The story of lactose tolerance evolving multiple times has blown up a bit, thanks to Nick Wade at The New York Times. Some people are making analogies to light skin evolving via different genetic architectures (remember, skin color is a polygenic trait, albeit dispersed over ~4 loci of large effect). But there is a difference, light skin color emerges via loss of functionality or expression on the loci which result in pigment production. There are many ways to lose function, but it generally is considered more difficult to gain function. And yet this is what lactose tolerance is. Or is it? Remember, infants and toddlers can digest lactose fine, the issue is that
tolerance persists in some populations
. In other words, whatever pathways shift so that metabolic shunts are closed which drive lactose breakdown, all you need to do is short circuit this abolishment of gene expression on the ...
