Genetic variation at the MCIR Locus and the time since loss of human body hair

The melanocortin I receptor (MCIR) locus makes a protein that affects the color of skin and hair. At this locus, amino-acid differences are entirely absent among African humans, abundant among non-Africans (especially Europeans), and abundant in chimpanzee/human comparisons (Rana et al. 1999, Harding et al. 2000). Previous studies have found no evidence of either directional or diversifying selection, so the pattern in these data has been attributed to tight selective constraint within Africa--a defense against the strong sunlight there--and relaxed constraint in Eurasia (Harding et al. 2000).