Bryc and others associated with the laboratory of Carlos Bustamante provide, in Chapter 8, a detailed case study involving mostly Hispanic/Latino populations in Central and South America. The authors compile and analyze genotypic information for several thousand individuals at several tens of thousands of SNPs (single-nucleotide polymorphisms) scattered across the two human genomes (nuclear and mitochondrial). The results reveal a complex genetic signature of recent sex-biased admixture superimposed on a potentially ancient substructure involving source populations of Native American, European, and West African ancestry. In addition to illuminating the genealogical heritage of particular human populations, genomic surveys of this sort, when interpreted in combination with detailed epidemiological data, should also be helpful in studies of the spatial distributions and evolutionary-genetic etiologies of particular human heritable diseases.
In Chapter 9, Nina Jablonski and George Chaplin show how, even in the age of genomics, much can still be learned about adaptive human evolution from comprehensive geographical analyses of phenotypes, in this case involving the most obvious of all human polymorphisms: skin pigmentation. Although the precise mechanistic action of the full suite of pigmentation genes underlying human skin-color variation remains incompletely known, the authors erect a compelling adaptationist scenario for why humans generally evolved dark skins near the equator and depigmented but tannable skins at intermediate and higher latitudes. This striking latitudinal pattern appears to reflect selection-mediated responses to two distinct challenges related to exposure to ultraviolet radiation (UVR), major forms of which (UVA and UVB) vary predictably with latitude and season. In the tropics, where UVA is high year-round, dark pigmentation tends to be selectively advantageous because it protects the body against damaging UVR exposure. At higher latitudes, where UVB levels generally are lower and peak only once per year, natural selection has tended to favor light but tannable skin that can capture UVB for the cutaneous production of vitamin D, which otherwise must come from a suitable diet. As detailed by Jablonski and Chaplin in their opening comments, this modern understanding of skin color variation in humans is strikingly different not only from some of the racially prejudiced ideas formerly in vogue, but also from the sexual-selection hypothesis for skin pigmentation favored by Darwin in The Descent of Man.
Before Darwin, most scientists as well as theologians accepted what seemed obvious: that divine intervention must have underlain nature’s design. The traditional “argument from design” traces back at least to the classical Greek philosopher Socrates more than 400 BC [see Sedley (2008)], and it was expressed again in a thoughtful treatise entitled Natural Theology by the Reverend William Paley (1802). Darwin later recalled in his