No one understands the targets of selection in humans (except disease)

I’m proposing on an upcoming episode of The Insight that we should talk about natural selection in the context of humans. The reason is that there seems to be a lot of it. It may even be ubiquitous. But, in most cases which aren’t trivial, we have no good idea what’s going on.

By not being trivial, I mean when there is selection on loci implicated in immunological variation in response, it’s pretty clear what’s going on. Infection by pathogens is arguably the reason that humans have sex, where some sort of frequency-dependent selection is obvious.

When it comes to something like lactose tolerance (lactase persistence), the genomic evidence seems indisputable that there was natural selection (a very long haplotype in Eurasia sweeping up in frequency recently). And there’s a reasonably plausible story. The adoption of agro-pastoralism by disparate populations across the world has produced similar adaptions, albeit via different genetic pathways. But, it isn’t as if we have experimental or ecological evidence as to the differential fitness of humans “in the wild” on this trait. Does milk sugar really make that huge of an impact? (disease kills, its selective power is clear)

Then, you have cases like pigmentation where there are numerous explanations which part of the story, but not most of it. And finally, you have situations like the EDAR variant among East Asians and Amerindians where selection seems likely, but there is zero plausible explanation of what the target of selection is.

8 thoughts on “No one understands the targets of selection in humans (except disease)

  1. I had wondered myself about the benefit of milk sugar – that alone doesn’t seem that differentially beneficial that it would drive such strong selection.

    I am undoubtedly going to try to explain this very badly – but I wonder if the real benefit of lactase persistence was that it enabled older children and adults to drink milk as a regular part of diet, which therefore enabled them to benefit from other nutrients in milk, which were denied to people who were not lactase persistent, just because drinking milk made them so sick. So, in other words, the main benefits from drinking milk were to get the other good stuff – the lactase persistence just unlocked the access to them.

    I say that advisedly – my wife can drink a glass of milk with no ill effects; the adverse consequences to lactose intolerant people seem to vary quite a bit in severity. She doesn’t do it because she is deluded by that to think that she can get energy from the milk sugar, she doesn’t need that at all, but because she listens to health advice that Chinese women need to get more calcium to avoid osteoporosis. And in the process she incidentally gets protein and other good stuff, although that is not why she drinks it.

    What I am saying must be stupid; if it wasn’t, someone else would have thought of it. But I can’t think why else lactase persistence seems to have been so strongly selected, where it was. But then, I can’t think of any plausible enough reason for selection of the EDAR variant either.

  2. Fermenting milk gets rid of most of the lactose intolerance problems anyway, while preserving almost all nutritional value. But people often instinctively prefer sweet to sour.

    My pet theory is that lactose intolerance was selected again as a mild disease with occasional incidences of stronger symptoms. Like even moderately intolerant humans couldn’t resist sweetness of fresh milk, and drank it. But occasionally they developed diarrhea, or had stronger diarrhea in conjunction with other diseases, and in those days severe diarrhea could kill

  3. I was actually going through Mathieson et al 2015 paper and wondering if several signals of recent selection in Europe are all inter-related in relation to a dietary switch in the absence of UV light AND lots of clothing. Lactose persistance allows one to digest the lactose, but that isn’t necessarily the selective force behind it. If you can drink milk you can access all the other nutrients milk has, and I point particularly to calcium, for example). I wonder if the combo light skin color variants and lactose persistance and the DHCR7 variants involved in higher vitamin D status’ allowed early human migration to northern latitudes by allowing them to avoid rickets.

  4. A quick online review of nutrition literature indicates no difference between milk, cheese and yoghurt. Lipid analyses of pottery show European consumption of dairy products since 8000 years ago, but ancient genomes show lactase persistence only spreads late in the Bronze Age, after 3500 years ago. And hunting-foraging people had been in far northern Europe since ca. 11,000 BP (and in Siberia 45,000 yrs), so calcium and vitamin D from dairy was unnecessary for survival at high latitudes.

  5. I don’t believe dairy products need to be “beneficial”, I just think they need to be a thing people are doing. Once you grant that, then the selection kicks in on milk-drinking people, causing the population to have fewer of the alleles for which milk-drinking is a major problem, and more of the alleles for which it is less of a problem.

    It’s obvious that big tails are not “beneficial” to the peacock species, but the quirks of natural selection are such that a preference for big tails is a thing with peahens, purely at random, and so that’s a thing that happens.

    Selection begins to work on the peahens too; once peacocks respond with big tails, a peahen who preferred smaller tails would have male offspring at a disadvantage. But as far as anyone knows, the whole situation is of no adaptive advantage. Natural selection doesn’t always work to the advantage of the population being selected on.

    African people don’t have genes for malaria resistance because hanging around with malaria is a great idea.

  6. Africa, I think it would be better to suggest the vitamin D in milk might have helped allow early *farmers* to migrate successfully to high northern latitudes, by allowing them to avoid rickets. Humans per se have always lived in high northern latitudes any time the ice would let them. But they were hunters living at low population densities who could eat meat.

    It gets more interesting if you imagine what it was like, say, 6,000 years ago as middle eastern farmers (or the practice of farming if you prefer) spread north, with the concommitant increase in population densities and the poor nutritional value of grain. Now you get a need for various ways of making up that lack. Super whiteness and milk drinking could each help in their own way.

  7. And, lactose ‘intolerance’ is an odd thing. Japanese supermarkets have large dairy sections, for example, and ice cream is quite popular. People who ‘should’ be lactose intolerant often have no reports of any problems. My half-Japanese daughter had her genome done for a class recently and was told she is lactose intolerant on BOTH sides, my western white and mom’s East Asian. All of us eat dairy daily.
    Given the strength of the sweep, something else seems to be going on. Milk doesn’t make any of us vomit or give us diarrhea, so what is the advantage of the tolerance? Different lactose tolerance mutations have appeared in different groups which have also spread throughout those groups, so obviously there is some advantage. Is it just at the edges? Is it something more subtle? Maybe we intolerant folks are just all a bit more sickly at the margins.

  8. Handy reference:
    http://www.stevecarper.com/li/list_of_lactose_percentages.htm

    Japanese have invented some (to me) really odd flavoured ice creams, which in itself suggests that ice cream must be fairly popular in Japan. Green tea ice cream seemed like a strange idea to me when I first tasted it, but now seems normal compared to, for example, squid flavoured ice cream. But then, Japanese sized dessert servings are notably small (certainly compared to Australian sized servings – really huge differences), so I guess very few Japanese are really pigging out on the stuff.

    With lactose intolerance, dose size evidently makes a big difference, which logically it should.

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