No one understands the targets of selection in humans (except disease)

I’m proposing on an upcoming episode of The Insight that we should talk about natural selection in the context of humans. The reason is that there seems to be a lot of it. It may even be ubiquitous. But, in most cases which aren’t trivial, we have no good idea what’s going on.

By not being trivial, I mean when there is selection on loci implicated in immunological variation in response, it’s pretty clear what’s going on. Infection by pathogens is arguably the reason that humans have sex, where some sort of frequency-dependent selection is obvious.

When it comes to something like lactose tolerance (lactase persistence), the genomic evidence seems indisputable that there was natural selection (a very long haplotype in Eurasia sweeping up in frequency recently). And there’s a reasonably plausible story. The adoption of agro-pastoralism by disparate populations across the world has produced similar adaptions, albeit via different genetic pathways. But, it isn’t as if we have experimental or ecological evidence as to the differential fitness of humans “in the wild” on this trait. Does milk sugar really make that huge of an impact? (disease kills, its selective power is clear)

Then, you have cases like pigmentation where there are numerous explanations which part of the story, but not most of it. And finally, you have situations like the EDAR variant among East Asians and Amerindians where selection seems likely, but there is zero plausible explanation of what the target of selection is.