This is a somewhat unusual example but has been well studied, and would seem to satisfy the criteria of the question. Let me start by quoting the Wikipedia entry for those unfamiliar with the phenomenon:
Lactase persistence is the continued activity of the enzyme lactase in adulthood. Since lactase’s only function is the digestion of lactose in milk, in most mammal species, the activity of the enzyme is dramatically reduced after weaning. In some human populations, though, lactase persistence has recently evolved as an adaptation to the consumption of nonhuman milk and dairy products beyond infancy.
Studies on the geographical distribution of lactase persistence (e.g. AJHG (2014) vol 94, pp. 496–510) show that lactase persistence is associated with cultures which practice pastoralism (specifically herding bovines), supporting the hypothesis that the trait evolved because of the benefit it conveyed to such populations in allowing them to use bovine (or caprine or ovine) milk to survive in adulthood.
There is no doubt that lactase persistence is a heritable — i.e. genetic — trait, as can be seen from consulting the OMIH (Online Mendelian Inheritance in Man) entry. This has extensive documentation, including description of the base changes associated with the trait:
Enattah et al. (2002) found a complete association between biochemically verified lactase nonpersistence in Finnish families and a C/T(-13910) polymorphism of the MCM6 gene (601806.0001) roughly 14 kb upstream from the lactase gene locus (LCT; 603202), located on 2q21. It was the C allele that associated with hypolactasia.
The molecular mechanism of lactase nonpersistence (the putative original human condition) — affected by mutation at this position — is still not completely clear. Recent work suggests:
Epigenetically controlled regulatory elements accounted for the differences in lactase mRNA levels among individuals, intestinal cell types and species.
Finally, I refer to the evidence for the Wikipedia statement that “lactase persistence has recently evolved”. This is a paper by Bersaglieri et al. in The American Journal of Human Genetics (Am. J. Hum. Genet. 74:1111–1120, 2004). I am not a population geneticist, so I shall reproduce the relevant section of their summary unedited:
In northern European–derived populations, two alleles that are tightly associated with lactase persistence (Enattah et al. 2002) uniquely mark a common (∼77%) haplotype that extends largely undisrupted for 11 Mb. We provide two new lines of genetic evidence that this long, common haplotype arose rapidly due to recent selection: (1) by use of the traditional FST measure and a novel test based on pexcess, we demonstrate large frequency differences among populations for the persistence-associated markers and for flanking markers throughout the haplotype, and (2) we show that the haplotype is unusually long, given its high frequency—a hallmark of recent selection. We estimate that strong selection occurred within the past 5,000–10,000 years, consistent with an advantage to lactase persistence in the setting of dairy farming; the signals of selection we observe are among the strongest yet seen for any gene in the genome.