Selection, but for what???

Gene Expression
By Razib Khan
Feb 29, 2008 12:27 PMNov 5, 2019 9:28 AM


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In my post below about a possible locus to look at to explain the normal variation in hair form we see around us a reader asked:

I was once suckered into giving a course on animal ecophysiology (I was told it was basic ecology until after it was too late to back out) which was a traumatic experience as I've only taken one university-level animal physiology course in my life. One of the students asked what the advantage is of kinky hair. I wondered if it might be a better insulator against the heat of the sun but that's just guessing. I also said that perhaps the question should be the other way round - why do people have straight hair. Perhaps to shed rain more readily? I was hoping this post would answer the question. Are there any suggestions?

Well, I didn't have one. I've read the insulation-from-climate thesis before, and I'm skeptical. I think that these are pretty much the sorts of adaptationist tales which stimulated the counter-reaction in the 1970s. There aremanywaysto detect natural selection in the genome, or at least identify regions which are possible candidates. There are also more classic techniques derived from ecological genetics.

But that doesn't mean we know exactly why a region of the genome was subject to natural selection, or the exact ultimate cause of an observed fitness differential with a natural population.

The lactase persistance story is probably one of the best ones we have. LCT is the region of the genome you want to look at if you want to make sure your test for selection actually works in humans (well, at least if you have sample of European humans). Not only that, we have a good idea about the phenotypic consequences of the genetic variation, we can also relate that phenotype to adaptive value, and hook up the function to cultural and historical correlates. Everything is just lined up there for you, it's too easy. Unfortunately, not everything is like that. Consider the likely selection around HERC2-OCA2, we know that the variation on this locus is a pretty good indicator of whether you have blue eyes or not. Well, obviously that means blue eyes are selected for, right? (the blue eye causing mutant has been increasing in frequency in Europe a great deal in the last 10,000 years) Perhaps. You could, for example, posit a sexual selection model. Unfortunately, sexual selection models can be introduced to explain almost anything that is physical, and often emerge when there isn't anything else on the table plausible. Additionally, as I've noted, since blue eyes are a recessive trait it seems likely that it would have been at rather low frequencies for a long period (because most copies wouldn't be expressed) which should have left particular stamp on the genome which I don't see (in short, I think the haplotype is too long for selection on a recessive trait). This is why I have suggested that lighter skin color is the real adaptation that is driving selection on HER2-OCA2, on this trait the genetic effect does not exhibit recessivity, and so is amenable to the power of selection from the get-go. But there are more details which need to be filled in here. For example, I have been having a difficult time getting specific reflectance values from the researchers who reported the complexion trend; they simply binned their observations into 3 categories (fair, medium and olive). We need to know how large of an effect this particular locus has on skin color, as after all there are around half-a-dozen other major candidate regions, and some of them likely have a far stronger effect. Could a small effect have resulted in the third longest non-rare haplotype in the European genome? Additionally, why was the selection recent? With LCT we know that cattle showed up right before this locus was subject to selection, there's a pretty obvious causal relation here. But if light skin allows one to synthesize Vitamin D at higher latitudes, why was it not selected for until relatively recently? I have some ideas, but the honest truth is that we aren't totally sure, and there's a lot of confusion right now. More questions than answers. Offering a tight adaptive explanation for selection on the region around HER2-OCA2 is contingent upon many parameters which haven't been locked down yet. I wanted to focus on blue eyes and HER2-OCA2 because though I don't think anyone can be totally confident, we have some parts of the picture. We can start the process of hypothesis generation informed by a few plausibilities. On the other hand, for a lot of the genome we don't have any clue as to why it might show up on these tests which imply selection. And there are many traits, like kinky hair, which don't have an obvious adaptive explanation right now. Unfortunately there are many gaps in the superstructure into which we would like to slot our neat explanatory frameworks. I suspect that hypotheses will come more easily once we know a lot more. All the biochemical pathways, their dependencies, more history and archaeology, ethnography which might give us a grip of contemporary fitness differentials, and so forth. Until then, we do the best we can. And try not to be too embarrassed to say we have no clue when that's all we can say.

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