Mammalian babies drink milk. Almost all non-human adult mammals do not drink milk. Some human adult mammals drink milk (northern Europeans, East Africans), while many adult human mammals (East Asian, Afro-American, Basque) cannot drink milk without suffering lactose intolerance symptoms (lactose, a milk sugar, cannot be metabolized).
Here’s the story: mammalian infants have an enzyme, lactase that enables lactose to be metabolized. This enzyme is normally lost after the infants are weaned. However in certain groups where dairy farming has been carried out, a gene enabling lactase persistence enables the enzyme to continue to be present. This gene was not present in these groups (Northern Europeans/East Africans) 8000 to 10,000 years ago, but is now (77% or higher in Northern Europeans). Therefore it must have appeared as a mutation and by enhancing survivability, spread.
This latter was the story I heard in John Hawks audiobook, “The Rise of Humans”, (see also this Nature article), and it seemed plausible, but as physicist I wanted numbers. The best science according to Fr. Stanley Jaki is quantitative, so that predictions or retrodictions can be assessed quantitatively. And I have found such a mathematical test in an article by Todd Bersagiliari and many others, Genetic Signatures…
Dancing on a tightrope of very involved statistical and Markov chain calculations (I have to confess I don’t fully understand all the math, but accept it) the authors find that this mutated gene that allowed persistence of the lactase enzyme action enhanced survivability by between .09 and 0.19 for the Scandinavian population and between 0.014 and 0.15 for the East African. I take this “coefficient of selection” to mean that those Scandinavians carrying this dominant gene produced between 1.09 to 1.19 more children per generation than those who did not have the gene.
Critical comments solicited, if I have misunderstood the conclusions of the cited papers. Finally, let me say that I’m happy to join or at least not throw brickbats at the Neo-Darwinians who argue for survival of the fittest as a mechanism for evolution.