Most Chinese are lactose intolerant and China doesn't fit this scenario at all. So I guess Darwinianism doesn't apply to 1/5 of humanity.
Are there any native cattle (or other milk producing animals) which were dometicated in China? If not, then ability to digest lactose wouldn't be a survival benefit. On the other hand, if you have a Germanic tribe where Johann's family can drink milk but Wilhelm's family can't, that could be the difference between living and dying during a tough year growing crops.