra skin flap provides?
Not a definitive one. It should be noted that certain African population (the Bushman, for example) also have epicanthic folds, so this condition may have evolved in Africa, and not in harsher, Asian climates. Or it may have evolved more than once, like lighter skin appears to have done.

Last edited by John Mace; 12-15-2013 at 03:36 PM.
#4
Old 12-15-2013, 03:40 PM
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Originally Posted by John Mace View Post
Not a definitive one. It should be noted that certain African population (the Bushman, for example) also have epicanthic folds, so this condition may have evolved in Africa, and not in harsher, Asian climates.
The trait could be homologous between the Bushmen and Andamanese, since the Andamanese are thought to be close to the first populations that migrated out of Africa. It's not out of the question that it could be an ancestral survival in northern Asians, but this would be surprising since it is less prevalent in southeast Asians (which would presumably have had a more direct relationship with the populations ancestral to the Andamanese).
#5
Old 12-15-2013, 03:48 PM
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Epicanthic fold. All humans have it as fetuses. In white people sometimes it survives into childhood and disappears as the bridge of the face develops.

Warner Oland, who played Charlie Chan, was Swedish.
#6
Old 12-15-2013, 04:07 PM
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Also occasionally present in Downs Syndrome.
#7
Old 12-15-2013, 04:10 PM
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Quote:
Originally Posted by Stink Fish Pot View Post
Just wondering about this....

Asian people have unique-looking eyes, and as I understand it, one of the reasons (and maybe the only reason, I don't know) that there is an extra fold of skin in the eye-lid, giving the eye its unique shape and appearance.

I assume this was due to an evolutionary need...
Things don't evolve due to a need. Changes can be beneficial, negative or neutral but they aren't need based.
#8
Old 12-15-2013, 04:13 PM
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Originally Posted by sailor View Post
Epicanthic fold. All humans have it as fetuses.
Is that true? The wikipedia article doesn't say that.
#9
Old 12-15-2013, 04:44 PM
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Originally Posted by John Mace View Post
Is that true? The wikipedia article doesn't say that.
From here.

Quote:
The epicanthic (or epicanthal) fold is a normal feature of fetuses of all races but is present in a pronounced form and in high concentrations in humans of certain geographic races and subraces.
As has been mentioned, the fold is retained in Down's Syndrome (which for this reason at one time was known as Mongolism) and in some other congenital conditions such as fetal alcohol syndrome.

Since in populations that don't have the fold as adults it disappears as the bridge of the nose develops, this opens the possibility that the fold may not be adaptive in itself, but instead may be associated with differences in the nasal structure between some populations.

Last edited by Colibri; 12-15-2013 at 04:44 PM.
#10
Old 12-15-2013, 06:40 PM
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Originally Posted by rsat3acr View Post
Things don't evolve due to a need. Changes can be beneficial, negative or neutral but they aren't need based.
Is this accurate?

As I understand evolution theory, something that is tweaked in the gene pool will not propagate fast enough to take hold if it didn't have some functional value...

So, if this was indeed just a random occurrence with no functional value, wouldn't it have just disappeared within the population over time? Certainly it wouldn't be likely to be seen more and more in subsequent generations, correct?

Or am I wrong here?
#11
Old 12-15-2013, 06:45 PM
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A beneficial change propagates. But "need" has naught to do with it.
#12
Old 12-15-2013, 06:48 PM
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Originally Posted by Stink Fish Pot View Post
Is this accurate?

As I understand evolution theory, something that is tweaked in the gene pool will not propagate fast enough to take hold if it didn't have some functional value...
Like different eye colors?

Genetically influenced physical structures don't have to be beneficial. But if they are detrimental, they'll be selected against. That doesn't mean they'll disappear, but they're less likely to propagate.
#13
Old 12-15-2013, 06:55 PM
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Quite a few white kids retain it until age 5~10.

A couple years ago I showed a friend a photo of a pretty Asian woman with beautiful eyes. She was showing her lower girly bits directly to the camera and holding the labia open with her hands. I said "One thing I love about Asian women is their epicanthic fold". The guy had no idea what it was and assumed the fold it must be down there somewhere but did not want to ask and went along with the conversation uneasily. He finally admitted he did not know what I was talking about and smiled sheepishly when I told him what the epicanthic fold is.
#14
Old 12-15-2013, 09:09 PM
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Originally Posted by JRDelirious View Post
A beneficial change propagates. But "need" has naught to do with it.
Quote:
Originally Posted by Great Antibob View Post
Like different eye colors?

Genetically influenced physical structures don't have to be beneficial. But if they are detrimental, they'll be selected against. That doesn't mean they'll disappear, but they're less likely to propagate.
I understand this point about eye color. However wouldn't asians be considered to have homogeneous eye and hair color? Black hair, and either dark brown or black eyes is all i've ever seen. And is this the same as the epicanthic fold? So if the fold did not provide any advantage in the evolution of the asian population's eye, how/why did it develop only in the asian population, and why is it observed in close to 100% of the population?

Because I would think that the fold was either a change in the asian genetic code and propagated through population selection, or the rest of the world's people who don't have the fold selected it out over time if it didn't have any real advantage or disadvantage and they didn't find it appealing

Is this a correct assumption?

I would understand this in comparison to eye color if only a small percentage of asian people had this trait... Say the same percentage of asian people who have green eyes. (i don't know the population well enough to know if there are any variations... The only green eyed asian girl I ever saw was in the movie "Big Trouble In Little China" (starring Kurt Russell!)). If eye color doesn't matter, why do almost all asians have dark eyes (I'd say extremely dark brown or black) and black hair? Was there an advantage to have these hair/eye colors for the region they populated, or was this a trait that was self-selected because it was appealing to the reproducing population?


Or is there no answer to this?
#15
Old 12-15-2013, 09:49 PM
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Originally Posted by Stink Fish Pot View Post
I understand this point about eye color. However wouldn't asians be considered to have homogeneous eye and hair color? Black hair, and either dark brown or black eyes is all i've ever seen. And is this the same as the epicanthic fold?
Ever see any Ainu from Northern Japan? Many don't have dark hair or eyes.

Just because a trait is passed on doesn't mean it is advantagous, just passed on more frequently.

Last edited by Colibri; 12-15-2013 at 10:51 PM. Reason: fix format
#16
Old 12-15-2013, 10:41 PM
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Quote:
Originally Posted by Stink Fish Pot View Post
Is this accurate?

As I understand evolution theory, something that is tweaked in the gene pool will not propagate fast enough to take hold if it didn't have some functional value...

So, if this was indeed just a random occurrence with no functional value, wouldn't it have just disappeared within the population over time? Certainly it wouldn't be likely to be seen more and more in subsequent generations, correct?

Or am I wrong here?
In short, and with respect, you are wrong.

A [dominant] genetic change related to an independent phenotype characteristic that is not advantageous will be selected against.

A change that is neutral, or associated with more than one characteristic, or, for that matter, just recessive, will/might not be selected against.

Always remember that 'survival of the fittest' was an economic theory, and would lead to too many evolutionary dead ends. The most viable species have wide ranges of critical characteristics, to improve that chances of survival in shifting environments.
#17
Old 12-15-2013, 10:42 PM
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[QUOTE=rsat3acr;16932677]
Quote:
Originally Posted by Stink Fish Pot View Post
I understand this point about eye color. However wouldn't asians be considered to have homogeneous eye and hair color? Black hair, and either dark brown or black eyes is all i've ever seen. And is this the same as the epicanthic fold?

Ever see any Ainu from Northern Japan? Many don't have dark hair or eyes.

Just because a trait is passed on doesn't mean it is advantagous, just passed on more frequently.
No, I never have seen any Ainu. Ever. I will do a search and see what you are referring to.

I understand what you are saying. And yes, a trait can be passed on without being advantageous. But over time, a trait like that could probably be something appealing to the reproducing pair.
#18
Old 12-15-2013, 10:43 PM
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I have heard it protected the eyes from blowing sands.
#19
Old 12-15-2013, 11:08 PM
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I'm not sure if it's been mentioned yet, but Asian eyes are actually two traits. one is epicanthic folds, which are from the fact that Asians have smaller noses. This could in part be a neotenous trait. Additionally, east Asians also have fat deposits on their eye lids. The combination of these two traits is what tends to give Asian eyes their distinctive look. The fat deposits are believed to have spread out of Mongolia from the last I heard. There's some speculation that it could have something to do with keeping sand out of your eyes. But I'm not sure how much I buy it.
#20
Old 12-15-2013, 11:21 PM
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I have always wondered about it also. Here in the Yucatan peninsula, most of the indigenous people's children have Asian eyes. As they get older, this feature disappears. I always assumed it was because their ancestors came via the Bering Strait.
#21
Old 12-15-2013, 11:37 PM
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Originally Posted by j666 View Post
In short, and with respect, you are wrong.

A [dominant] genetic change related to an independent phenotype characteristic that is not advantageous will be selected against.

A change that is neutral, or associated with more than one characteristic, or, for that matter, just recessive, will/might not be selected against.

Always remember that 'survival of the fittest' was an economic theory, and would lead to too many evolutionary dead ends. The most viable species have wide ranges of critical characteristics, to improve that chances of survival in shifting environments.
Ok, i can buy that. And I do understand it. But I think what you are saying is that if the trait isn't harmful in some way, that it can/will be propagated without issue, which I have no problem with. It's the eye color discussion. however, if the trait IS harmful in some way, and causes failure of the carrier of the gene, it will die off eventually. If not, it will condemn the species.
#22
Old 12-15-2013, 11:56 PM
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Originally Posted by Stink Fish Pot View Post
So if the fold did not provide any advantage in the evolution of the asian population's eye, how/why did it develop only in the asian population, and why is it observed in close to 100% of the population?
As already mentioned upthread, it is not limited to Asians. The Khoi-San of sub-Saharan Africa often have epicanthic folds, it shows up in the Americas, and it's also found in Northwestern Europe among the Irish and other Celts.

There is also something called "founder effect" when someone in a small group enjoys disproportionate reproductive success, leaving a trait or two among their many descendants that is basically there by luck rather than it being useful. If in ancient times a group with epicanthic folds, dark hair, and dark eyes did particularly well in Asia and expanded greatly in number those traits might become extremely common even without being advantageous. After enough time, people who didn't have them might be considered such oddballs as to be at a disadvantage in reproducing, in which case we're getting into sexual selection.
#23
Old 12-16-2013, 12:13 AM
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Originally Posted by Broomstick View Post
As already mentioned upthread, it is not limited to Asians. The Khoi-San of sub-Saharan Africa often have epicanthic folds, it shows up in the Americas, and it's also found in Northwestern Europe among the Irish and other Celts.

There is also something called "founder effect" when someone in a small group enjoys disproportionate reproductive success, leaving a trait or two among their many descendants that is basically there by luck rather than it being useful. If in ancient times a group with epicanthic folds, dark hair, and dark eyes did particularly well in Asia and expanded greatly in number those traits might become extremely common even without being advantageous. After enough time, people who didn't have them might be considered such oddballs as to be at a disadvantage in reproducing, in which case we're getting into sexual selection.
Just to go full-on crazy talk, how much influence from Genghis Khan is there?
#24
Old 12-16-2013, 07:28 AM
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I have never been able to figure out exactly what the epicanthal/epicanthic fold actually looks like. I've studied loads of Google Images supposedly illustrating them, with arrows and everything... and they just look like eyes to me! I must be really unobservant...
#25
Old 12-16-2013, 07:30 AM
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A few things:

Neutral alleles are just that - neutral. Thus, they are predicted to persist at roughly the same frequency from generation to generation (assuming Hardy-Weinberg equilibrium). The only way that an allele's frequency can predictably change over time is through selection, either positive or negative. Without selection, nothing changes.

That said, this is biology, so of course there are exceptions. Without selection, the propagation of an allele is dependent on randomness. It's like flipping a coin. You expect to get 50/50 heads/tails over long trials, but each flip is a random chance. If you only do, say, 10 flips, there's a pretty good chance you won't get 5 and 5. Similarly, in a small population size, alleles can become more or less common simply due to random chance. This is called genetic drift. You can even genetic drift to fixation, which is where every gene in the population is of the same allele.

It's important to understand that not each and every feature of a human population - even physical, genetic traits - are necessarily the direct result of evolution by natural selection. That is, just because everyone in Asia has the epicanthic fold doesn't mean that it provides some sort of survival benefit. I want to clarify at this point that I have no idea what the current research actually is on this topic - I'm merely listing alternative explanations that may or may not apply.

Genetic drift is one way in which this trait could have arisen. A small population containing people with and without the fold first spread into Asia. Over time, through random chance, the epicanthic fold trait went to fixation.

Sexual selection is another, related, possibility. That would be where the fold trait went to fixation not through random chance, but because it was considered sexually attractive. Those without the fold were perceived to be ugly, and were bred out of the population. Again - no actual physical adaptive advantage was conferred.

Founder effect, which has been mentioned, is another idea related to genetic drift. If the small founding population all happened to have the epicanthic fold, then all of their descendents would also have it.

Slightly more complex is the idea of a selective sweep. In this scenario, one person in the population was born with an advantageous mutation that happened to be physically located on the chromosome very very close to his epicanthic fold gene, but which is in a completely unrelated gene. It could be, I don't know, a handy liver enzyme, or a healing factor, or whatever. The key is just that it's physically linked to the epicanthic fold gene. Over time, this new beneficial mutation will be selected for and spread rapidly through the population. As it does, the epicanthic fold gene is sort of hijacked and carried along with it, and also spreads through the population to fixation. Over long periods of time, the beneficial mutation could be separated from the epicanthic fold gene through recombination, but if the two are very close to each other, those recombination events will be very very rare, and could be expected to take longer than the sweep to fixation.

Quote:
how much influence from Genghis Khan is there?
Minimal. I know that he had lots of children, and most Asians can expect him to be in their genealogy, but he's still a single ancestor out of many. The idea that he would be responsible for a widespread trait like this is implausible.
#26
Old 12-16-2013, 09:38 AM
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A "gene" is really just a nickname for a given sequence of genetic code that creates a given predictable result in the organism. Evolutionary "direction" is driven by only one thing: Does a chance variation of sequence create more successful reproduction?

However, highly penetrated genetically-driven characteristics are not necessarily highly penetrated because the genetic sequence coding for that exact characteristic created the successful reproduction. As mentioned above, maybe that part of the gene (or even the whole "gene") is just hitching a ride on a more important part (where "important part" means sequences that do drive more successful reproduction). Or maybe the number of alleles in a founding population was so small and the reproductive advantage so slight that genetic drift drove frequency by random chance instead of evolution's "directional" preference.

If a founder of a given group of descendants has a sequence coding for an outcome that is linked to a sequence that's important for successful reproduction, then the trait will be highly penetrated even if it's not itself important. For these kinds of reasons, we don't know if epicanthal folds are incidental, or selected for because they are functional. (And of course, one kind of "functional" might happen if a given group decides epicanthal folds look great, so appearance itself rather than physiologic function is driving the successful reproduction.)

Some mathematical modeling efforts have been done to sort out evolutionary ("Darwinian" in the paper I link) selection from effects such as a population bottleneck that creates a paucity of founders and therefore a paucity of genetic architectures to pass on.

It seems intuitive to me that a more protected inner canthus is a good idea, but I don't think it's much more than speculation to say so. As Colibri points out, reasonably prominent epicanthal folds are found in pretty ancient lineages. If its such a great idea, why isn't it more highly penetrated everywhere?

Last edited by Chief Pedant; 12-16-2013 at 09:39 AM.
#27
Old 12-16-2013, 09:49 AM
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Originally Posted by Stink Fish Pot View Post
however, if the trait IS harmful in some way, and causes failure of the carrier of the gene, it will die off eventually. If not, it will condemn the species.
Well, a trait can be harmful without being fatal. Our bodies and brains are full of flaws that can cause us harm now and then but haven't killed us off as a species yet.
#28
Old 12-16-2013, 09:52 AM
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Originally Posted by Chief Pedant View Post
It seems intuitive to me that a more protected inner canthus is a good idea, but I don't think it's much more than speculation to say so. As Colibri points out, reasonably prominent epicanthal folds are found in pretty ancient lineages. If its such a great idea, why isn't it more highly penetrated everywhere?
Because it's not the only good idea. Someone a long time ago was born without one, but with other traits that enhanced survival, so the eye thing was left behind. Or perhapts it's a trait that only makes a difference in certain parts of the world.
#29
Old 12-16-2013, 09:58 AM
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Originally Posted by Colibri View Post
As has been mentioned, the fold is retained in Down's Syndrome (which for this reason at one time was known as Mongolism)....
BTW, Dr. Langston Down, the guy for whom Down syndrome is named, thought that Asians ("Mongols" to him) and people with Down syndrome were somehow related, not just similar. He based this on the fact that (in his typical Victorian racist view) both have such limited intelligence as well as similar physical characteristics.
#30
Old 12-16-2013, 12:17 PM
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Originally Posted by Stink Fish Pot View Post
Ok, i can buy that. And I do understand it. But I think what you are saying is that if the trait isn't harmful in some way, that it can/will be propagated without issue, which I have no problem with. It's the eye color discussion. however, if the trait IS harmful in some way, and causes failure of the carrier of the gene, it will die off eventually. If not, it will condemn the species.

Such a harmful trait may die off eventually.

Or environmental changes could happen that will turn the tables and later favor that trait.

Or a trait that is harmful in the homozygous recessive may actually be advantageous in the heterozygous state. This is assumed to be the case with the CFTR loci with multiple such alleles that have been stable for many generations.
#31
Old 12-16-2013, 03:12 PM
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Originally Posted by lance strongarm View Post
BTW, Dr. Langston Down, the guy for whom Down syndrome is named, thought that Asians ("Mongols" to him) and people with Down syndrome were somehow related, not just similar. He based this on the fact that (in his typical Victorian racist view) both have such limited intelligence as well as similar physical characteristics.
Observations on an Ethnic Classification of Idiots.

He also recognized Ethiopian and Malay idiots, as well as those resembling American Indians.

Quote:
Apart from the practical bearing of this attempt at an ethnic classification, considerable philosophical interest attaches to it. The tendency in the present day is to reject the opinion that the various races are merely varieties of the human family having a common origin, and to insist that climatic, or other influences, are insufficient to account for the different types of man. Here, however, we have examples of retrogression, or at all events, of departure from one type and the assumption of the characteristics of another. If these great racial divisions are fixed and definite, how comes it that disease is able to break down the barrier, and to simulate so closely the features of the members of another division. I cannot but think that the observations which I have recorded, are indications that the differences in the races are not specific but variable.

These examples of the result of degeneracy among mankind, appear to me to furnish some arguments in favour of the unity of the human species.
Note that "idiot" once referred to a specific grade of mental retardation, below "moron" and "imbecile."
#32
Old 12-16-2013, 03:42 PM
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Originally Posted by Colophon View Post
I have never been able to figure out exactly what the epicanthal/epicanthic fold actually looks like. I've studied loads of Google Images supposedly illustrating them, with arrows and everything... and they just look like eyes to me! I must be really unobservant...
It's an extra fold around the corner of the eye (the corner closest to the nose), that creates a more hooded upper eye, and in more extreme cases such as north-east Asians, completely shielding the upper eyelid.

No epicanthic fold.

Epicanthic fold.

A diagram of the fold.

If you wanted to give someone without the fold the appearance of one, you'd gather some skin slightly away from the corner of the eye, on the bridge of the nose, and "pin" it to the corner of the eye. You'd see, with a little imagination, how this would create another fold radiating from the tear duct, and "hooding" the upper eye as in natural epicantic folds.

Hope that helps some.

Last edited by cmyk; 12-16-2013 at 03:44 PM.
#33
Old 12-16-2013, 03:52 PM
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Originally Posted by Stink Fish Pot View Post
however, if the trait IS harmful in some way, and causes failure of the carrier of the gene, it will die off eventually. If not, it will condemn the species.
Few traits are universally good or bad. Example: sickle cell trait. Can be potentially very bad (especially at high altitudes). It's not bad for areas with malaria.

Even outside the tropics in areas where it is more negative than positive, there's no real reason it should "condemn" the carriers by its presence.

I do note you modified this statement with "causes failure of the carrier of the gene". Well, that's an awfully high bar, and a much higher one than you originally set. Few genes make that much of a difference on their own.
#34
Old 12-16-2013, 04:33 PM
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Originally Posted by Colibri View Post
Note that "idiot" once referred to a specific grade of mental retardation, below "moron" and "imbecile."
Yes, and to put a finer point on it, "mental retardation" was the term used to replace those...until it became bad and was replaced again...and on it will go.
#35
Old 12-16-2013, 04:34 PM
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Originally Posted by chacoguy View Post
Just to go full-on crazy talk, how much influence from Genghis Khan is there?
Genghis Khan is too recent. My guess is that the wide geographic spread of the East-Asian "look" comes from the first development of agriculture in east Asia. A small group of people with a fairly uniform set of physical characteristics experienced a population explosion and spread their genes throughout the region and beyond.
#36
Old 12-16-2013, 10:02 PM
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Originally Posted by Stink Fish Pot View Post
Ok, i can buy that. And I do understand it. But I think what you are saying is that if the trait isn't harmful in some way, that it can/will be propagated without issue, which I have no problem with. It's the eye color discussion. however, if the trait IS harmful in some way, and causes failure of the carrier of the gene, it will die off eventually. If not, it will condemn the species.
Not necessarily. Remember, not all genes are a simple as the examples we are given in grammar school.

A gene that 'causes failure of the carrier' under certain circumstances might be advantageous under others.

Also:
Few genes 'cause failure'; most have little effect on survival; most of those that do have marginal effects.

Genes can be recessive.

Genes may not be expressed.

Genes may have multiple phenotypic effects - genotype does not effect survival, phenotype does, I want to stress this, although you do not argue otherwise, because a lot of people don't think of this.
#37
Old 12-17-2013, 09:32 AM
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Then you've got your epigenetics, which massively complicates things, and is also mind-blowing when you first learn about it.
#38
Old 12-17-2013, 10:39 AM
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Originally Posted by j666 View Post
Not necessarily. Remember, not all genes are a simple as the examples we are given in grammar school.

A gene that 'causes failure of the carrier' under certain circumstances might be advantageous under others.

Also:
Few genes 'cause failure'; most have little effect on survival; most of those that do have marginal effects.

Genes can be recessive.

Genes may not be expressed.

Genes may have multiple phenotypic effects - genotype does not effect survival, phenotype does, I want to stress this, although you do not argue otherwise, because a lot of people don't think of this.
Thank you for the clarifications. I know I HAVE simplified my questions/comments. However, this is far from a simplistic subject. I think your notes (as well as the notes of others) have demonstrated that the success or failure of a gene, its retention in the genetic code and its ability to propagate throughout a population relies on a number of factors.
#39
Old 12-17-2013, 11:56 AM
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Originally Posted by Stink Fish Pot View Post
Ok, i can buy that. And I do understand it. But I think what you are saying is that if the trait isn't harmful in some way, that it can/will be propagated without issue, which I have no problem with. It's the eye color discussion. however, if the trait IS harmful in some way, and causes failure of the carrier of the gene, it will die off eventually. If not, it will condemn the species.
Only if it is "harmful" wrt to reproduction. It's quite "harmful" for a male peacock to have a ginormous tail, but that's outweighed by the advantage said tail gives him in wooing females and spreading his genes.
#40
Old 12-17-2013, 12:11 PM
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Originally Posted by John Mace View Post
Only if it is "harmful" wrt to reproduction. It's quite "harmful" for a male peacock to have a ginormous tail, but that's outweighed by the advantage said tail gives him in wooing females and spreading his genes.
Aside from direct attractiveness to females, features such as the peacock's tail are often considered to be expressions of the "handicap principle": if a male can survive despite having to drag around that enormous tail, he must be exceptionally fit in other ways, and hence represent a good bet for a female.
#41
Old 12-17-2013, 01:00 PM
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Originally Posted by Colibri View Post
Aside from direct attractiveness to females, features such as the peacock's tail are often considered to be expressions of the "handicap principle": if a male can survive despite having to drag around that enormous tail, he must be exceptionally fit in other ways, and hence represent a good bet for a female.
I don't see an animal capable of making that kind of distinction. It would seem to me that if a female of the species decides that a trait is attractive and therefore wants to mate with that male, THAT is what will help a trait spread. But I don't see how an animal could say "that tail is ridiculous, but he must be fit to drag it around."

Do animals have that capability?
#42
Old 12-17-2013, 06:16 PM
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Originally Posted by Stink Fish Pot View Post
I don't see an animal capable of making that kind of distinction. It would seem to me that if a female of the species decides that a trait is attractive and therefore wants to mate with that male, THAT is what will help a trait spread. But I don't see how an animal could say "that tail is ridiculous, but he must be fit to drag it around."
Do animals have that capability?
They don't make that distinction consciously. Here's the scenario I imagine:
Before the showy tail feathers evolved, males had just...tail feathers. But males that were diseased, or starving, or had some hormonal imbalance had bedraggled looking feathers. Females then evolved an aversion to nasty feathers as a way of avoiding bad mates. This was just a simple mental trigger: smooth, long, colored = sexy. Once this trigger was in place in the females, it provided a selective pressure in the males to over-stimulate the trigger by producing longer, more colorful tail feathers. Now the pressure was on the females to be more selective in their triggers. Iterate.
#43
Old 12-17-2013, 06:31 PM
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Originally Posted by Stink Fish Pot View Post
Do animals have that capability?
No, but they don't have to. The actual trigger for a female to find a male attractive may be the size and color of the tail. The handicap principle is just an explanation for why an attractive tail may be what is known as an "honest signal" of the physical fitness of the male.
#44
Old 12-18-2013, 10:38 AM
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Originally Posted by Stink Fish Pot View Post
I don't see an animal capable of making that kind of distinction. It would seem to me that if a female of the species decides that a trait is attractive and therefore wants to mate with that male, THAT is what will help a trait spread. But I don't see how an animal could say "that tail is ridiculous, but he must be fit to drag it around."

Do animals have that capability?
Absolutely - a large part of "attractiveness" is a judgement of health and fitness, including the ability to raise young or protect a family. It may not be a conscious reasoned thing, but it's part of what makes a potential mate "attractive."
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