FWIW the lactase gene is not a matter of life or death — people lived at high latitude for millennia before it evolved. When it happened though it clearly was selected for. Most genes don’t select Individually on a life-or-death basis, especially in long-living species as its time scale is wrong (slight exception: pinch points like pandemics). It merely adds a bit of evolutionary advantage aggregated with all others. Thus, for example, your sickle cell example: it’s pervasive but hardly universal among its carrier population even though that population was until very recently all exposed to the same (malarial) risk.

In any case my point was that mutations certainly do evolve and stabilize within the span of human civilization (however one might define that). Evolution is a big, parallel, path-dependent process indifferent to which particular kinds of selection (physical or social, endemic or peculiar) happen to favor reproduction.