Friday, August 19, 2005
It's really frustrating when you can't find information via google, but, it just reminds you how shallow the the data mining of search engine crawlers can be. On this weblog people have mentioned blondeness among Australian Aboriginals multiple times, and ultimately we really haven't gotten anywhere (no one has brought up novel data) because no one has any information to offer aside from what they read in C.S. Coon's books when they were younger. There isn't much out there on the web.
Luckily, I decided to check the local college library, and I found Joseph Birdsell's Microevolutionary Patterns in Aboriginal Australia, which has a large section addressing the issue of blondeness among the indigenous people of the antipodal continent. Below, I will summarize most of Birdsell's data and analysis so that google will at least have this to crawl now.
But first, I want to address a minor point that often comes up. One hypothesis about Australian Aboriginal blondeness is that it is due to admixture with Europeans, in particular Dutch sailors who entered into undocumented liasons with native women prior to British colonization. This to me seems like a ludicrous assertion for the following reason: if the blonde alleles introgressed from another population, they can be thought of as proxies for the ancestral admixture of Western Europeans into these tribes. Though a very high frequency of tribal members exhibit preadult blondeness, there are almost no other European diagnostic phenotypes in evidence! That is, their skins are rather dark and their features classically Australian Aboriginal. Most people talk about European blondeness as if it is a recessive trait. I have issues with that simple idea, but, taking it at face value the frequency of blonde alleles in a panmictic population should be higher than the frequency of the blonde phenotype,1 so we are talking about a rather high level of admixture if the blondeness is due to European ancestry. On the other hand, there are no other visible signs of this ancestry. One could hypothesize of course that the initially low frequency (attained via admixture) spread through the population because of positive directional selection on the trait. So in that case the alleles are of European origin, but the frequency of blondeness is not diagnostic of ancestry because it is not a neutral trait. But Birdsell's data points away from a European origin for blondeness, and many of the recollections of readers of GNXP are correct as to the character of this trait among Australian Aboriginals.
To review, there are two primary melanin pigments, dark eumelanin and red-gold pheomelanin. The dosage of these two pigments results in the various hair colors we see in people. Redheads tend to have a great amount of pheomelanin, but almost no eumelanin. Ash blonde people are the reverse when it comes to pheomelanin, while golden blonde individuals tend to be somewhere in the middle. People with auburn hair have relatively high levels of both. But note that pheomelanin is more diffuse and less abundant, and it is no a surprise that black haired individuals may simply mask their "red" pigment. Many people with black hair (including yours truly) go through a "red blonde" phase during hair bleaching, as the dense eumelanin granules are stripped away by the bleaching agents first. It seems that the expression of the phenotype is dependent on many genes, though a few, like MC1R, have an outsized influence (perhaps through regulation of other loci). This is probably one reason that despite the typological division of Europeans into "blondes," "brunettes" and "redheads," there tends to be a continuous gradation of color. Not only do the combinations of eumelanin and pheomelanin dosage add "mixed" categories (strawberry blonde, auburn) to the triplet, the expression of these pigments is not an "on" or "off" matter as one would expect if one locus was at the heart of the process. I have made the repeated argument that the "recessive" character of blondism and the "dominant" character of brunette hair is partially an artifact of how we classify hair color. All the various non-blonde hair colors, from brown to black are slotted into the "dominant" category, when I would argue that even among black haired people there is a wide variance of pigment concentration of eumelanin that visual inspection might miss, for example between a light skinned Japanese individual and someone from southern India or Africa (basically, one can not get below a certain level of reflective, so all the extra melanin does not register any change in color).
Now, to the Australian Aboriginals.
1) The perception (based I assume in color plates in older anthropology books) that the blonde Aboriginals were ash in their coloration is correct. The reason, according to Birdsell, is that they exhibit very little pheomelanin in their hair. Of course there is a lack of eumelanin in the hair samples as well. Unfortunately Birdsell did not assay the concentration of granules quantitatively, but inspected them visually under a microscope. Nevertheless, he saw what was going on at the proximate level pretty well. It wasn't, to consider an outlandish example, a case where a yellow pigment was being produced that obscured the eumelanin.
2) There is both sexual dimorphic and paedomorphic tendencies to the trait. In short, pre-pubescent children are blonder, as are females.
3) This is not a rare trait that is expressed by a few individuals in many tribes. Rather, the frequency of the phenotype can approach 90-100% in children, and still remain significant even in adult males. Also, the "darkening" is often to a brown color, rather than black.
4) Birdsell suggests that the allele which causes this blondeness, in reality the loss of function or expression of both traits (dark and red pigment), is characterized by "incomplete dominance." The frequencies for the expression of the trait are extremely high. If it was a "recessive" trait the allele(s) must be close to fixed. I don't find his arguments persuasive because he didn't mention crosses between dark haired aboriginals and blonde aboriginals, in part because the unmixed peoples of this sort (that is, without European ancestry) are also not likely to go on cross-continental searches for husbands or brides from other Aboriginal groups. But, that being said, Birdsell offers the following observation: hybrids between Europeans and dark-haired (eastern) Aboriginals never exhibit hair that is lighter than brown. Obviously, not all Europeans are blonde, or carry blonde genes, but the conclusion of blonde phenotypic recessiveness is hammered home. Hybrids between blonde Aborigines and Europeans almost always exhibited the ash blonde phenotype of the Aborigines as children. I don't put too much stock in terms like "incomplete dominance," aside from that it is saying "hey, we don't know much about this gene." Nevertheless, I think the hybrid phenotype is a strong line of evidence that it isn't localized on the same part of the genome as the blonde loss-of-function alleles in Europeans. Crosses between dark haired Europeans and blonde Europeans do not almost always result in blonde children (many times the children are blonde and they become dark haired as they develop, but, Birdsell seems to suggest that inheritance pattern is more deterministic when one of the parents is an Australian Aboriginal blonde).
5) Birdsell notes that the blonde phenotype does not apply to all body hair. Almost all the rest of the body hair is rather dark, the only exception being the hairs on the forearm, which tend to be even blonder (that is, those who darken with adulthood retain blonde forearm hair).
I would like to end with a tentative hypothesis. Obviously Birdsell is trying to convey the impression that this is a trait that is "incompletely dominant," even though it is a "loss of function" trait (eumelanin and pheomelanin seem to not be found in the hair). The "incompletely dominant" part suggests that there is a locus of large effect at work here. Additionally, Birdsell only mentions gradation in hair color as a function of development or maturation, not population. What I mean by this is that one doesn't get the impression of individuals with light brown or dark blonde shades as youth who become black haired as adults. Continuity (normalish distribution) is a feature of polygenic traits, while discrete or binary tendencies are exhibited by classical mendellian traits. With this in mind, I offer that perhaps these Australian Aboriginals carry an allele which results in the synthesis of a trans-acting factor which suppresses gene expression on the loci which control for melanin production (or, it could be interfering with a crucial regulatory step). This suppression is obviously dependent on factors relating to development and cell-cell differentiation, because the melanin is found in copious amounts in other body hairs as well as in the skin. A sequencing of the loci which we know affect melanin dosage might not turn up anything out of the ordinary in comparison to other dark skinned people. In contrast, I suspect many Europeans have multiple polymorphisms which result in the overall reduction in melanin production via melanocytes throughout their skin, their body hair as well as their irises.
So why is this trait expressed in frequencies of 90%+ (that is, adults who started out ash blonde as youth) in the west-central deserts of Australia? Birdsell doesn't offer any selectionist reason, and I can't think of any environmental ones. There was obviously constraint on skin color, which makes sense in light of the protection that dark skin confers against radiation. The only thing I can come up with is sexual or social selection (ie; it might have been preferences for a particular type of child as opposed to males and females choosing each other for this trait). But it is basically a default hypothesis (I do not credit genetic drift in this case, but I do not know the demographic history of these tribes, so that is a possibility I suppose). Also, blondism might just be a byproduct of the allele's function, which we do not know yet (or, we know it, but have not made the connection).
I was going to scan the map up, but I'm having some driver issues, so no go in that direction (if someone wants to find the book and scan it up and put it on flickr I will link to it-it's on page 196). Descriptively, you have a modal frequency of this phenotype in the middle of western Australia of 90-100%. The frequency drops off to around 50% by the southwest coast and the geographic center of the continent, and more sharply north toward Arnehm Land until the phenotype is almost nonexistent on the north coast. The phenotype is absent from the eastern third of the continent. Overall, one can imagine an area of the map where the phenotype is absent like a crescent, thick and rotund in the southeast, and becoming a relative sliver as it arcs around the zone of blondeness around its northern edge.
Related: Black and strawberry.
1 - p2 + 2pq + q2 = 1. The "recessive" allele is usually signified by q. The q2 is the frequency of expression of the recessive phenotype, so for example, if the blonde allele is present in a frequency of 0.5 throughout the random mating population, 1/4 of the individuals will express it. If a population is 1/2 blonde, than 70% of the alleles floating in the population are blonde. So, if you had a tribe that was 50% blonde, if blonde alleles are neutral (no selective advantage), ignoring drift one could assume that 70% of the ancestry was European if the alleles had to have come from that source population. Of course, I don't think that the dominance-recessive concept really works well a lot of the time, and I certainly don't think that blondism is a one locus mendellian trait, contrary what they taught us in high school.