12 Malaria and Rickets Represent Selective Forces for the Convergent Evolution of Adult Lactase Persistence

摘要

Most of the world's adults cannot drink milk without digestive disturbances because the gut enzyme lactase-phlorizin hydrolase (LPH), responsible for hydrolyzing lactose into easily digestible glucose and galactose, does not persist long after weaning. Notable exceptions to this global generalization are most northern Europeans and certain African and Bedouin pastoralists with high rates of adult lactase persistence (ALP) (Swallow 2003). Accordingly, it is commonly accepted that the high prevalence of ALP in these populations arose rapidly by selection for the LPH encoding gene (LCT) in the 5,000 to 10,000 years since their adoption of dairying (Swallow 2003, Bersaglieri et al. 2004, Tishkoff et al. 2006). Clearly, gene flow from dairying populations with pre-existing high rates of ALP may underlie intermediate ALP frequencies in societies with no history of dairying. Nevertheless, the ability to digest milk is considered an archetypal example of gene-culture co-evolution whereby dairying confers selective benefit to those adults who can drink milk ad libitum without digestive discomfort. The selection coefficient (s) for putative LCT alleles in sub-Saharan Africans (0.035 to 0.097) (Tishkoff et al. 2006), Europeans (0.014 to 0.15) (Bersaglieri et al. 2004), and Scandinavians (0.09 to 0.19) (Bersaglieri et al. 2004) has been labeled: (1) "among the strongest yet seen for any gene in the genome" (Bersaglieri et al. 2004) and (2) "one of the strongest genetic signatures of natural selection yet reported in humans" (Tishkoff et al. 2006).