Why did blue eyes proliferate?

[TheVat]

https://www.thearchaeologist.org/blog/blue-eyed-humans-have-a-single-common-ancestor

 

The most interesting question this study raises is asked at the end of the article: how did humanity go from zero blue eyes 10,000 YA to now 20-40% of European countries?  What was the selective advantage(s)?  Aesthetics?  Better night vision?  

[Moontanman]

Sexual selection? 

[CharonY]

Short answer is that it is not know specifically. Longer answer is a bit generic and encompasses arguments of weak natural selection, bottleneck in ancestral populations and sexual selection as Moon mentioned. There are a handful of more specific speculations but the genetics of pigmentation is somewhat complex. Some genetic variants influence pigmentation on different parts (i.e. not only the iris) and could for example be co-selected.

[Daniel_J]

1. Mutation.

2. Selection. Can be nature but for blue eye it does not seems to have a strong proof. Human selection: possible, maybe the blue eys want to have children with other blue eyes. Then the gene stablized in the community.

3. Not-too-bad gene. Well, if it is too bad then it will be eliminated from the nature. Extention of 2.

[J.C.MacSwell]

Single common ancestor...so from the simplest blue/brown model that ancestor could not have had blue eyed kids...but at earliest could have blue eyed grandkids given an incestuous relationship of those kids...assuming they had at least two with the recessive blue gene.

[Peterkin]

It probably was caused by a mutation that prevented melanin forming in the iris. It still shows up once in a while in brown-skinned people. It may have been part of a genetic package along with desirable other traits and was not itself harmful, except in that blue eyes are more sensitive to light. This would not be a disadvantage in northern climates. In small, isolated populations, blue-eyed (and probably grey-eyed) people naturally occurred and met one another. It didn't need to be specifically selected for, though it may have been attractive as a novelty, and there was no reason to reject grey- or blue-eyed people as mates.   

[TheVat]

13 hours ago, J.C.MacSwell said:

Single common ancestor...so from the simplest blue/brown model that ancestor could not have had blue eyed kids...but at earliest could have blue eyed grandkids given an incestuous relationship of those kids...assuming they had at least two with the recessive blue gene.

Cousins marrying is not uncommon in many societies.  Einstein married his first cousin, Elsa, and no one called it incest.  Wisely, they did not have children.

I have wondered if blue eyes were co-selected, due to the complexity of pigmentation as @CharonY mentioned.  Lesser pigmentation in the skin certainly had a strong selective advantage in the north, in getting sufficient vitamin D.  Blue peepers could be like blonde hair, a trait that piggybacks on that adaptive train of lower melanin, but then also does have a slight sexual advantage - anthropologists sometimes refer to an overall set of traits that simply make someone more noticeable.  In a group of brunette girls, male suitors might take more notice of the blonde blue-eyed cutie.

Like Van Morrison, I've always had a soft spot for brown eyes.  

12 hours ago, Peterkin said:

It didn't need to be specifically selected for, though it may have been attractive as a novelty...

Yes, for a few generations there would be the novelty factor and so a temporary sexual selection.  

Have also wondered if an element of tender feelings about babies was in play, too.  Pale hair and blue eyes are prevalent in infants, so when those phenotypes start to emerge in a group, there would be the novelty of attractive "babes."  (sorry - this is a bit tongue-in-cheek)

[Peterkin]

2 hours ago, TheVat said:

Have also wondered if an element of tender feelings about babies was in play, too.  Pale hair and blue eyes are prevalent in infants, so when those phenotypes start to emerge in a group, there would be the novelty of attractive "babes."  (sorry - this is a bit tongue-in-cheek)

I hope it's way deep down in the cheek, because the implications are not auspicious for the psychological health of such a society.

[J.C.MacSwell]

5 hours ago, TheVat said:

Cousins marrying is not uncommon in many societies.  Einstein married his first cousin, Elsa, and no one called it incest.  Wisely, they did not have children.

 

Cousins it could well have been but through cousins it would take longer for the second blue eyed human to show up, based on the simplest model and your link.

Your OP suggests a single common ancestor. Under the simplest blue/brown model with blue genes recessive cousins would require an extra generation.

I was wrong though suggesting the single common ancestor could not have had blue eyed kids. They could have had them if sexual bonding with their own children, again assuming the simplest gene model.

What's interesting is that a recessive gene, whether advantageous or not, might gain some traction before much testing through the fitness/survival filter.

[Peterkin]

1 hour ago, J.C.MacSwell said:

What's interesting is that a recessive gene, whether advantageous or not, might gain some traction before much testing through the fitness/survival filter.

I doubt it would figure in that filter. Some other traits with which it is associated might but there is no evident survival drawback to eye colour. On the contrary, since it does appear to be sexually attractive, especially among people where it's uncommon, blue or grey eyes could well be an asset.

Edited January 1 by Peterkin

[Sensei]

In some people, the eyes turned blue overnight:

https://nypost.com/2023/09/05/babys-brown-eyes-turn-blue-overnight-after-covid-19-treatment/

 

[swansont]

25 minutes ago, Sensei said:

In some people, the eyes turned blue overnight:

https://nypost.com/2023/09/05/babys-brown-eyes-turn-blue-overnight-after-covid-19-treatment/

 

Temporarily. The topic here is genetics.

[Sensei]

59 minutes ago, swansont said:

Temporarily. The topic here is genetics.

Accident. Next time it may not be case if someone takes gene-altering drugs.

ps. I introduced this as novel information in case it was not noticed by forum members..

[J.C.MacSwell]

3 hours ago, Peterkin said:

I doubt it would figure in that filter. Some other traits with which it is associated might but there is no evident survival drawback to eye colour. On the contrary, since it does appear to be sexually attractive, especially among people where it's uncommon, blue or grey eyes could well be an asset.

Hard to say though.

A recent study was done on primates with regard to eye colour and the ambient light of the areas they inhabit:

https://phys.org/news/2022-10-eye-variation-primates.html

[Peterkin]

2 hours ago, J.C.MacSwell said:

A recent study was done on primates with regard to eye colour and the ambient light of the areas they inhabit:

i had mentioned that there are more light-skinned and light-eyed people in northern latitudes. Yet the de-pigmentation of irises still occurs in Africa and the affected individuals seem to manage all right. It's one of those genetic situations that prevail without making very much difference to survival ; it can be studied, recorded and quantified, but it's ultimately not significant. 

[J.C.MacSwell]

1 hour ago, Peterkin said:

i had mentioned that there are more light-skinned and light-eyed people in northern latitudes. Yet the de-pigmentation of irises still occurs in Africa and the affected individuals seem to manage all right. It's one of those genetic situations that prevail without making very much difference to survival ; it can be studied, recorded and quantified, but it's ultimately not significant. 

It might not be over quite a number of generations but ultimately it can be significant. Luck and random chance are of course involved but if blue eyes are more sensitive to light that could affect the odds of survival differently in different regions and ever so slightly effect a change given enough time.

I don't think it is as conclusive as you suggest. 

Of course with modern adaptations dark and light skinned individuals do fine in both northern and southern latitudes (or closer to and further from the equator, polar region and other extremes notwithstanding)

Edited January 1 by J.C.MacSwell

[Arthur Smith]

On 1/27/2023 at 5:35 PM, TheVat said:

The most interesting question this study raises is asked at the end of the article: how did humanity go from zero blue eyes 10,000 YA to now 20-40% of European countries?  What was the selective advantage(s)?  Aesthetics?  Better night vision? 

A skeleton found in Gough's Cave, Cheddar (of the cheese), UK recently had remnants of DNA extracted and sequenced. According to press reports, this fully modern human, when living around 10,000 years ago, had dark skin and blue eyes.

Fixation by drift? Sexual selection? Niche change and population explosion from neolithic to now?

ETA: and there's no reason to suppose "either or" here.

On 1/27/2023 at 5:38 PM, Moontanman said:

Sexual selection? 

Ah, missed that. There was a discussion on Larry Moran's blog a while ago that might be of interest.

Edited January 1 by Arthur Smith

[Arthur Smith]

https://sandwalk.blogspot.com/2014/05/answering-creationist-questions-about.html

Ten years ago. Some good comments, including Joe Felsenstein.

Edited January 1 by Arthur Smith

[Peterkin]

20 hours ago, J.C.MacSwell said:

It might not be over quite a number of generations but ultimately it can be significant. Luck and random chance are of course involved but if blue eyes are more sensitive to light that could affect the odds of survival differently in different regions and ever so slightly effect a change given enough time.

They have survived. No filter sieved them out.

[J.C.MacSwell]

11 hours ago, Peterkin said:

They have survived. No filter sieved them out.

Hadn't realized they had all survived. They should interview that single common ancestor...

Or did you mean they all successfully reproduced....which is not quite as unlikely....

Or did you mean any that didn't...failed for some other reason...which is slightly plausible but impossible for you to know...

If you simply meant te relevant gene or genes survived, what in any of my posts might suggest it didn't?

There is certainly no evidence that the gene/s proliferated fully or equally everywhere.

Simply put, no evidence of any disadvantage is not evidence there is none.

[CharonY]

On 12/30/2023 at 8:56 PM, J.C.MacSwell said:

Single common ancestor...so from the simplest blue/brown model that ancestor could not have had blue eyed kids...but at earliest could have blue eyed grandkids given an incestuous relationship of those kids...assuming they had at least two with the recessive blue gene.

It does not have to be. It is possible that other variants existed, but they vanished with this specific haplotype being the sole survivor.

[J.C.MacSwell]

52 minutes ago, CharonY said:

It does not have to be. It is possible that other variants existed, but they vanished with this specific haplotype being the sole survivor.

I was only assuming a single common ancestor based on the claim in the link, I was going to later ask how that was known.

Is it determined that that specific haplotype can only have come from a single mutation event and not been duplicated by some other identical mutation rather than only from being passed on?

Edited January 2 by J.C.MacSwell

[TheVat]

Eye color genetics turns out to be complex....

https://www.news-medical.net/health/Genetics-of-Eye-Color.aspx

Quote

To date, researchers have discovered more than 150 genes that influence eye color, a number of which have been discovered through studies of genetic disorders. Others have been identified during genomic studies of mice and fish.

Some genes play a major role in determining eye color, while others only have a small contribution.

One region of chromosome 15 contains two genes located near to each other that play major roles in determining eye color. One gene, called OCA2, codes for a protein called P protein, which is involved in melanosome maturation and affects the amount and quality of melanin stored in the iris. A number of genetic variations (polymorphisms) in this gene reduce how much P protein is produced and result in a lighter eye color.

The other main gene involved is called HERC2. Intron 86 on this gene controls the expression of OCA2, activating it or deactivating it as required. At least one polymorphism in this intron reduces the expression and activity of OCA2,which reduces how much P protein is produced.

A number of other genes play smaller roles in eye color. The roles of the genes ASIP, IRF4, SLC24A4, SLC24A5, SLC45A2, TPCN2, TYR, and TYRP1 are thought to combine with those of OCA2 and HERC2....

 

[Peterkin]

4 hours ago, J.C.MacSwell said:

Hadn't realized they had all survived.

The ones that are alive today survived: blue eyes have not been bred out of the human population. Nor are they restricted to northern latitudes:  while less common in southern climates, they do occur and the odd pair still appears in Africans.

 

4 hours ago, J.C.MacSwell said:

Or did you mean they all successfully reproduced

Of course not! Nor have all brown-eyed people. My partner and I both have hazel eyes and no intention to reproduce. How, in light of all the recombinations that take over time in an ever-increasing gene-pool relevant to the continued existence of a non-harmful variant? 

 

4 hours ago, J.C.MacSwell said:

If you simply meant te relevant gene or genes survived, what in any of my posts might suggest it didn't?

I meant the trait was not filtered out. I can't remember what specifically prompted my response and don't consider it important enough to search for.

 

Edited January 2 by Peterkin

[CharonY]

2 hours ago, J.C.MacSwell said:

I was only assuming a single common ancestor based on the claim in the link, I was going to later ask how that was known.

Is it determined that that specific haplotype can only have come from a single mutation event and not been duplicated by some other identical mutation rather than only from being passed on?

Just very quickly and generally (not really specific to the trait in question). We can start at the locus (site) of the mutation that provides the trait of interest and then look at the surrounding regions, which presumably are not under the same selective pressures. If the mutation arose in different persons independently, we would expect to see some variations in those surrounding areas (think of it as different persons providing different backgrounds for the mutation).

Blue eyes arose likely not due to a mutation in a  gene associated with eye color (OCA2)  but in an upstream regulatory element. The interesting bit is that the surrounding area is also conserved in all (tested) folks with blue eyes suggesting that they all share a common ancestor providing this mutation. However, as this analysis relies on testing of folks who are still alive, it obviously cannot tell us if there had been other mutations with the same phenotype or even independent cases of the same mutations.

I.e. we can say all currently living folks with blue eyes (who have been tested) have the same common ancestor, we we cannot say that there were no other cases of blue eyes in the past.