Complex traits & evolution

Gene ExpressionBy Razib KhanDec 6, 2008 10:06 AM


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Adaptive Complexity takes issue at a post over at Information Processing over race & genetics. On that specific topic, let me just quote Luigi Luca Cavalli-Sforza:

[my question] Question #3 hinted at the powerful social impact your work has had in reshaping how we view the natural history of our species. One of the most contentious issues of the 20th, and no doubt of the unfolding 21st century, is that of race. In 1972 Richard Lewontin offered his famous observation that 85% of the variation across human populations was within populations and 15% was between them. Regardless of whether this level of substructure is of note of not, your own work on migrations, admixtures and waves of advance depicts patterns of demographic and genetic interconnectedness, and so refutes typological conceptions of race. Nevertheless, recently A.W.F. Edwards, a fellow student of R.A. Fisher, has argued that Richard Lewontin's argument neglects the importance of differences of correlation structure across the genome between populations and focuses on variance only across a single locus. Edwards' argument about the informativeness of correlation structure, and therefore the statistical salience of between-population differences, was echoed by Richard Dawkins in his most recent book. Considering the social import of the question of interpopulational differences as well as the esoteric nature of the mathematical arguments, what do you believe the "take home" message of this should be for the general public? [Answer] Edwards and Lewontin are both right. Lewontin said that the between populations fraction of variance is very small in humans, and this is true, as it should be on the basis of present knowledge from archeology and genetics alike, that the human species is very young. It has in fact been shown later that it is one of the smallest among mammals. Lewontin probably hoped, for political reasons, that it is TRIVIALLY small, and he has never shown to my knowledge any interest for evolutionary trees, at least of humans, so he did not care about their reconstruction. In essence, Edwards has objected that it is NOT trivially small, because it is enough for reconstructing the tree of human evolution, as we did, and he is obviously right.

But that's not what I want to talk about. Michael at Adaptive Complexity says:

Complicated traits, involving many genes with likely pleiotropic effects are, given human population history, much less likely to differ significantly (at least the genetic component) between different populations.

Quantitative polygenic phenotypes such as intelligence and height would be "Complicated traits, involving many genes with likely pleiotropic effects." Earlier he says:

there has been a fairly substantial amount of gene flow among human populations over the last 50k years, which tends to act against population differentiation. Given what we know about human population history, and the likelihood that a very complex trait like intelligence is controlled by many genes with pleiotropic effects, I find it implausible that significant differences in intelligence between populations have arisen over the past 50k years.

I need to open my quantitative genetics textbooks again perhaps, but I'm a little confused by this. We already have a great deal of extant variation on quantitative traits like intelligence and height. In other words, there's a lot of variation out there that doesn't seem to have deleterious consequences for selection to work from. I don't get why 50,000 years just isn't long enough for natural selection to shift the distribution of phenotypes when we already have a great deal of heritable variation across our species. The rate of phenotypic change is rather simple for a quantitative trait; it is proportional to heritability (which is a nice proxy for the correlation between the phenotype and genotype on the population level) and strength of selection. Because this process is reducible the same can be said of traits dominated by genes of large effect, though here we would would probably substitute the term penetrance for heritability. What am I missing? (a citation to a page of Mike Lynch's Genetics and Analysis of Quantitative Traits would be nice)

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