Human Diversity review: altitude, introducing the ACE model

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@@ -44,11 +44,13 @@ Another mechanism of recent human evolution is _introgression_: early humans int
 
 Population differences are important when working with genome-wide association studies, because a model "trained on" one population won't perform as well against the "test set" of a different population. Suppose you do a big study and find a bunch of SNPs that correlate with a trait, like schizophrenia or liking opera. The frequencies of those SNPs for two populations from the same continent (like Japanese and Chinese) will hugely correlate (Pearson's _r_ ≈ 0.97), but for more genetically-distant populations from different continents, the correlation will still be big but not huge (like _r_ ≈ 0.8 or whatever).
 
-What do these differences in SNP frequencies mean in practice?? We ... don't know yet. At least some population differences are fairly well-understood: I'd tell you about sickle-cell and lactase persistence, except [then I would have to scream](/2017/Dec/interlude-xi/). There are some cases where we see populations independently evolve different adaptations that solve the same problem: people in Tibet and Peru and Ethiopia have 
+What do these differences in SNP frequencies mean in practice?? We ... don't know yet. At least some population differences are fairly well-understood: I'd tell you about sickle-cell and lactase persistence, except [then I would have to scream](/2017/Dec/interlude-xi/). There are some cases where we see populations independently evolve different adaptations that solve the same problem: people in Tibet and Peru have both adapted to high altitudes, but the Tibetans did it by breathing faster and the Peruvians did it with more hemoglobin!
 
-[Tibet, Peru, Ethiopia all have high-altitude adapatations, but they're different adaptations, p. 198]
+Sorry, "the Tibetans did it with ..." is sloppy phrasing on my part; what I actually mean is that the Tibetans who weren't genetically predisposed to breathe faster were more likely to die without leaving children behind. That's how evolution works!
 
-The third part of the book is about genetic influences on class structure!
+[TODO: link, Cynthia M. Beall, "Two Routes to Functional Adaptation", double-check what "resting ventilation" means]
+
+The third part of the book is about genetic influences on class structure! Untangling the true causes of human variation is a really hard technical philosophy problem, but behavioral geneticists have at least gotten started on the problem with their simple _ACE_ model.
 
 [p. 212-4: A + C + E model and comparing identical and fraternal twins (different from twins raised apart)]
 [ACE model assumes no assortative mating, which leads to an underestimate of A: because it makes fraternal twins resemble each other for non-environmental reasons]