Natural selection in modern humans

Last year, John Hawks and team wrote about how evolution has been accelerated in recent human evolution, esp. in the past 40 kya or so. A day or two ago Barreiro ed amici wrote about recent natural selection in modern humans. When it comes to genetics (and to science, as long as we’re being candid) I’m fairly green, so if you want to read a good news brief about this interesting paper, check out Ann Gibbons’s coverage in Science. Basically, the selection-seeking squad analyzed 2.8 million single nucleotide polymorphisms (SNPs) from 210 people from Africa, Asia and Europe. They identified hundreds of genes that appear to have been the targets of fairly recent selection. In some areas of the genome, esp. those related to diseases, negative selection appears to have reduced differentiation between the three continents. Conversely, positive selection has increased diversity, presumably as populations have adapted to local environments.

The authors point out that their methods pave the way for important research to be done on diseases with genetic predispositions, esp. those correlated with ethnicity. It’s pretty cool stuff, but another thing I found really interesting was this paragraph:

Of note, among the highly differentiated genes with known functions, several control variable morphological traits in humans (Table 1). Furthermore, most of these genes are pleiotropic: that is, they are individually involved in several different traits. for example, EDAR regulates hair follicle density and the development of sweat glands and teeth in humans and mice [24, 25]. In humans, selective pressures on EDAR favoring changes in body temperature regulation and hair follicle density in response to colder climates may have influenced tooth shape, although this trait probably does not affect population fitness. This anecdotal example shows how ‘phenotypic hitchhiking’ in genes under positive selection may have substantially increased the observed number of physiological and morphological traits differentiating modern human populations. (p. 4)

One of the shortcomings of paleoanthropology is poor understanding of the genetic basis for various aspects of skeletal morphology, which is just about all the benevolent fossil record gives us; that is to say, it can be difficult to determine whether characters (i.e. “brow ridges”) are determined by genes, environment, complex interaction between genes and environment, complex interaction between different genes (epistasis), etc. Because of the pleiotropic effects of EDAR, discerning which aspects of the phenotype were shaped by selection becomes difficult. In the course of human evolution, did the loss of body hair (compare most humans to any primate) coincide with dental evolution, and if so, how does this affect our understanding of selective forces shaping hominin evolution? Or, can hominin dental evolution tell us anything about the evolution of body hair or sweat glands? These are interesting questions that could really only be asked as the functional properties of the genome begin to be uncovered. Of course, they can probably only be best answered when we know more about the true functions and frivolities of genes.

References
Barreiro LB, Laval G, Quach H, Patin E, and Quintana-Murci L. 2008. Natural selection has driven population differentiation in modern humans. Nat Genet xx:1-6. [I need to review how to cite advance-online materials]

Hawks J, Wang ET, Cochran GM, Harpending HC, and Moyzis RK. 2007. Recent acceleration of human adaptive evolution. Proc Nat Acad Sci 104: 20753-20758.

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3 thoughts on “Natural selection in modern humans

  1. That’s a really good question. The seminal Cann et al (1986) study that looked at mtDNA in modern humans used only 147 people, out of which they identified 133 mtDNA haplotypes. I had a huge problem with this when I read it, but I haven’t heard any criticism of the sample, and people still tout it as evidence for a recent African origin of modern humans.

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