January 15, 2011

Selection in looks?

A new research paper finds that many genes involved in appearance have rather clear indications of positive selection.


From the Abstract:

Results

Here, we study the level of population differentiation among different populations of human genes. Intriguingly, genes involved in osteoblast development were identified as being enriched with higher FST SNPs, a result consistent with the proposed role of the skeletal system in accounting for variation among human populations. Genes involved in the development of hair follicles, where hair is produced, were also found to have higher levels of population differentiation, consistent with hair morphology being a distinctive trait among human populations. Other genes that showed higher levels of population differentiation include those involved in pigmentation, spermatid, nervous system and organ development, and some metabolic pathways, but few involved with the immune system. Disease-related genes demonstrate excessive SNPs with lower levels of population differentiation, probably due to purifying selection. Surprisingly, we find that Mendelian-disease genes appear to have a significant excessive of SNPs with high levels of population differentiation, possibly because the incidence and susceptibility of these diseases show differences among populations. As expected, microRNA regulated genes show lower levels of population differentiation due to purifying selection. 

While the paper does not seem to suggest why these patterns of inter-population differentiation are stronger in the genes of appearance mostly, I'd say that the selection involved is directly social and sexual and largely the same pattern by which races appeared and are maintained (to some extent). Intuitively people tend to favor statistically more those who look like themselves of people they know and have good opinion of. That way certain looks or family air are sustainedly favored within each population (the son who looks more like the father, the person who looks more sexy according to certain parameters, largely socio-cultural, etc.) producing eventually what we call races, more apparent than the actual underlying differences between populations, which are invariably much smaller than it looks. 

8 comments:

  1. "Other genes that showed higher levels of population differentiation include those involved in pigmentation, spermatid, nervous system and organ development, and some metabolic pathways"

    It's difficult to believe that for all these chararctes:

    "the selection involved is directly social and sexual"

    and a result of:

    "people tend to favor statistically more those who look like themselves"

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  2. I was not talking of them but of the mentioned genes which seem privative of looks and that nevertheless seem to have been selected more than your usual random gene.

    Why?, I wonder, and I can only conclude sexual or otherwise appearance-based selection, which relies only on how others see you.

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  3. "producing eventually what we call races, more apparent than the actual underlying differences between populations, which are invariably much smaller than it looks."

    That's not what this paper shows at all. Listed first in Figure 1: pituitary gland development. Second: dorsoventral neural tube patterning. Then some low-level cellular functions, more brain development, sperm motility, thyroid gland development. Pigmentation comes about halfway down the list and hair follicle development below that.

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  4. True but sperm motility is only logical to be subject to strong selection, right? On the other hand hair, facial cartilages, etc. are not explainable that way because they are trivial traits mostly with almost zero adaptive relevance.

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  5. "On the other hand hair, facial cartilages, etc. are not explainable that way because they are trivial traits mostly with almost zero adaptive relevance".

    They may not have 'zero adaptive relevance'. As n/a said:

    "Pigmentation comes about halfway down the list and hair follicle development below that".

    So those characters are part of a suite. Perhaps selection has been for some character linked to pigmentation or hair colour, although I can easily see why selection may act directly on those two characters.

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  6. "sperm motility is only logical to be subject to strong selection, right?"

    It depends on your social structure. If women are having sex with multiple men in short time frames, sperm motility is subject to powerful selection. If people have very firmly monogamous sexual relationships and a pretty marginal economic situation, the optimal level of sperm motility may be one that produces egg fertilization after a year or two of regular sexual relations, in order to better space children and give them each a better chance of survival (supplementing the parallel adaption of female ferility suppression while nursing). Well spaced children and their parents may survive better than the families whose children are all Irish twins in population bottleneck periods.

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  7. In all cases sperm motility is critical to long term survival of the lineage. Low sperm motility simply means much less descendants and even possibly none.

    Now this can be overcome with in vitro methods, cheating nature for the time being, but in normal conditions sperm motility may be the difference between conception and lack of any offspring.

    Regardless of monogamy or polygamy, because, even in polygamy, sperm seldom has to compete with other men's sperm directly, only in results.

    "If people have very firmly monogamous sexual relationships and a pretty marginal economic situation, the optimal level of sperm motility may be one that produces egg fertilization after a year or two of regular sexual relations"...

    That's nonsense because women have always spaced children (up to a point) by means of extended lactation and diverse anti-conceptive methods. San women use extended lactation to have children spaced by some 4 years, which is ideal for their nomadic lifestyle. Zoé women use certain herbs (secret to men) to prevent conception at will. Etc.

    This stuff (birth control) is better left to women, sincerely. It is that way in all original societies I know of.

    Relying that on sperm motility is flirting with total sterility and the reality-check of someone else getting your wife pregnant, with or without your knowledge.

    Because there's no such thing as strict monogamy in any case. We may have a monogamous tendency but it's nothing strict, not at all.

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  8. You may also be interested in this new paper, where selection in sperm is demonstrated in mice, both poly- and monogamous - more strongly in the polygamous lines, of course, but in both cases anyhow.

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