Humanities Homework Help

Some authors hypothesize that sexual selection may also affect the pigments in our skin, eyes, and hair. Sexual selection is when natural selection increases the frequency of certain traits because they increase the chances of attracting a mate. For example, Mel Peacock stuck in survive despite having a huge, cumbersome tail must have pretty good genes; sexual selection led to 1) the attractive trait (males showing off large tails) and 2) the attraction TO the trait (female peacocks finding large tails sexy). Could sexual selection also apply to human pigmentation?

Questions to answer:

1) The article suggest that environmental and dietary factors of a change in skin color among early European’s. Why can’t we use the same factors to explain eye and hair color?

2) The author suggests that the proportion of people with lighter hair and eye color could have increased because they were considered attractive. Explain how frequency dependent selection (Selection for traits that are either rare or common) could have been at work.

3) how might assortative mating (in other words, endogamy) contribute to the perpetuation of lighter hair and eye color?

4) The researchers suggest an alternative explanation for why hair and eyeball color became lighter in Europe. What is it? Why is it significant that pigmentation is polygenic in pleiotropic? 

5) The researchers note that people in some cultures prefer to Mary partners with skin color that is similar to their own. However, this is not the same thing as saying there was sexual selection for skin color (and other studies have found no evidence for it). How are cultural preferences different from natural (sexual) selection for what’s attractive? 

Our analysis indicates that positive selection on pigmentation variants associated with depigmented hair, skin, and eyes was still ongoing after the time period represented by our archaeological population, 6,500-4,000 y ago. This finding suggests that either the selection pressures that initiated the selective sweep during the Late Pleistocene or early Holocene were still operative or that a new selective environment had arisen in which depigmentation was favored for a different reason.

The high selection coefficients estimated for pigmentation genes HERC2, SLC45A2, and TYR are best understood in the context of estimates obtained for other recently selected loci. Using spatially explicit simulation and approximate Bayesian computation, selection on the LCT -13,910*T allele—which is strongly associated with lactase persistence in Europeans and southern Asians—was inferred to fall in the range 0.0259-0.0795 and to have begun around 7,500 y ago in the region between the Balkans and central Europe (37). However, another simulation-based study incorporating latitudinal effects on selection resulted in a lower estimate of S (0.008-0.018) (38). The selective advantage of the G6PD A- and Med deficiency alleles conferring resistance to malaria have been estimated at 0.019-0.048 and 0.014-0.049, respectively, in regions where malaria is endemic (39). These alleles are estimated to have arisen ~6,357 y ago (G6PD A-) and 3,330 y ago (G6PD Med) (39). Thus, the estimates of S for the three pigmentation genes examined in this study are comparable to those for the most strongly selected loci in the human genome.

Although these estimated selection coefficients are high, they are comparable to previous estimates for genes in the pigmentation complex. The selective sweeps favoring the SLC45A2 derived allele, as well as the derived alleles of SNPs in SLC24A5 and TYRP1, which are also implicated in the lightening of skin pigmentation, are estimated to have begun between 11,000 and 19,000 y ago, after the separation of the ancestors of modern Europeans and East Asians (the ages of the selective sweeps affecting HERC2 and TYR have not yet been estimated) (14, 40). Beleza et al. (14) recently estimated the coefficient of selection at the SLC45A2 locus to be 0.05 under a dominant model of inheritance and 0.04 under an additive model. Selection favoring the derived alleles of SNPs in SLC24A5 and TYRP1 was found to be similarly strong.

Estimating selection coefficients using the ancient DNA-based simulation approach presented here offers considerable advantages over traditional methods based on allele age and frequency estimates (1): Selection coefficients are estimated over a defined period; selection acting on standing variation can be accommodated; and our approach is insensitive to the frequently unaccounted for uncertainties associated with allele age estimation using molecular or recombination clocks. This latter advantage is likely to result in considerable improvements in precision. However, our approach does require the assumption of population continuity and will not provide direct estimates of when a selective sweep began.

Although the strength of the selection coefficients in a certain time window can be estimated with improved precision using our ancient DNA-based simulation approach, the actual nature of the selection pressure remains unknown. However, temporal and geographical information from the prehistoric skeletal population under study can help in formulating reasonable hypotheses. Geographic variation in many functional skin pigmentation gene polymorphisms (13), and lighter skin pigmentation more generally, correlate strongly with distance from the equator in long-established populations, suggesting that selective pressure also occurred along a latitudinal gradient. The samples in our study were from between 42°N and54°N, a latitudinal belt in which yearly average UVR is insufficient for vitamin D3 photosynthesis in highly melanized skin (4, 41). Constraints on the ability to photosynthesize vitamin D3 imposed by low incident UVR intensity may have provided significant selective pressure favoring lighter pigmentation populations in high-latitude regions such as the northern Pontic steppe belt. The need to admit UVB radiation to catalyze the synthesis of vitamin D3, together with the decreased danger of folate photolysis at higher latitudes, may account for the observed skin depigmentation from prehistoric to modern times in this region (5).

Dietary change during the Neolithization process may have reinforced selection pressure favoring depigmented skin. The individuals analyzed in this study lived ~500-2,000 y after the arrival of farming in the region north of the Black Sea (42, 43). In many parts of Europe, the Mesolithic-Neolithic transition is associated with a switch from a vitamin D-rich aquatic or game-based hunter-gatherer diet (44) to a vitamin D-poor agriculturalist diet. In low-UV regimes such as the one prevailing in our study region, it is difficult to meet vitamin D requirements without the consumption of significant quantities of oily fish or animal liver (45, 46). The vitamin D recommended dietary allowance of 800-1,000 IU for adults requires the daily consumption of the equivalent of 100 g of wild salmon (the dietary input with the greatest measured vitamin D concentration). Isotopic evidence suggests that the populations sampled in our study continued to access aquatic resources, primarily river fish, in the Neolithic, Eneolithic, and Bronze Age, although there was considerable heterogeneity in fish consumption within the study region (47-50). However, any diminution in fish consumption may have been sufficient to generate additional selective pressure favoring depigmentation at this low-incident-UVR latitude.

Although ecological and environmental factors may be sufficient to explain the observed change in European skin pigmentation, these explanations are unlikely to hold for eye and hair color. The geographic distribution of iris and hair pigmentation variation does not conform as well to a latitudinal cline model, with much of the observed phenotypic variation restricted to Europe and closely related neighboring populations (51, 52). The blue iris phenotype characteristic of the HERC2 rs12913832 G allele, for example, is almost completely restricted to western Eurasia and some adjacent regions, its descendant populations, and populations containing European admixture (51, 52). It is possible that depigmented irises or the various human hair color morphs in Europeans are by-products of selection on skin pigmentation. There is evidence for gene-gene interaction within the polygenic system governing complex pigmentation traits; interactions between HERC2, OCA2, and MC1R, in particular, have been found to have a statistically significant effect on hair, iris, and skin color (36). There is also evidence for epistatic interactions between components of the melanin synthesis pathway in other mammalian model systems, including interactions between the products of ASIP, MC1R, and TYR (53). Additionally, many pigmentation genes, including TYR, HERC2, and SLC45A2 have pleiotropic effects on skin, hair, and eye color (11, 36).

Given that intraspecific pigmentation variability in other taxa, particularly avians, has been attributed to signaling and other factors associated with mate choice (54) it is possible that depigmented irises and the various hair colors observed in Europeans arose through sexual selection (7). Frequency-dependent sexual selection in favor of rare variants has been observed in vertebrates (55, 56), and such selection favoring rare pigmentation morphs could have driven alleles associated with lighter hair and eye colors to higher frequency. Once lighter hair and eye pigmentation phenotypes reached appreciable frequencies in European populations, these novel traits may have continued to be preferred as indicators of group membership, facilitating assortative mating. Assortative mating based on coloration is common in vertebrates (57), and skin pigmentation has been observed as a criterion for endogamy in modern human populations (58, 59). In addition, there is some evidence that lighter iris colors, because of their recessive mode of inheritance, may be preferred by males in assortative mating regimes to improve paternity confidence (60). Consistent with positive assortative mating, an exact test of Hardy-Weinberg equilibrium reveals an excess of HERC2 rs12913832 homozygotes in both the modern (P = 0.0543) and ancient (P = 0.0084) East European samples genotyped here (Table S3 (Links to an external site.)), despite the relatively small sample sizes.

The observed excess of HERC2 rs12913832 homozygotes in the ancient sample might be explained by population stratification in a temporally heterogeneous population sample. Although we do not observe any chronological or spatial patterning of the pigmentation markers in our prehistoric sample, we cannot exclude population stratification in the absence of additional neutral SNPs. However, we note that neither the TYR nor the SLC45A2 SNPs investigated here, nor three additional SNPs investigated in the same ancient and modern samples, showed any significant observable excess of homozygotes (Table S3 (Links to an external site.)), suggesting that the excess of HERC2 rs12913832 homozygotes is less likely to be due to population stratification.

In sum, a combination of selective pressures associated with living in northern latitudes, the adoption of an agriculturalist diet, and assortative mating may sufficiently explain the observed change from a darker phenotype during the Eneolithic/Early Bronze age to a generally lighter one in modern Eastern Europeans, although other selective factors cannot be discounted. The selection coefficients inferred directly from serially sampled data at these pigmentation loci range from 2 to 10% and are among the strongest signals of recent selection in humans.