Inheritance of eye color doesn’t follow a simple
Mendelian model. Although the blue-eye allele (C) is less dominant than the
brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC
homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes
blue-eyed. There is also a sex difference, with women having a more diverse palette
of eye colors. (source)
Most humans have black hair and brown eyes.
Europeans display a much more diverse range of hues, their hair being also
brown, flaxen, golden, or red and their eyes being also blue, gray, hazel, or
green.
This diversification has gone farther in European
women than in European men. According to a twin study, women have a more
diverse palette of hair colors, with a greater prevalence of lighter shades,
particularly red hair (Shekar et al., 2008).
Women also have a more diverse palette of eye colors,
according to a recent study of six SNPs associated with eye color. When both copies
of the so-called blue-eye allele are present, the resulting phenotype is more
variable in women than in men (Martinez-Cadenas et al., 2013). This translates
into a greater range of female eye colors in regions, like northern and eastern
Europe, where blue eyes are the single most common phenotype (Walsh et al.,2012). As the study’s authors observe, “in populations with very high blue-eye
frequency, such as Iceland or Holland, females show greater proportion of green
eyes at the expense of blue eyes” (Martinez-Cadenas et al., 2013). The authors
also confirmed earlier findings that eye color doesn’t follow a simple
Mendelian model. Although the blue-eye allele (C) is less dominant than the
brown-eye allele (T), CT heterozygotes aren’t necessarily brown-eyed and CC
homozygotes aren’t necessarily blue-eyed. Even TT homozygotes are sometimes
blue-eyed.
Thus, both hair color and eye color tend to be more
diverse in women. There is, however, one difference. With hair color, the “derived”
alleles are more fully expressed in women than in men. With eye color, they are
less fully expressed. This seems to argue against the theory that hair and eye
color diversified through a process of sexual selection that acted more
strongly on women than on men. Since blue eyes are the derived phenotype, they
should be more fully expressed in women because the female sex is the main
target of this selection pressure. Yet the reverse is actually true.
The reason may be physiological. It seems easier to
produce new eye colors by modifying the way the blue-eye allele is expressed
than by simply creating new alleles. Thus, to produce a range of hues that
extends beyond brown and blue, the so-called blue-eye genotype must be more
common than the actual incidence of blue eyes. As a result, “more females
bearing the ‘blue eye genotype’ (HERC2/OCA2 CC homozygous genotype) end up
having green or intermediate eyes” (Martinez-Cadenas et al., 2013).
Other recent
studies
A recent paper has confirmed that European eye color
diversified through some kind of selection pressure, and not random factors
like genetic drift or founder effects. Blue-eye alleles show a very strong
signal of selection (Donnelly et al., 2012). Another study, however, has failed
to find any preference for blue eyes over other colors, an indication that all
eye colors are at selective equilibrium, at least for the German population under
study. This finding may be related to the already high frequency of blue eyes in
that population:
Perhaps the frequency of eye
colors plays a role. In most countries, blue eyes are less prevalent than other
eye colors and may have the image of something special and more valuable. If
this assumption is true, brown eyes should be preferred in countries where the
majority of the population has blue eyes. (Gründl et al, 2012).
In this case, sexual selection is
frequency-dependent, shifting to whichever eye color is least frequent. Eventually,
an equilibrium is reached where color novelty is in balance with other
characteristics, such as color brightness, that may increase sexual
attractiveness.
This last finding shows the opportunistic nature of
sexual selection. When too many of one sex have to compete for mating
opportunities with too few of the other sex, there will be selection for any
traits that increase mating success. In many cases, these traits will hyperstimulate
a mental algorithm that is used for sex recognition. In other cases,
hyperstimulation will simply involve use of bright or novel colors that can
better engage visual attention and remain longer in memory.
References
Donnelly, M.P., P. Paschou, E. Grigorenko, D. Gurwitz, C. Barta, R-B. Lu,
O.V. Zhukova, J.-J. Kim, M. Siniscalco, M. New, H. Li, S.L.B. Kajuna, V.G.
Manolopoulos, W.C. Speed, A.J. Pakstis, J.R. Kidd, and K.K. Kidd. (2012). A
global view of the OCA2-HERC2 region and pigmentation, Human Genetics, 131,
683–696.
http://europepmc.org/articles/PMC3325407
Gründl, M., S. Knoll, M. Eisenmann-Klein, and L.
Prantl. (2012). The blue-eyes stereotype: Do eye color, pupil diameter, and
scleral color affect attractiveness? Aesthetic
Plastic Surgery, 36, 234–240.
Martinez-Cadenas, C., M. Pena-Chilet, M.
Ibarrola-Villava, and G. Ribas. (2013). Gender is a major factor explaining
discrepancies in eye colour prediction based on HERC2/OCA2 genotype and the
IrisPlex model, Forensic Science
International: Genetics, 7,
453–460.
Shekar, S.N., D.L. Duffy, T. Frudakis, G.W.
Montgomery, M.R. James, R.A. Sturm, & N.G. Martin. (2008).
Spectrophotometric methods for quantifying pigmentation in human hair—Influence
of MC1R genotype and environment, Photochemistry
and Photobiology, 84, 719–726.
Walsh, S., A. Wollstein, F. Liu, U. Chakravarthy, M.
Rahu, J.H. Seland, G. Soubrane, L. Tomazzoli, F. Topouzis, J.R. Vingerling, J.
Vioque, A.E. Fletcher, K.N. Ballantyne, and M. Kayser. (2012). DNA-based eye
colour prediction across Europe with the IrisPlex system, Forensic Science International: Genetics, 6, 330–340.
http://www.fsigenetics.com/article/S1872-4973(11)00144-X/abstract
Unusually colored eyes and hair may have given ancestral humans a boost not only when it came to marriage, but also when it came to surviving potential infanticide. I don't know about non-white babies, but white babies tend to be born with blue eyes which later darken. Strikingly blue eyes might promote better care responses out of parents, and the persistence of this neotenous trait could encourage longer-term care, increasing the child's chance of survival.
ReplyDeleteAlso, I am a little skeptical of a study which apparently just showed people close-ups of the eyes themselves. I know that's removing some extraneous variables, but it also seems kind of creepy to just look at giant eyeballs.
The sex difference could be explained by the so called blue eye allele being expressed differently with a feminine prenatal hormone balance, thereby becoming green, gray, or violet in women more often than in men. Face shape is less masculine in blue eyed men. I think green eyed men would tend to have an even less masculine face shape than blue eyed men.
ReplyDeleteIt's been suggested that "estrogenization is overdetermined in females, i.e., almost all girls are exposed to enough estrogen in the womb to feminize their face shape". I wonder about women with green eyes; wouldn't they tend to have been exposed to a more feminising environment before their birth?
Maybe "sexual selection is frequency-dependent, shifting to whichever eye color is least frequent", but there are countries where blue eyes are ten a penny; how did they get that way? The Baltic is an area where the digit ratio is low. Maybe blue eyes could have become more common when the average level of prenatal oestrogenisation in the Baltic region decreased through mixing with the Saami. I wonder if blue eyes may be more common now than they have ever been, and green eyes are more unusual in Europe now that they were 10,000 years ago.
Only two posts still? Let me stimulate discussion by suggesting that the greater variability observed in CC homozygous females doesn't necessarily indicate anything regarding a genetic mechanism favoring diversity. It may be the straightforward result of sex-linked darkening processes.
ReplyDeleteAnd these sex-linked processes wouldn't even need to arise because of sexual selection; people with blue eyes have better night vision, which would have benefited hunters and fighters (who were of course disproportionately male).
Avery,
ReplyDeleteWhat study are you referring to? (i.e., the one that showed close-ups of eyeballs).
Sean,
I agree. The sex linkage seems to be mediated by the prenatal surge of estrogen in female fetuses. I'm speculating here, but the differences in eye-color frequencies may simply reflect gene flow from areas to the south and east.
Anon,
But we also see the same thing with hair color, i.e., hair color is more diverse in women than in men.
If blue eyes are an asset for hunting at night, we should especially see them in the northern hunting peoples of Asia and North America. Those regions have very little sunlight during half of the year.
It may be relevant that red hair is more common in women, and more common in north-west Europe.
ReplyDeletePeter,
ReplyDeleteThat hair color is more diverse in women can also be explained by pointing to the fact that their hair is overall lighter. In Europeans, eyes are generally lighter than hair. Lighter eyes (much like lighter skin) push against a pigmentational ceiling, while lighter hair does not.
Skin, hair, and eye pigmentation are all strongly correlated. In all likelihood, a few genes regulate pigmentation throughout the entire body. That the hunting peoples of Asia and North America have brown eyes may relate to the fact that their overall pigmentation isn't very light.
Moreover, as Terman & Terman (1999) point out, pale eyes definitely confer sensitivity to light:
"Among patients in New York, approximately one third of whites have been blue-eyed. Interestingly, they show lower depression scores and can be classified by discriminant analysis as less fatigued than their darker-eyed counterparts (N. Goel et al, unpublished data, 1999). Reduced ocular pigmentation in blue-eyed people causes hypersensitivity to bright light (van den Berg et al 1991). They may also respond at lower light levels to artificial dawn simulation used to treat winter depression and delayed sleep-phase syndrome (Terman 1997)."
And as a pale eyed individual, I can definitely attest to the benefits and drawbacks of light sensitive eyes. I need to be careful about wearing sunglasses, but I have good night vision.
Once I was walking in the wilderness behind a brown-eyed companion with normal vision. We were following a trail, but my companion suddenly stopped dead. When I asked him why, he said the trail had ended. I simply walked around him and continued down the trail. Had we lived a few thousand years ago, he would have been at a profound disadvantage not only on hunting excursions, but on night raids. His daughters, had he fathered any, would likely be at a similar disadvantage under such circumstances - but they wouldn't have cared.
Reference:
Terman, J. S., & Terman, M. (1999). Photopic and scotopic light detection in patients with seasonal affective disorder and control subjects. Biological psychiatry, 46(12), 1642-1648.
Anon,
ReplyDeleteYour explanation doesn't explain why red hair is more common in women than in men. Red hair results from very specific alleles (to varying extents, the same is true for other hair colors). You seem to think that there is only one allele for "lighter hair" and that this allele is expressed differently under different conditions. That isn't true. The MC1R gene has at least 11 functionally different alleles in Europeans.
In any case, what selective advantage are you proposing for lighter hair?
Skin, hair, and eye pigmentation are loosely intercorrelated. Light skin is associated with the full range of hair and eye colors. And even that correlation appears to be due to common selection pressures and not genetic linkage.
In each case, the genes involved are different. Diversification of European hair color occurred primarily at the MC1R gene. Diversificaton of European eye color occurred primarily in the HERC2-OCA2 region. European skin whitened through allelic substitutions at the TYRP1, SLC24A5, and SLC45A2 genes.
You didn't answer my question about blue eyes. If blue eyes were favored because they helped hunters see their prey under conditions of dim light, why don't we see blue eyes in the northern hunting peoples of Asia and North America? Many of them lived under conditions of dimmer light than those of the hunting peoples of early Europe.
Avery, babies of other races also tend to be born with blue eyes, and they later darken. My former bio teacher says that most non-African babies will be born with blue eyes that later darken; African-American babies most commonly are born with grey eyes that later darken.
ReplyDeleteUh, yes I did answer your question about blue eyes. Again, it could be that we don't see blue eyes in non-European populations because those are more darkly pigmented overall, presumably due to weaker sexual selection for feminine traits on those populations.
ReplyDeleteAnd no I don't think there is only one allele for lighter hair. You're being annoying, Peter. Talk to you some other time.
Anon,
ReplyDeleteI'm sorry. I should have written something like "My question to you remains unanswered." I was writing fast and sloppily.
I was hoping to read your explanation as to why blue eyes have not been favored in the northern hunting peoples of Asia and North America. Yes, the incidence of blue eyes does correlate inversely with darkness of skin, but this inverse correlation is due to common selection pressures and not to genetic linkage. Blue eyes can occur in dark-skinned individuals (who are almost always of mixed background).
In the case of hair color, I must confess I have no idea what your argument is. You seemed to be arguing that hair, eye, and skin color are controlled by the same genes and that these genes are limited in number. If that isn't your position, then please accept my apologies.
Oh, sorry for being unclear. The Grundl study--from the linked abstract: "The stimulus material comprised images of the eye areas of 60 women ages 15–65 years."
ReplyDeleteActually, I was wondering how eye color was determined in these studies. Because sometimes--speaking from personal experience--women may lie about their eye color.
Hi peter, I find this fascinating, even just being made aware of the diversity is something most euros don't think about and yet the variability is immense. I met two women in the last couple of days - the first told me that her eyes can change from brown to green to grey to blue. That surprised me because usually people either vary green-brown or blue-green, I didn't know some eyes could do both. She said her mother is icelandic (grey) and her father norwegian (brown). She said, 'look at the ring around my iris, it's yellow', and it was. Today i met a woman with rich dark brown eyes and a baby with bright blue eyes who looked too old to still have blue eyes and she told me that she'd been told that European brown doesn't necessarily over-ride blue whereas African and Asian brown does, always. And when I looked closer I could see that she also had yellow near the centre. Maybe you've covered this already, but is this yellow thing part of the mutability we see?
ReplyDeleteRegarding red hair. What I love about red is the way it lurks beneath and I think maybe alot of men grow red beards who would not be classed as red heads. Blonde hair that goes reddish in the sun is also quite common in men. And it seems that once red is in the family it doesn't go away and can come back as recessive in the right marriage. Blonde on the other hand seems just to darken and to get back to blonde is much rarer once brown is in the family. Just my impressions but red hair seems to be on the increase in the UK, and I wondered if that may be due to red's ability to lurk?