Lactase persistence

Lactose tolerance is the ability to drink milk past infancy. Most mammals can't do this,Swallow, D. M. (2003). "Genetics of lactase persistence and lactose intolerance". Annual Review of Genetics 37: 197–219. doi:10.1146/annurev.genet.37.110801.143820. PMID 14616060Cats like small amounts, but it's basically candy for them, and too much induces diarrhoea. Commercial "cat milk" has added lactase. but, in a startling example of recent evolution, some populations of humans have evolved the beneficial mutation of lactase persistence, the continued activity of the enzyme into adulthood.
Most humans remain lactase non-persistent and hence suffer varying degrees of lactose intolerance (failure to digest milk accompanied by various undesirable gastrointestinal symptoms). However, in areas where humans herd mammals, there are obvious advantages to being able to drink their copious milk. A mutation that appeared in Europe around 10,000 years agoCoelho, M., Luiselli, D., Bertorelle, G., Lopes, A. I., Seixas, S., Destro-Bisol, G. and Rocha, J. 2002. Microsatellite variation and evolution of human lactase persistence. Human Genetics 117(4): 329–339.Bersaglieri T., Sabeti P. C., Patterson N., Vanderploeg T., Schaffner S. F., Drake J. A., Rhodes M., Reich D. E. and Hirschhorn J. N. 2004. Genetic signatures of strong recent positive selection at the lactase gene. American Journal of Human Genetics 74(6): 1111–20. and a mutation that appeared separately in east Africa a mere 3-6,000 years ago each allowed humans to consume milk and products derived from it throughout their lives (convergent evolution).Study Detects Recent Instance of Human Evolution (Nicholas Wade, New York Times, 2006-12-10)Tishkoff et al. "Convergent adaptation of human lactase persistence in Africa and Europe." Nature Genetics 39, 31-40 (2007). doi:10.1038/ng1946 Lactase persistence presents stronger selection pressure than any other known human gene. doi:10.1016/j.bbagen.2005.02.003 
Creationists are not happy at such blatant and striking direct evidence of evolution in humans.
==Global spread==
Adult human populations before the Neolithic revolution were overwhelmingly lactose intolerant.Swaminathan, N. 2007. Not Milk? Neolithic Europeans Couldn't Stomach the Stuff. Scientific American. doi:10.1186/1471-2148-10-89
The lactase persistence haplotype experienced very strong positive selection doi:10.1086/421051 during the rise of dairy farming, around 5,000 to 10,000 years ago. doi:10.1371/journal.pcbi.1000491 It can be found in 80 percent of European-descended humans. Lactase persistence also sprang up separately in Kenya around 3,000-6,000 years ago. It appears the mutation sprung up fairly often, but only benefited from natural selection where it was advantageous, whereas it was eliminated by genetic drift or even actively selected against elsewhere.
Correspondingly, in places with a lack of dairy farming, lactase persistence is much rarer. It is very low in southeast Asia and sub-Saharan Africa, and is absent in the Bantu of South Africa and most Chinese populations ...except amongst the nomads on the borders of China, who drank horse milk regularly (and horse milk is even higher in lactose than cow's milk). The nomads also make an alcoholic beverage, kumis, from mare milk.
The most important factor for lactose persistence is that cow or goat herders in the middle of a steppe have only a few possible sources of food: namely, their animals and whatever sparse vegetation may be fit for human consumption. Being able to consume milk and milk products is hugely beneficial and eliminates the need to slaughter an animal for every meal which could allow for a consistent food source during the entire time the animal produces milk. However, some herding nomads have also taken to drinking blood (in addition to milk) from their animals which they gather (mostly) in non-lethal ways.
Being able to drink milk throughout life turned out to be useful. In Northern Europe milk was a good source of Vitamin D, a shortage of which leads to the particularly nasty and painful disease The 2009 British Women's Heart and Health Study reported more hip and wrist fractures, more osteoporosis and more cataracts in the lactase non-persistent group. doi:10.1038/ejhg.2008.156 They also were on average very slightly shorter (4–6mm) than the other women, as well as weighing slightly less. These were after correcting for metabolic traits, socioeconomic status, lifestyle, and fertility.
== The creationist response ==
Unsurprisingly, creationists had difficulty dealing with this incredibly blatant and powerful evidence of evolution, particularly when it hit the popular press.http://www.nytimes.com/2006/12/11/science/11evolve.html The standard response was to claim that this evidence for evolution is evidence against evolution, on the basis that the failure to switch off lactase is a loss of information (in the special creationist sense of "information", which is biologically meaningless).
The other response was to claim nothing is happening and hey, look over there:
* "Lactase persistence has nothing to do with molecules-to-man evolution or ape-to-human evolution or even information-adding evolution. It is just a switched-on variation within the human genome."https://answersingenesis.org/human-evolution/cavemen/does-paleo-diet-hold-evolutionary-key-good-nutrition/
* "If anything, the prevalence of lactase persistence is a testimony to the fact an all-knowing Creator designed the human genome with the ability to change."https://answersingenesis.org/genetics/dairy-products-early-saharan-inhabitants/
Creationists also accused scientists of racism for having dared to previously think that lactose tolerance was the normal thing and lactose intolerance the oddity,http://creation.com/lactose-intolerance as if that has anything to do with the question.
Creation science research also indicates that "Rather than being an example of 'evolution in action,' adaptive mutation is an awesome witness to God’s design of bacteria." Georgia Purdom and Kevin Anderson. "Analysis of Barry Hall’s Research of the E. coli ebg Operon: Understanding the Implications for Bacterial Adaptation to Adverse Environments." Proceedings of the Sixth International Conference on Creationism, pp. 149–163 (2008) Apparently.
== Alt-right appropriation ==
The ability to consume milk and products derived from it has eventually become a part of the alt-right glossary, since by omitting any mention of the East African mutation one can portray lactase persistence as an exclusive trait among the almighty White Race. A notable occurrence of this was captured on the "He Will Not Divide Us" stream in early 2017, with alt-righters downing gallons of milk (and possibly some poor soul who secretly had lactose intolerance getting diarrhoea in the process).Why White Supremacists Are Chugging Milk (and Why Geneticists Are Alarmed), The New York Times, 17 October 2018 (paywall)
The alt-right are less tolerant of lactose when it's in a milkshake, though.
==See also==
*The false microevolution vs. macroevolution divide that creationists love so much
==References==
Category:Evolution