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Two interesting studies of skin color, one in North India and the other in Malaysia. Both studies showed variability in skin color caused by different factors.
quote: Abstract
Background:
Remarkable diversity of skin tones among Indians ranging from pale pinkish to dark brown appears to be an ideal choice for the assessment of skin pigment variation.
Aim:
The present study was designed to assess the variation observed in melanin and erythema indices among north Indians.
Material and Methods:
Skin reflectance data n = 574- was collected from six diverse populations of north India using DermaSpectrometer -DSM II ColorMeter- followed by statistical analysis to investigate the impact of geographical location and gender on constitutive skin pigmentation.
Results:
The melanin index -MI- varied between 17.93 and 56.92 (Mean (M) = 35.80 ± 6.26) whereas the erythema index -E- varied between 4.92 and 18.82 -M = 10.48 ± 2.6 -. MI and EI of females were found to be significantly lower than males -P < 0.001-. Geographical location exhibited a significant association with MI and EI -P < 0.001-. Furthermore, we have noted a positive correlation between MI and EI -P < 0.001-.
Conclusion:
The study has refined our understanding of skin pigmentation variation among north Indians in terms of significant association with geographical location -MI: F -5,568- = 31.07, P < 0.001; EI: F -5,568= 73.37, P < 0.001} and gender -MI: t -386 = −4.06, P < 0.001; EI: t -386- = −11.96, P < 0.001- and rendered opportunities for further studies.
Pigmentation is a readily scorable and quantitative human phenotype, making it an excellent model for studying multifactorial traits and diseases. Convergent human evolution from the ancestral state, darker skin, towards lighter skin colors involved divergent genetic mechanisms in people of European vs. East Asian ancestry. It is striking that the European mechanisms result in a 10–20-fold increase in skin cancer susceptibility while the East Asian mechanisms do not. Towards the mapping of genes that contribute to East Asian pigmentation there is need for one or more populations that are admixed for ancestral and East Asian ancestry, but with minimal European contribution. This requirement is fulfilled by the Senoi, one of three indigenous tribes of Peninsular Malaysia collectively known as the Orang Asli. The Senoi are thought to be an admixture of the Negrito, an ancestral dark-skinned population representing the second of three Orang Asli tribes, and regional Mongoloid populations of Indo-China such as the Proto-Malay, the third Orang Asli tribe. We have calculated skin reflectance-based melanin indices in 492 Orang Asli, which ranged from 28 (lightest) to 75 (darkest); both extremes were represented in the Senoi. Population averages were 56 for Negrito, 42 for Proto-Malay, and 46 for Senoi. The derived allele frequencies for SLC24A5 and SLC45A2 in the Senoi were 0.04 and 0.02, respectively, consistent with greater South Asian than European admixture. Females and individuals with the A111T mutation had significantly lighter skin (p = 0.001 and 0.0039, respectively). Individuals with these derived alleles were found across the spectrum of skin color, indicating an overriding effect of strong skin lightening alleles of East Asian origin. These results suggest that the Senoi are suitable for mapping East Asian skin color genes.
Skin color is a highly visible and variable trait across human populations. It is not yet clear how evolutionary forces interact to generate phenotypic diversity. Here, we sought to unravel through an integrative framework the role played by three factors—demography and migration, sexual selection, and natural selection—in driving skin color diversity in India.
Methods
Skin reflectance data were collected from 10 diverse socio-cultural populations along the latitudinal expanse of India, including both sexes. We first looked at how skin color varies within and between these populations. Second, we compared patterns of sexual dimorphism in skin color. Third, we studied the influence of ultraviolet radiation on skin color throughout India. Finally, we attempted to disentangle the interactions between these factors in the context of available genetic data.
Results
We found that the relative importance of these forces varied between populations. Social factors and population structure have played a stronger role than natural selection in shaping skin color diversity across India. Phenotypic overprinting resulted from additional genetic mutations overriding the skin lightening effect of variants such as the SLC24A5 rs1426654-A allele in some populations, in the context of the variable influence of sexual selection. Furthermore, specific genotypes are not associated reliably with specific skin color phenotypes. This result has relevance for DNA forensics and ancient DNA research.
Conclusions
India is a crucible of macro- and micro-evolutionary forces, and the complex interactions of physical and social forces are visible in the patterns of skin color seen today in the country.
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