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Why does human skin color vary so much? And what is the relationship between hair color, eye color and overall pigmentation? What genes control pigmentation in humans and other animals? Razib addresses all these questions in this episode of Unsupervised Learning, as he discusses the genetic basis and evolutionary origins of variation on this trait that has held such importance in our natural, social and cultural history. He notes that today we understand the genetic basis of pigmentation in terms of what variants control skin, hair and eye colors and how they relate to other traits, as well as their evolutionary trajectory over the past 100,000 years. Forensic pigmentation prediction tools in Europeans in particular are now excellent.

But Razib notes that it remains a mystery exactly how natural and sexual selection relate to variation in human pigmentation. In Descent of Man, Charles Darwin proposed that racial differences were driven by sexual selection, and this framework has been picked up by later scholars and often emerges as an almost deus ex machina when it comes to explaining variation in pigmentation. The tempting explanation of Vitamin-D synthesis at high latitudes suffers from the reality that light skin has evolved recently in much of Europe, and many northern peoples like the Inuit remain comparatively dark.