Adaptation of human skin color in various populations

Abstract

Background: Skin color is a well-recognized adaptive trait and has been studied extensively in humans. Understanding the genetic basis of adaptation of skin color in various populations has many implications in human evolution and medicine.

Discussion: Impressive progress has been made recently to identify genes associated with skin color variation in a wide range of geographical and temporal populations. In this review, we discuss what is currently known about the genetics of skin color variation. We enumerated several cases of skin color adaptation in global modern humans and archaic hominins, and illustrated why, when, and how skin color adaptation occurred in different populations. Finally, we provided a summary of the candidate loci associated with pigmentation, which could be a valuable reference for further evolutionary and medical studies.

Conclusion: Previous studies generally indicated a complex genetic mechanism underlying the skin color variation, expanding our understanding of the role of population demographic history and natural selection in shaping genetic and phenotypic diversity in humans. Future work is needed to dissect the genetic architecture of skin color adaptation in numerous ethnic minority groups around the world, which remains relatively obscure compared with that of major continental groups, and to unravel the exact genetic basis of skin color adaptation.

Keywords: Archaic hominin; Genetic adaptation; Modern humans; Natural selection; Skin color.

Fig. 1

Correlation between skin color and latitude (from Barsh (2003) [5]). (a) A map of human skin color distribution. (b) A plot of skin reflectance against latitude

Fig. 2

Evolutionary model of human pigmentation in three continental populations. The rooted tree shows the genetic phylogeny of human populations from Africa, North Europe and East Asia, with the colors of the branches roughly indicating the generalized skin pigmetation level of these populations (adapted from McEvoy et al. (2006) [39]). Genetic loci reported to be under positive selection in the common ancestor of modern Eurasians are represented by rs1881227 in KITLG, and those independently evolved in Europeans and East Asians, indicating possible convergent evolution, are represented by rs12913832 in OCA2 and rs885479 in MC1R, respectively. The maps of allele frequency were drawn using R (version 3.2.1, https://www.r-project.org), based on these loci in 53 global populations provided by the Human Genome Diversity Panel CEPH (HGDP, http://www.hagsc.org/hgdp/index.html). Blue and red colors denote the ancestral and derived alleles, respectively

Fig. 3

A framework of admixture mapping to detect positive selection. The average faces of African, European, and African America were downloaded from http://www.mediadump.com/hosted-id167-average-faces-from-around-the-world.html#.WLkMU-kfU1A

Fig. 4

Skin color of aboriginal people in the Equatorial zone and the Arctic. (a) Skin color comparison between Bateq (a subgroup of Negrito) and Malay from Peninsular Malaysia. (b) Skin color comparison between Inuit and Swedish from similar latitudes. Portraits of Malay and Swedish individuals are provided by the Joshua Project (http://joshuaproject.net), the Bateq portrait is from http://www.businessinsider.my/, and the Inuit portrait is from http://www.arcticphoto.co.uk/