Once again, my oversimplified discussion of molecular motion doesn’t affect the point I intended: I have never denied that temperature (or other statistical science) is scientific - the point is that individual molecules don’t have a temperature, or rather that their individual kinetic energy is unknown when temperature is being considered.
My overall point is only that what is unknown, or unrepeatable, should not properly be considered “science”, though statistical generalisations about them may be true, and speculations about their properties, used to model such generalisations, may be useful, though often not true.
Natural selection as an intuitive principle is fine - as “design” as an intuitive principle is fine. As a scientifically defined entity it is less so. Considering the lactose story, in its (relative) simplicity, “selection” stands for processes that are known, or at least believed to be likely and testable, and so in this case the details, as well as the population genetics models, are indeed scientific.
So here one is talking about a known single and specific “fitness” criterion, the ability to digest lactose, and not an abstract “fitness”. Various specific reasons are postulated why this should be advantageous, from additional sources of calories to Vitamin D deficiency in sunless climes, but “able to digest” is intuitively more likely to confer benefit than “inability to digest” - yet that intuition alone is speculation, not science: there may be other factors at play.
This lactose tolerance is correlated positively with a known cultural practice, dairying, and a known (if currently incompletely known) set of genetic markers: it is not just a generalised “environment”.
So in this specific example, both the environmental factors and the fitness criteria are known, and can be modeled. But even in this case, the modelling can be deceptive where speculation replaces true scientific knowledge.
From Wikipedia’s article on lactase persistence, I chased an article which, it was suggested, said that in Northern Europe a form of genetic drift was involved. This seems to be an exaggeration, but the article did suggest at least a couple of important qualifications to the “clearcut case of selection” conclusion.
The first is that niche construction is involved - they suggest that coevolution of the genetic tolerance and the culture of dairying was responsible for the phenomenon: dairying would be more attractive to lactose tolerant groups, and the development of tolerance would be more likely in pastoral societies. When there is a cultural chicken and a genetic egg, at the very least ones model will have to be changed to take that into account, and (I suppose) the apportionment of causation will thereby be less secure.
Since adult consumption of fresh milk was only possible after the domestication of animals, it is likely that lactase persistence coevolved with the cultural practice of dairying, although it is not known when lactase persistence first arose in Europe or what factors drove its rapid spread.
This leads to a second complication (and maybe the origin of Wiki’s “drift” allusion): the increase in dairying in Europe was largely due to migration of pastoral groups, and this would have spread lactase persistence across Europe regardless of selection (and the paper does suggest its spread to non-dairying nearby people groups). This too needs to be apportioned.
As inferred here, the spread of a LP allele in Europe was shaped not only by selection but also by underlying demographic processes; in this case the spread of farmers from the Balkans into the rest of Europe.
Nevertheless, the paper does not dispute, and neither do I, that natural selection occurred, though they do point out the inherent dangers of all models not based on actual observation:
However, as with any simulation model of population history, many simplifying assumptions have to be made and the extent to which these assumptions may lead to erroneous conclusions remains unknown.
In other words, the more unknown contingencies underlie ones science, the more likely it is that the resulting science will also err - and that the error will remain unrecognised.
At one time, one simplifying assumption was that all evolution occurred through natural selection, considered as a general principle rather than the specific interactions between dairying and lactase persistence of your example. But even now, in that case, there’s a real danger of assuming ones conclusion. I mentioned that the advantages of tolerating milk are intuitively obvious, but that doesn’t establish them as true. As my cited article says (last quote):
The reasons why LP, in conjunction with dairying, should confer such a strong selective advantage remain open to speculation.
One suggestion was the prevalance of Vit D deficiency in sunless northern Europe - but this paper found no evidence for what seems a very plausible thing in the year that the UK government has advised all us Brits to take Vitamin D supplements. The more general assumptions about better nutrition leading to higher birthrates or survival really ought to be tested against historical records comparing the survival of milk-drinkers and milk-intolerant in the same societies as lactase persistence was spreading. But such data are not, of course, available - though I am aware of some studies that early agricultural societies were less well nourished than hunter-gatherers.
To the extent that plausible assumptions about selective advantage are made, rather than historical data input, other possible mechanisms will be missed. In the lactase case, this risk is relatively small, but in the original focus of evolutionary theory, and the one that causes controversy - the origin of species - speculations about unknown contingencies (unknown selective pressure and unknown environments expressed as “fitness” in the abstract) are mostly what have been used to construct instances of natural selection.
This is not the application of known scientific laws to new circumstances, but of indefinable general concepts like “natural selection” to intuitive speculations. I don’t consider that ought to be called science, if science is about “scientia” (knowledge), though that doesn’t in any way make it an invalid human activity.