I've always wondered. Why are the genes for blue/green eyes or red/blonde hair recessive? Do you know the genetic explanation why we can't have blue eye genes to be dominant?
You should write about African hair sometime. Most Black women in the US spend a lot of money trying to make their hair straight. A lot of them just wear wigs or straight hair extensions (yes Michelle Obama also). I've personally observed Black women just hate staring at White and Asian women's straight hair.
Heat in Africa can't be the only explanation. Australian aborigines or South Asians live in hot areas and they don't have those tight curls.
Contrary to widespread belief, eye color does not follow a simple two-allele model of inheritance where brown eyes are dominant and blue eyes recessive. A single copy of the blue-eye allele usually produces an intermediate hue, like green or hazel, and even two copies do not always produce blue eyes (Branicki et al. 2009; Eiberg et al. 2008; Martinez-Cadenas et al. 2013).
It would be more accurate to say that "European" alleles are not dominant (such as those for blue/green eyes and red/blonde hair). By mixing these alleles, it's possible to create many gradations of color. If the same alleles were dominant, that would not be possible.
Keep in mind that we are attracted to a color by three qualities: its brightness, its purity (narrow band width on the spectrum), and its novelty. The last quality is the one that has driven selection for an increasing variety of eye and hair colors.
I've written about the evolution of human head hair. Straight hair seems to have begun as an infant trait that was mimicked by women in a context where sexual selection acts on women.
Branicki, W., U. Brudnik, and A. Wojas-Pelc. (2009). Interactions between HERC2, OCA2 and MC1R may influence human pigmentation phenotype. Annals of Human Genetics 73(2): 160-170. https://doi.org/10.1111/j.1469-1809.2009.00504.x
Eiberg, H., J. Troelsen, M. Nielsen, A. Mikkelsen, J. Mengel-From, K.W. Kjaer, and L. Hansen. (2008). Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression. Human Genetics 123(2): 177-187. http://doi.org/10.1007/s00439-007-0460-x
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(4): 453-460. https://doi.org/10.1016/j.fsigen.2013.03.007
The reason seems to be the higher rate of maternal neglect. I address that topic in one of the chapters of my latest book: Frost, P. (2022). European Hair, Eye, and Skin Color: Solving the Puzzle. Washington: Academica Press. https://www.academicapress.com/node/549
The following is a cut-and-paste:
As with Europeans, blondism varies by age and sex. The Aborigines of central Australia are about 85% fair-haired and 15% dark-haired up to ten years of age in both sexes. Male hair then darkens, its color ranging from medium brown to black during adolescence and being almost always dark beyond the age of twenty. In contrast, female hair darkens only after twenty and is seldom darker than light brown even in old age. We see similar age and sex differences in Solomon Islanders. Thus, in both regions, blond hair is widespread in children of both sexes and then becomes much more specific to women. This is similar to the European pattern, but more exaggerated.
Why did blond hair evolve in Oceania? One reason may simply be an aesthetic preference:
"In Samoa, all fair hair is considered 'ena'ena, a word that is usually translated as brown, although when English-speaking Samoans use this term in reference to hair, they typically gloss it as 'blond'. This makes sense since, when one is bleaching Polynesian hair, it goes through a series of reddish- brown shades prior to arriving at blond, and even then retains a reddish hue. When describing hair, Samoans specify the actual shade of 'blond' by using certain modifiers with 'ena'ena, such as 'ena'ena manaia, which literally means 'really nice brown hair', but which refers to a very fair reddish colour."
"The hair of female spirits is most commonly said to be 'ena'ena manaia, and they are wont to decorate it with a red hibiscus."
Blond hair may also protect children from neglect. Papua New Guinea has a relatively high rate of child malnutrition—35% on average and up to 78% in some regions, including regions where adults are well fed. The key factor seems to be neither environmental nor ecological but cultural. In some cultures of Papua New Guinea, mothers may deliberately withhold parental care:
"These women take a guarded attitude toward infants, extending the greatest amount of their affection and parental care toward children who are physically strong and who survive the first couple of years of life."
Because a newborn child has uncertain chances of survival, it remains unnamed for a few weeks after birth, and the father's kin will be ritually thanked only after a six-month wait. Much will depend on the mother’s feelings for the child, and those feelings may select for “cute” features, like blond hair.
Do Europeans have a greater degree of sexual dimorphism than other descent groups? (Depends on how you might measure sexual dimorphism - difference in height/weight between the sexes, perhaps-- is there a way to measure this?). Qualitatively, in terms of appearance in and observed behavior and cognition, I would say yes. Your stats about differences in rates of lighter hair and eye color support this.
For some characteristics, but not for others. Keep in mind two things:
- sexual dimorphism can be reduced by biological constraints. The sex difference in skin color is actually smaller in northern Europeans than in most other populations. This is because sexual selection for lighter-skinned women pushed skin depigmentation in both sexes to the biological limit.
- in ancestral Europeans, sexual selection of women was intensified by limits on polygyny (because of the high cost of providing for a second wife and her offspring) and by high male mortality (because of longer hunting distances and, hence, a higher rate of hunting-related mortality). The reverse situation occurred in tropical regions, particularly among the "horticultural" farming peoples of sub-Saharan Africa. Because women could largely provide for themselves, the costs of polygyny were low and men were incentivized to mate with as many women as possible. Also, there was much less difference between male mortality and female mortality in tropical regions than in non-tropical regions.
I've always wondered. Why are the genes for blue/green eyes or red/blonde hair recessive? Do you know the genetic explanation why we can't have blue eye genes to be dominant?
You should write about African hair sometime. Most Black women in the US spend a lot of money trying to make their hair straight. A lot of them just wear wigs or straight hair extensions (yes Michelle Obama also). I've personally observed Black women just hate staring at White and Asian women's straight hair.
Heat in Africa can't be the only explanation. Australian aborigines or South Asians live in hot areas and they don't have those tight curls.
Contrary to widespread belief, eye color does not follow a simple two-allele model of inheritance where brown eyes are dominant and blue eyes recessive. A single copy of the blue-eye allele usually produces an intermediate hue, like green or hazel, and even two copies do not always produce blue eyes (Branicki et al. 2009; Eiberg et al. 2008; Martinez-Cadenas et al. 2013).
It would be more accurate to say that "European" alleles are not dominant (such as those for blue/green eyes and red/blonde hair). By mixing these alleles, it's possible to create many gradations of color. If the same alleles were dominant, that would not be possible.
Keep in mind that we are attracted to a color by three qualities: its brightness, its purity (narrow band width on the spectrum), and its novelty. The last quality is the one that has driven selection for an increasing variety of eye and hair colors.
I've written about the evolution of human head hair. Straight hair seems to have begun as an infant trait that was mimicked by women in a context where sexual selection acts on women.
Frost, P. (2015). Evolution of long head hair in humans. Advances in Anthropology 5(4): 78-88. https://doi.org/10.4236/aa.2015.54021
Other references
Branicki, W., U. Brudnik, and A. Wojas-Pelc. (2009). Interactions between HERC2, OCA2 and MC1R may influence human pigmentation phenotype. Annals of Human Genetics 73(2): 160-170. https://doi.org/10.1111/j.1469-1809.2009.00504.x
Eiberg, H., J. Troelsen, M. Nielsen, A. Mikkelsen, J. Mengel-From, K.W. Kjaer, and L. Hansen. (2008). Blue eye color in humans may be caused by a perfectly associated founder mutation in a regulatory element located within the HERC2 gene inhibiting OCA2 expression. Human Genetics 123(2): 177-187. http://doi.org/10.1007/s00439-007-0460-x
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(4): 453-460. https://doi.org/10.1016/j.fsigen.2013.03.007
What about the yellow hair of some Australian Aborigines? Why did they develop differently than S-S Africans despite a similar enviroment?
The reason seems to be the higher rate of maternal neglect. I address that topic in one of the chapters of my latest book: Frost, P. (2022). European Hair, Eye, and Skin Color: Solving the Puzzle. Washington: Academica Press. https://www.academicapress.com/node/549
The following is a cut-and-paste:
As with Europeans, blondism varies by age and sex. The Aborigines of central Australia are about 85% fair-haired and 15% dark-haired up to ten years of age in both sexes. Male hair then darkens, its color ranging from medium brown to black during adolescence and being almost always dark beyond the age of twenty. In contrast, female hair darkens only after twenty and is seldom darker than light brown even in old age. We see similar age and sex differences in Solomon Islanders. Thus, in both regions, blond hair is widespread in children of both sexes and then becomes much more specific to women. This is similar to the European pattern, but more exaggerated.
Why did blond hair evolve in Oceania? One reason may simply be an aesthetic preference:
"In Samoa, all fair hair is considered 'ena'ena, a word that is usually translated as brown, although when English-speaking Samoans use this term in reference to hair, they typically gloss it as 'blond'. This makes sense since, when one is bleaching Polynesian hair, it goes through a series of reddish- brown shades prior to arriving at blond, and even then retains a reddish hue. When describing hair, Samoans specify the actual shade of 'blond' by using certain modifiers with 'ena'ena, such as 'ena'ena manaia, which literally means 'really nice brown hair', but which refers to a very fair reddish colour."
"The hair of female spirits is most commonly said to be 'ena'ena manaia, and they are wont to decorate it with a red hibiscus."
Blond hair may also protect children from neglect. Papua New Guinea has a relatively high rate of child malnutrition—35% on average and up to 78% in some regions, including regions where adults are well fed. The key factor seems to be neither environmental nor ecological but cultural. In some cultures of Papua New Guinea, mothers may deliberately withhold parental care:
"These women take a guarded attitude toward infants, extending the greatest amount of their affection and parental care toward children who are physically strong and who survive the first couple of years of life."
Because a newborn child has uncertain chances of survival, it remains unnamed for a few weeks after birth, and the father's kin will be ritually thanked only after a six-month wait. Much will depend on the mother’s feelings for the child, and those feelings may select for “cute” features, like blond hair.
Awesome! I'll check out those papers.
Do Europeans have a greater degree of sexual dimorphism than other descent groups? (Depends on how you might measure sexual dimorphism - difference in height/weight between the sexes, perhaps-- is there a way to measure this?). Qualitatively, in terms of appearance in and observed behavior and cognition, I would say yes. Your stats about differences in rates of lighter hair and eye color support this.
For some characteristics, but not for others. Keep in mind two things:
- sexual dimorphism can be reduced by biological constraints. The sex difference in skin color is actually smaller in northern Europeans than in most other populations. This is because sexual selection for lighter-skinned women pushed skin depigmentation in both sexes to the biological limit.
- in ancestral Europeans, sexual selection of women was intensified by limits on polygyny (because of the high cost of providing for a second wife and her offspring) and by high male mortality (because of longer hunting distances and, hence, a higher rate of hunting-related mortality). The reverse situation occurred in tropical regions, particularly among the "horticultural" farming peoples of sub-Saharan Africa. Because women could largely provide for themselves, the costs of polygyny were low and men were incentivized to mate with as many women as possible. Also, there was much less difference between male mortality and female mortality in tropical regions than in non-tropical regions.