Gene Expression

Humans Have Spread Globally, and Evolved Locally (The New York Times):

No one yet knows to what extent natural selection for local conditions may have forced the populations on each continent down different evolutionary tracks. But those tracks could turn out to be somewhat parallel. At least some of the evolutionary changes now emerging have clearly been convergent, meaning that natural selection has made use of the different mutations available in each population to accomplish the same adaptation.

This is the case with lactose tolerance in European and African peoples and with pale skin in East Asians and Europeans.

Nothing new to readers of this weblog, but Wade does a good job surveying the various angles. Anyone with a model of evolution in their head shouldn’t be surprised, the range of human variation is to be expected; we are a species which spans Arctic and tropical biomes, evolutionary pressures generally reshape populations into localized ecotypes. Dogs are similar except their selection pressure was our species, and our preferences (as opposed to environmental conditions) served as evolution’s sculpting tool.

Note that Wade mentions that selection seems notable on both disease and metabolically salient genomic regions. This illustrates the dynamic and multi-layered texture of evolutionary processes, pathogen resistance is always something which all complex species are always tinkering with as we attempt to stay ahead of the race. In regards to the changes in metabolism Wade alludes to the shift between hunter-gather and farming lifestyles. In most of the world this transition occurred between 5 to 10 thousand years before the present, suggesting rapid and recent evolutionary processes which allow for localized adaptation. Lactose tolerance is another example of this, it seems to have emerged in northern Europe only in the last 5 to 6 thousand years, but is now the dominant phenotype across broad swaths of northwest Eurasia. While particular evolutionary dynamics are always bubbling in the background, others are responses to local conditions in time and space. Shifting from a high protein low starch diet to a low protein high starch diet as populations transitioned between modal hunting and gathering toward agriculture was a definite shock to our metabolic systems. Though there is evidence that peasant populations were always physiologically sub-optimal compared to hunter-gatherers (they were smaller for example, and showed more stress during growth in their bone development) their metabolic systems co-evolved with their lifestyles to produce a “good enough” solution and result in more natural increase than non-agricultural peoples. It is a classic illustration that all evolution cares about is replication, not quality of life or some idealized perfection.

Finally, note that Wade emphasizes the convergent evolutionary patterns throughout the world. Selection operates upon traits, the phenotype, the underlying genetic architecture that produces this is irrelevant. In the case of light skin for example alternative alleles (genetic variants) arose which produced the same phenotype via mutation. This might not be too difficult if you imagine that light skin is simply a loss of function and there are many ways to do that. One can imagine that these mutants were always bubbling in the background but in the higher latitudes the dampening pressure of selection was removed (and perhaps reversed), and out of the random sample spaces of mutants western and eastern Eurasia exhibited different clusters. This is likely to have happened quite a bit in the past 10,000 years as cultural innovations, such as agriculture and mass societies, have swept across the world to a far greater extent than populations. The implication being that cognate selective pressures arose at about the same time across disparate regions.

Related articles on recent human evolution.

Comments

  1. #1 Ted G.
    June 27, 2007

    Razib,
    The one thing that puzzles me about Wade’s presentation is this: how can there be a “continental” division between Caucasians and East Asians, when they’re on the same landmass? Especially when you consider that he includes most of the people on the Indian subcontinent in Caucasians.

    The whole concept of race seems artificial once you see intermediate cases. For example, I suppose the Dravidians of south India and Pakistan are not “Caucasians.” But if not, what are they? Clearly not East Asians.

    I’m sure there’s some plausibility to the idea of race if you only mean groups that are highly isolated from one another. If they’re isolated, they will come to be more and more different. But since we’re all human and can reproduce, that isolation is limited, and there are many intermediate cases.

  2. #2 razib
    June 27, 2007

    how can there be a “continental” division between Caucasians and East Asians, when they’re on the same landmass? Especially when you consider that he includes most of the people on the Indian subcontinent in Caucasians.

    i think he means continental scale. in any case, take an instrumentalist view here: there was obviously genetic exchange: but we know that the exchange was of a small enough degree that alternatively positively selected alleles arose which coded for the same phenotype. to make it concrete, most of the alleles which code for light skin in eastern asia and western europe seem totally different. why? if there was a lot of genetic exchange positively favored alleles would spread and reduced the selective value of mutants which arose later. as it is, it seems that the two populations can be modeled as distinct when it comes to response to selection for this trait. that’s what wade is talking about. also, the use of “populations” can be deceptive when you are talking about different dynamics on different genes. various gene trees will give different results because we’re not talking about populations or individuals as units.

    The whole concept of race seems artificial once you see intermediate cases. For example, I suppose the Dravidians of south India and Pakistan are not “Caucasians.” But if not, what are they? Clearly not East Asians.

    you’re being platonic. thing in terms of average genetic differences and clusters. no matter the label you give them population substructure exists. you can do away with the term race if you want, it doesn’t really matter. see my posts here.

  3. #3 Charlie
    June 28, 2007

    It’s interesting that you’ve posited that canine evolution has been directed by human preference, yet human evolution is “undirected” or directed by environmental factors.

    Personally, I agree with Darwin that the primary factor in recent human evolution has been sexual selection, so I see humans and dogs as being subject to related and similar forces of cultural preference.

    It’s likely that people evolved lighter skin further from the equator because people who had it were perceived to be sexually desirable. In climates where such skin is a handicap, lighter skinned people present a rather ugly parboiled appearance and suffer from skin cancer and actinic keratinosis (speaking as a light skinned person who has endured both).

    Spend a few days in France or England and you can tell a French or English girl at a glance. There are physical traits that have been selected for in each case – how many almond-eyed Scottish girls are there? How often do you see almond-eyed Parisians?

  4. #4 razib
    June 28, 2007

    It’s likely that people evolved lighter skin further from the equator because people who had it were perceived to be sexually desirable.

    no it isn’t. it’s a hypothesis.

  5. #5 Charlie
    June 28, 2007

    I was under the impression that my colloquial style makes it clear that I am stating my own conclusions, not the accepted wisdom of some higher authority. I’ll endeavor to be more clear in the future, if I am to be held to such a high standard. :)

    In all of the studies that I have read that have tried to quantify peoples’ hypothetical preference for lighter skin, the authors of the studies entered with preconceived notions of why such a preference might exist, usually involving evil cultural influences, and did not attempt to prove or disprove these notions. Rather, they attempted to show (in whatever population they were studying) that a preference for lighter skin does (or doesn’t) exist.

    My own experiences and observations of the last 30 years indicate to me that most people’s ideals of beauty are more congruent with smooth, unlesioned flesh (such as I have when in Scotland or Maine) than with the broiled and blistered appearance that I take on after twenty minutes of exposure to equatorial climates. If the humidity and heat are high enough, I don’t even have to be exposed to sunlight to turn patchy and red, and my son and spouse have an even stronger reaction (they are freckled redheads whilst I am blond and blue-eyed). My daughter (who is of entirely separate genetic stock and has both African and Native American ancestors) turns a rather lovely coffee color after ten minutes of summer sun and never gets blotchy or blistered skin regardless of heat.

    So, to recap – in my personal anecdotal experience, I find that lighter skinned people are perceived by others to be more attractive when they are not exposed to tropical climes. I have seen no science to disprove this. Darwin hypothesized that the primary factor in recent human evolution has been sexual selection, and I agree with his reasoning and conditionally accept this view as correct given the absence of evidence to the contrary and the examples available to my naked eye (I’ve got quite a few distinct “races” and “hybrids” in my immediate family, including Russian/Tuvan, Chinese, and Puerto Rican). Therefore, it seems appropriate for me to believe that sexual selection is likely responsible for differences in skin tone among population groups, and I hypothesize tenatively that a mating preference for lighter skin that was universal across all human genetic stock would still result in sexual selection for darker skin in tropical climes due to the inability of lighter skinned people to maintain their sexual attractiveness in such places.

    Was that better? Sorry to be so long winded.

    I thought this post said a lot of interesting things.

    http://www.sepiamutiny.com/sepia/archives/001324.html#comment7447

  6. #6 razib
    June 28, 2007

    I thought this post said a lot of interesting things.

    since that was my comment (the post you link to) i would tend to agree ;-)

  7. #7 razib
    June 28, 2007

    also, if you are interested in the topic, read fair women, dark men. the sexual selection hypothesis is hard to falsify, so i don’t think it should be accepted without overwhelming evidence. there are functional rationales for light skin at higher latitudes (the vit D + folic acid theory for example).

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