Lactose tolerance is the trait that allows children to digest their motherâ€™s milk, and this makes it an essential character during infancy. However, the gene for making lactase (the enzyme responsible for converting lactose to digestible products) may be switched off after weaning. Thus, many adults in the world have no interest in consuming milk. Research into adult lactose tolerance in sub-Saharan Africa is the theme of a news feature by Erika Check in Nature. This work reveals that people groups who rely on cattle for survival all have lactose tolerance. This has led to a reconstructed history of the lactose persistence mutation driven by selection. It involves rapid establishment of the trait in genetically related groups and, with three lactase persistence mutations, the phenomenon is described as a good example of â€œconvergent evolutionâ€. This research is being hailed as a â€œtextbook exampleâ€ of evolutionary processes affecting humans.
Distinguishing between different meanings of the word â€œevolutionâ€ does not seem to be widespread outside ID literature. It is relevant to this case: lactose persistence is not a novelty but the retention of a trait present in infancy. There is no new genetic information: although the word â€œmutationâ€ is used, it refers to switch inactivation. These genetic changes do not, for example, do anything to explain the origin of lactase, nor do they illuminate the origin of any genetic novelty. Consequently, they fit into a category of â€œevolutionâ€ known as â€œmicroevolutionâ€ or â€œminor genetic variationsâ€. They have no bearing on mechanisms for achieving large scale evolutionary transformations.
What can a design perspective contribute to this? At very least, it allows some interesting questions to be asked: are we dealing here with a lactose persistence mutation or a lactose intolerance mutation? Are there different mechanisms for achieving lactose intolerance? Why should lactose tolerance be lost in hunter/gatherers? Is the cost of producing lactase so high? Why a genetic switch when the trait could be kept as a vestigial genetic system? Put another way, is it â€œnormalâ€ or â€œabnormalâ€ for adults to have milk with their muesli?
Human evolution: How Africa learned to love the cow
Nature, 444, 21/28 December 2006), 994-996. Â¦ doi:10.1038/444994a
Lactose Tolerance in East Africa Points to Recent Evolution
By NICHOLAS WADE
New York Times: December 11, 2006
Convergent adaptation of human lactase persistence in Africa and Europe
Sarah A Tishkoff, Floyd A Reed, Alessia Ranciaro, Benjamin F Voight, Courtney C Babbitt, Jesse S Silverman, Kweli Powell, Holly M Mortensen, Jibril B Hirbo, Maha Osman, Muntaser Ibrahim, Sabah A Omar, Godfrey Lema, Thomas B Nyambo, Jilur Ghori, Suzannah Bumpstead, Jonathan K Pritchard, Gregory A Wray & Panos Deloukas
Nature Genetics, Published online: 10 December 2006 | doi:10.1038/ng1946
A SNP in the gene encoding lactase (LCT) (C/T-13910) is associated with the ability to digest milk as adults (lactase persistence) in Europeans, but the genetic basis of lactase persistence in Africans was previously unknown. We conducted a genotype-phenotype association study in 470 Tanzanians, Kenyans and Sudanese and identified three SNPs (G/C-14010, T/G-13915 and C/G-13907) that are associated with lactase persistence and that have derived alleles that significantly enhance transcription from the LCT promoter in vitro. These SNPs originated on different haplotype backgrounds from the European C/T-13910 SNP and from each other. Genotyping across a 3-Mb region demonstrated haplotype homozygosity extending >2.0 Mb on chromosomes carrying C-14010, consistent with a selective sweep over the past ~7,000 years. These data provide a marked example of convergent evolution due to strong selective pressure resulting from shared cultural traitsâ€”animal domestication and adult milk consumption.
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