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Shedding light on skin color
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[QUOTE]Originally posted by xyyman: [QB] I wonder if the hypertension theory is true?? See below Climatic adaptation Early migration out of Africa exposed ancestral populations to colder environments, with less incident sunlight. The most obvious response is in pigmentation, due to the quantity, type, and distribution of melanin. Skin colour is strongly geographically differentiated, with darker-skinned populations concentrated in the tropics, and lighter-skinned populations in more northerly latitudes. In tropical climates the melanin in dark skins protects against sunburn by scattering and absorbing UV radiation; it may also limit photodegradation of nutrients such as folate. Where sunlight is low, depigmentation may be favoured because UV penetration is necessary for vitamin D synthesis (26). A balance between these factors can largely explain the global pattern of pigmentation (27). However, there are discrepancies: while these could be due to recent migration or admixture, an alternative view (downplaying the importance of light skin in vitamin D synthesis) suggests that they may reflect sexual selection, in particular male preference for light-skinned females (28). The last idea predicts that sexual dimorphism should increase with increasing latitude, but in practice this is not borne out (29). The genetic basis of pigmentation variation is complex (30). Candidate gene approaches based on information about melanosome development, abnormal pigmentation phenotypes and orthologs in other species, together with analyses of selection signals in HapMap, have identified a set of about twenty genes that are likely to play important roles (18,30–32). The finding of selection signatures in Europeans and East Asians suggests that there really is some selective advantage for light skin, and the fact that they are in different genes shows that evolutionary routes to similar phenotypes were distinct – convergent evolution (30,32,33). Residence of ancestral populations in tropical Africa also necessitated heat adaptation, including cooling through efficient sweating. The considerable salt loss, combined with low dietary salt availability, led to selection for salt retention; at the same time, there was likely selection for increased arterial tone and cardiac contraction force when blood volume was depleted by water loss. After migration into temperate climates, these adaptations became maladaptive, and may be responsible for increased blood pressure. In the CEPH-HGDP samples, the global distribution of the functional (heat-adapted) alleles of seven genes involved in blood pressure regulation shows a latitudinal cline (34) unmatched by neutral markers. Furthermore, a combination of latitude and the frequency of one of these alleles in the G protein ß3 subunit gene explains a remarkable 64% of global variation in blood pressure. Initially, heat-adapted African populations expanded northward, undergoing selection for cold adaptation. Subsequently, cold-adapted north Asians expanded southward into the Americas less than 20 KYA, undergoing selection for heat adaptation, so that Native Americans show similar salt retention and cardiovascular phenotypes to Africans living at the same latitudes. In more recent migrations, particularly that of Africans to the temperate climate and high-salt environment of North America (Fig. 1C), previously adapted genes may be particularly maladapted, leading to high frequencies of hypertension (35). Dietary adaptation The diversity of environments occupied by hunter–gatherer humans after the early migrations are mirrored by a diversity of diets. Early heterogeneity of resources may still have an impact today: for example, as judged by modern Y chromosome diversity (36), population expansions occurred earlier in the northern part of East Asia, probably because of the abundant megafauna of the ‘Mammoth Steppe’, but later in the south, due to poorer resources. In terms of influence on diet, the most important development in human prehistory was agriculture, beginning 10 KYA in the Near East, with distinct varieties emerging later in China, the Americas and West Africa (Fig. 1B). Increased population densities, reduced dietary diversity, sedentary lifestyle and exposure to animal pathogens together represented a major set of challenges. A common view is that post-Neolithic humans are adapted, through a ‘thrifty genotype’ (37,38) to a hunter–gatherer lifestyle of feast and famine (39), and that the arrival of agriculture signalled the start of an era of dietary maladaptation, leading to high incidences of type 2 diabetes. Later colonization events, like those of the Pacific islands (Fig. 1C), may have involved particularly strong selection for thrifty genotypes (40) – possibly causing subsequent extreme levels of diabetes. These views have not gone unopposed, however (41,42), and recent studies of diabetes susceptibility loci suggest that reality is not so simple. A variant in the transcription factor 7-like 2 gene (TCF7L2) is responsible for 17–28% of the risk of type 2 diabetes in Europeans (43), but, contrary to expectations of the thrifty genotype hypothesis, is associated with reduced body mass index (BMI) in diabetics (44). In the HapMap samples, the frequency of another variant of the same gene, associated with increased BMI, has been driven by selection to near fixation (95%) in East Asians, with lower frequencies elsewhere. The ages of the variant in the different populations correspond approximately to the times of origin of agriculture, suggesting that it conferred some advantage in the post-agricultural environment. However, the nature of this advantage is unclear. A clear example of genetic adaptation to cultural innovation is the selection of alleles of LCT permitting persistence of lactase expression into adulthood. This allows the drinking of milk without adverse effects, and the distribution of the phenotype correlates well with that of populations with a history of cattle domestication and milk drinking (45). In the HapMap samples (16), LCT in Europeans shows the strongest signal of positive selection, reflecting a powerful advantage that may have been more related to milk as a source of uninfected water than as a source of nutrition. Studies in European populations identified a causative regulatory variant 14 kb upstream of the LCT gene (46), with an estimated age of 2000–20 000 years (47). However, lactase-persistent populations elsewhere, including Africa, do not carry this variant. Studies of Tanzanians, Kenyans and Sudanese (48,49) reveal three further nearby variants causing lactase persistence. Examination of surrounding haplotypes show that the three African variants arose independently of each other and of the European variant (a further example of convergent evolution), within the last 7000 years. The known variants still do not account for all of lactase persistence; so further examples are likely to exist. Further dietary adaptations remain to be discovered, and signals of selection around genes involved in the metabolism of other carbohydrates, fat and alcohol (18) are interesting. Cognitive adaptation? While many factors have been crucial to the success of Homo sapiens, the defining innovation has been culture – the capacity to communicate and transfer knowledge, and to deal with novel environments by creating new technologies, including the development and exploitation of new food resources. Some have made the argument that the out of Africa migration entailed novel challenges that favoured the selection of enhanced cognitive ability, and have supported this using comparisons of IQ and brain size (50). However, it is unclear why the cognitive challenges in this gradual migration should be greater than those facing non-migrants subsisting in the diverse and changing environments of Africa. Genetic studies in this area differ from those in, for example, pigmentation, because the interest in a particular gene is stimulated not by any known phenotypic effect, but simply by its expression in the brain, and some unexpected pattern of population differentiation. The identification of an underlying selected phenotype may not be straightforward, or without controversy. Undoubtedly, brain size (and presumably associated cognitive capabilities) increased rapidly in the human lineage over the past 3–4 million years. A long list of functional candidate brain genes has been produced (51), though few have been studied. Most interest has focused on two genes that, when mutated, result in microcephaly – a small brain, but with normal neural architecture. Both abnormal spindle-like microcephaly associated (ASPM) and Microcephalin (MCPH1) show signatures of adaptive selection both during the emergence of the human lineage (52,53), and subsequently (54,55). Globally, both genes show young, high frequency haplotypes that are rare in Africa, and could reflect recent regional selection. There has been argument over whether these patterns could be explained by demographic processes rather than selection (56,57). Selection on these genes is expected to be through some aspect of intelligence, rather than brain size (58). However, the common derived alleles for both genes are unlinked to standard measures of IQ (59), suggesting either that the gene variants were not being selected at all, or that the selected phenotype is something other than intelligence, as measured by the simple single metric of IQ. Information about the transcript and protein expression patterns of the different allelic variants would be helpful. http://hmg.oxfordjournals.org/cgi/content/full/16/R2/R134#SEC3 [/QB][/QUOTE]
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