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https://en.wikipedia.org/wiki/Lactase_persistence

In Northern European populations, the spread of the lactase persistence allele is most closely correlated with positive selection due to added vitamin D into the diet. Whereas in African populations, where vitamin D deficiency is not as much of an issue, the spread of the allele is most closely correlated with the added calories and nutrition from pastoralism.

Confounding Factors There are some examples of factors that can cause the appearance of lactase persistence without actually having the alleles. For example, cultures may have a large reliance on milk products but lack the alleles for lactase persistence because they process their milk first (such as in with cheese). This breaks down the lactase and can allow people to more easily digest the food when they otherwise might not have been able to tolerate it. Also, it has been hypothesized that healthy colonic gut bacteria can also aid in the breaking down of lactase, allowing those without the genetics for lactase persistence to gain the benefits from milk consumption.

Final Draft of Paper
The Evolution of Lactase Persistence among Human Populations

Many think of evolution as a force that has been incredibly influential on the past but has had little effect relatively recently. Long ago humans may have been different, but for thousands of years not much has changed. However, there is much evidence to disprove this and show evolution as force that has been and continues to act on humans, in everything from 10,000 years ago to the present day. One such interesting topic is lactase persistence in humans. Most mammals, and many humans, lose the ability to digest lactose (a primary component of milk) after childhood; this is known as lactase inpersistence. However there are many human populations where the percentage of those with a mutated gene to allow for lactase persistence in adults is high. This shows evolutionary forces at work on human populations extremely recently. Looking at the circumstances surrounding the development of lactase persistence in human populations offers an amazing look into the selective pressures, cultural affects, and evolutionary mechanisms at work.

It is perhaps first beneficial to look at what exactly lactase persistence means in humans. Lactase persistence is the ability to break down lactose, and therefore milk products, into adulthood due to the creation of an enzyme, lactase. Without that enzyme, the lactose cannot be broken down and a person will feel ill or even get sick eating or drinking large amounts of milk or milk products. The genetics of lactase persistence are complicated but are beginning to be understood. First of all, there are only two phenotypes involved. A person is either lactase persistent or inpersistent, there is no in-between. Having even one copy of the allele allows enough lactose digestion to be considered lactase persistent. Lactase persistence is a dominant trait, so those who cannot digest lactose must be homozygous for the recessive allele (Troelson et al, 19). This increases the spread of the phenotype but is not on its own enough to account for the fixation of lactase persistence among many populations. There are clearly strong selective forces at work.

In mammals the ability to digest lactose lasts almost no longer than weaning does for those animals. Typically within about six months to a year animals have lost all ability to digest milk. In all humans it typically lasts beyond weaning by at least a year or two, and in some populations it continues for life. This is lactase persistence in adults, and it is a thoroughly human trait found nowhere else in the animal kingdom.

The exact genetic underpinnings of lactase persistence are being revealed through the scientific study of the human genome. In an example of convergent evolutions it appears multiple human populations evolved lactase persistence independently of each other through separate mutations (Tiskoff et al, 32). In Europe the mutation for lactase persistence is tightly connected with a mutation at the SNP −13,910 C/T (generally linked to −22,018 G/A). The mutation is different in populations in Northern Africa and the Middle East, however. In those places the mutation is associated with the −13,915 T/G location on the genome. Finally in East Africa it is associated with the −14,010 G/C location (Tiskoff et al, 32-33). These three separate instances show that convergent evolution is at work. The similar phenotype (lactase persistence) is not related to gene flow between these populations or because it arose from a common ancestor of these groups. Instead each group had an independent mutation that then arose due to selection in each population. From there, it is possible for other factors to be at work. For example, some postulate that the spread of lactase persistence in Europe might also be caused by genetic drift, or the random spread of alleles within and between populations (Ingram et al, 589). Latctase persistence is an excellent example of a beneficial mutation arising to prominence within a population and can obviously be a textbook example of evolutionary mechanisms at work. Now that the genetics have been at least moderately understood it is possible to take a look at the global spread of lactase persistence in human populations. Overall, it might be surprising to find that about two-thirds of the global population is lactase inpersistent (Ingram et al, 578). Milk drinking is so prevalent in western culture that this fact could be easily missed by many people. This is due to European populations having very high rates of lactase persistence. In northern Europe they can be around 90% and though they decrease the further south one goes they still stay high throughout all of Europe (Ingram et al, 579). Americans of European descent have a similarly high rate of lactase persistence. In contrast, in many Asian populations lactase persistence is almost nonexistent. Chinese and South-East Asian rates are estimated around 90-95% inpersistent (Wang et al, 106). In contrast, the Mongolian diet relies extremely heavily on milk and milk products, implying some very high rates of lactase tolerance in Central Asia (Heyer et al, 380). Rates of lactase persistence among those populations are expected to be some of the highest in the world. Almost all Native Americans are lacking the lactase persistence allele (Kretchmer, 77). Persistence rates in Africa are very patchy and vary greatly between individual populations. There are some pockets with extremely high rates of lactase persistence and others with extremely low rates. African American rates can similarly vary, but are typically very high for lactase inpersistence (Ingram et al, 579). The spread of lactase persistence is a major clue as to its evolutionary effects on human populations.

Perhaps the most interesting aspect of lactase persistence in humans is the selection processes that would have allowed a mutation that emerged to become widespread in separate populations. The timing of the allele rising to fixation with the populations has been shown to always be after the domestication of animals (Platinga et al, 780). This makes sense, as before that there would be little possible milk to drink that would make the mutation beneficial. Populations that never had large reliance on domesticated animals would never have had positive selection for the lactase persistence allele. After the domestication of animals, however, the ability to drink their milk would have had strong nutritional benefits and thus those who could do so would have strong selection in that society (Burger et al, 3740). Furthermore, it is shown that animal products were in use before the rise of lactase persistence. Several cultural techniques can limit the need for lactase, such as fermentation of milk into cheeses and yogurts. In these foods bacteria break down some of the lactose before consumption allowing for even lactase inpersistant humans to be able to digest it more easily (Enattah et al, 60). This widespread use in some societies set the stage for the rise of lactase persistence to allow access to the nutrients of milk and milk products with less preparation. Recent evidence of European DNA shows that markers associated with lactase persistence show some of the strongest evidence of selection ever seen in modern populations (Bersaglier et al, 1118). This is excellent evidence of the amount to which dairy products were depended upon in many populations and the effect they had on the evolution of those populations.

There are a few theories as to what exactly caused the selection for the lactase persistence allele among certain populations. The high rates of lactase persistence among societies with domesticated animals are used as an example of gene-culture evolution (Ingram et al, 581). It is not necessarily the environment at play here, but the human created culture of animal domestication and the use of their products that is driving the evolutionary forces. Pastoralism is seen as the main driving force behind this change, where the entire subsistence strategy of a group is based around surviving off a herd of animals with widespread focus on their products as opposed to just their meat. Another hypothesis is that in higher latitudes where vitamin D deficiency is very possible animal milk offers an alternative (Gerbault et al, 17). There would be strong selection for populations that had alternative ways of supplementing vitamin D due to the possibilities that the environment might not provide it. These theories need not be competing. In Europe, the spread of lactase persistence is more highly correlated with the higher latitudes and genetic drift spreading it somewhat around the continent. In Africa, however, it is far more heavily associated with pastoralism over altitude (Gerbault et al, 20). These two theories can both work together to provide the most accurate picture. It is likely that Northern European populations did benefit enormously from the added vitamin D and also the general nutritional benefits of lactase persistence; but in Africa, where vitamin D is not an issue but food sources can occasionally be tenuous the dependence on herded animals might have been even more extreme. The high rates of pastoralism among traditional Mongolian culture caused extremely high rates of lactase persistence, whereas the relatively close Chinese populations have almost none of those alleles. The interplay of genetics and culture make lactase persistence a fascinating issue.

There are many benefits to be gained from the study of lactase persistence among humans. It is a wonderful example of many evolutionary principles as well as showing that evolution was active among humans extremely recently. It also showcases how among humans cultural factors must be taken into account to truly get an accurate representation of selective pressures. There is significant ongoing research into the exact genetic underpinnings of lactase persistence that may have already or will continue to reveal the specific regions of the genome with more accuracy. It is also a matter that affects the daily life of billions of people. The diet of western culture can be very unforgiving to a lactase intolerant individual due the high presence of milk and milk product among the foods that are eaten. Lactase persistence among adult humans is an incredibly complex and interesting developing example of the power of evolution, its widespread effects, and how humans are still under evolutionary pressures even into the modern day.

References Bersaglieri T et al.; Sabeti, Pardis C.; Patterson, Nick; Vanderploeg, Trisha; Schaffner, Steve F.; Drake, 	Jared A.; Rhodes, Matthew; Reich, David E.; Hirschhorn, Joel N. (June 2004). "Genetic Signatures 	of Strong Recent Positive Selection at the Lactase Gene". American Journal of Human Genetics. 74 (6): 1111–20. Burger, J., Kirchner, M., Bramanti, B., Haak, W., and Thomas, M. G. (2007). Absence of the lactase-	persistence-associated allele in early Neolithic Europeans. Proceedings of the National Academy 	of Sciences USA 104(10):3736-3741.

Enattah, N. S., Jensen, T. G., Nielsen, M., Lewinski, R., Kuokkanen, M., Rasinpera, H., El-Shanti, H., ... Peltonen, L. (January 01, 2008). Independent introduction of two lactase-persistence alleles into 	human populations reflects different history of adaptation to milk culture. American Journal of 	Human Genetics, 82, 1, 57-72.

Gerbault, Pascale; Moret, Céline; Currat, Mathias; Sanchez-Mazas, Alicia; O'Rourke, Dennis; Volm, M; 	Lorenz, WJ (24 July 2009). "Impact of Selection and Demography on the Diffusion of Lactase 	Persistence". PLoS ONE 4 (7): 16-21. Heyer, E., Brazier, L., Ségurel, L., Hegay, T., Austerlitz, F., Quintana-Murci, L., Georges, M., ... Veuille, M. (June 01, 2011). Lactase Persistence in Central Asia: Phenotype, Genotype, and Evolution. Human Biology, 83, 3, 379-392.

Ingram, C. J., Mulcare, C. A., Itan, Y., Thomas, M. G., & Swallow, D. M. (January 01, 2009). Lactose		 digestion and the evolutionary genetics of lactase persistence. Human Genetics, 124, 6, 579-91.

Kretchmer N (1972). "Lactose and lactase". Scientific American. 227 (4): 71–8. Plantinga, T. S., Alonso, S., Izagirre, N., Hervella, M., Fregel, R., van,. M. J. W., Netea, M. G., ... de,. R. 	C.(January 01, 2012). Low prevalence of lactase persistence in Neolithic South-West Europe. European Journal of Human Genetics : Ejhg, 20, 7, 778-82.

Tishkoff, S. A., Reed, F. A., Ranciaro, A., Voight, B. F., Babbitt, C. C., Silverman, J. S., Powell, K., 	Mortensen, H. M., Hirbo, J. B., Osman, M., Ibrahim, M., Omar, S. A., Lema, G., Nyambo, T. B., 	Ghori, J., Bumpstead, S., Pritchard, J. K., Wray, G. A., and Deloukas, P. (2007). Convergent adaptation of human lactase persistence in Africa and Europe. Nature Genetics 39:31-40.

Troelsen, Jasper T. Adult-type hypolactasia and regulation of lactase expression, Biochimica et 	Biophysica Acta (BBA) - General Subjects, Volume 1723, Issues 1–3, 25 May 2005, Pages 19-32.

Wang YG, Yan YS, Xu JJ, et al. (1984). "Prevalence of primary adult lactose malabsorption in three 	populations of northern China". Human Genetics. 67 (1): 103–6.

Oct 1st assignment.
Article:https://en.wikipedia.org/wiki/Lactase_persistence

Edit to main article: However, studies show that individuals lacking the lactase persistence allele may still be able to digest lactose due to colonic bacteria, showing some doubts about the selective advantage of the allele.

Talk Page: 1. Perhaps adding a section about the difference between drinking fresh milk (requires lactase persistence) and fermented products like cheese lessening the need for lactase persistence?

2. Adding that milk usage was likely in the population before the lactase persistence gene arose.

3. Talking about populations that were pastoral and yet have very low lactase persistence rates.

Topic: What are the factors involved in the rise and spread of lactase persistence among human populations?

Annotated Bibliography:

Ingram, C. J., Mulcare, C. A., Itan, Y., Thomas, M. G., & Swallow, D. M. (January 01, 2009). Lactose digestion and the evolutionary genetics of lactase persistence. Human Genetics, 124, 6, 579-91.

-This paper determines the genetic underpinnings of lactase persistence. It looks into the probabilities of multiple cases of the mutation arising within different human populations. It outlines the cases of selection for lactase persistence among certain human populations.

Heyer, E., Brazier, L., S&eacute;gurel, L., Hegay, T., Austerlitz, F., Quintana-Murci, L., Georges, M., ... Veuille, M. (June 01, 2011). Lactase Persistence in Central Asia: Phenotype, Genotype, and Evolution. Human Biology, 83, 3, 379-392.

-This paper outlines the percentages of lactase persistence among the many different populations in Central Asia. It lists not only frequency, but also cultural information on those populations and reasons for lactase persistence in those areas.

Plantinga, T. S., Alonso, S., Izagirre, N., Hervella, M., Fregel, R., van,. M. J. W., Netea, M. G., ... de,. R. C. (January 01, 2012). Low prevalence of lactase persistence in Neolithic South-West Europe. European Journal of Human Genetics : Ejhg, 20, 7, 778-82.

-This paper tests DNA of Neolithic populations in South-West Europe and tests the percentage of lactase persistence to modern populations in that area, helping to imply when lactase persistence arose and the reasons for it to be adopted.

Gallego, R. I., Basu, M. C., Liebert, A., Crivellaro, F., Chaubey, G., Itan, Y., Metspalu, M., ... Kivisild, T. (January 01, 2012). Herders of Indian and European cattle share their predominant allele for lactase persistence. Molecular Biology and Evolution, 29, 1, 249-60.

-This paper tests the assumptions of the rise of lactase persistence in India. It shows that Indian lactase persistency is identical to the genetic underpinnings of Europeans and that it is most highly focused in groups with a pastoralist history.

Enattah, N. S., Jensen, T. G., Nielsen, M., Lewinski, R., Kuokkanen, M., Rasinpera, H., El-Shanti, H., ... Peltonen, L. (January 01, 2008). Independent introduction of two lactase-persistence alleles into human populations reflects different history of adaptation to milk culture. American Journal of Human Genetics, 82, 1, 57-72.

-This paper reflects the convergent evolution among human populations of lactase persistence and hypothesizes that the cause is due to adapatations to different type of milk, in this case that of cattle versus camel milk.