User:Thomas.2553/sandbox

Note of Revisions
Thank you to those who helped revise my edits. I appreciate the use of the AWB to help edit some of the changes made by the other OSU student (London.34.osu.edu) and myself. While I had not looked in to the vitamin D deficiency angle, I fact checked this portion of the other student's addition and found the research to be quite interesting. I will continue to research and make minor edits to the page's format. I think there is some overlapping information within the sections, so I will go in to the page and try to remove repetitive information.

Addition to Lactase Persistence page
https://en.wikipedia.org/wiki/Lactase_persistence

The ability to digest lactose is not an evolutionary novelty in human populations. Nearly all mammals begin life with the ability to digest lactose. This trait is advantageous during the infant stage, because breast milk serves as the primary source for nutrition as weaning occurs, and other foods enter the diet, breast milk is no longer consumed. As a result, the ability to digest lactose no longer provides a distinct fitness advantage. This is evident in examining the mammalian lactase gene (LCT), which decreases in expression after the weaning stage, resulting in a lowered production of lactase enzymes. When these enzymes are produced in low quantities, lactose non-persistence (LNP) results.

The ability to digest fresh milk through adulthood is genetically coded for by LCT variants, which differed amongst populations. Individuals, who expressed lactase persistent phenotypes, would have had a significant advantage in nutritional acquisition. This is especially true for societies in which the domestication of milk producing animals, pastoralism, became a main way of life.

The combination of pastoralism and LP genes would have allowed individuals the advantage of niche construction, meaning, they would have had less competition for resources by deriving a secondary food source, milk. Milk as a nutrition source may have been more advantageous than meat, as it is renewable. Rather than having to raise and slaughter animals, one cow or goat could repeatedly serve as a resource with fewer time and energy constraints. The competitive advantage conferred on lactose tolerant individuals would have given rise to strong selective pressures for this genotype, especially in times of starvation and famine, which in turn gave rise to higher frequencies in lactase persistence within the populations.

In contrast, societies which did not engage in pastoral behaviors, no selective advantage exists for lactase persistence. Mutations which may have developed allelic variations which code for lactase production into adulthood are simply neutral mutations. They seemingly confer no fitness benefit to individuals. As a result, no selection has perpetuated the spread of these allelic variants, and the LP genotype and phenotype remains rare.

Final Paper: Got Milk?
Allelic variants within populations have long been the fascination of scientists far and wide, especially those interested in the study of evolutionary relationships. These variations in the genetic structure may be deleterious, neutral, or provide a fitness advantage to organisms. Over time, selective pressures may lead to the rapid increase of specific alleles within a population. The exploration of the genetic and selective factors which drive these frequency changes is an area of special interest, especially in regards to variations within the human species. The trait of lactase persistence amongst humans has long been a fascination for evolutionary scientists. Genetic evidence has shown it to be a polymorphic trait, which has arisen in several populations due to a combination of natural selection and favorable Mendelian inheritance patterns (Swallow 2003, Burger et. al 2006). Lactase persistence (LP) has emerged as a convergent evolution across different human subpopulations. However, the prevalence of this trait varies across the species, while some human populations exhibit high levels of lactase persistence, others exhibit little to none (Gerbault et. al 2011). The ability to digest lactose is not an evolutionary novelty in human populations. Nearly all mammals begin life with the ability to digest lactose, a milk based sugar. This trait is advantageous during the infant stage, because breast milk serves as the primary source for nutrition As weaning occurs, and other foods enter the diet, breast milk is no longer consumed. As a result, the ability to digest lactose no longer provides a distinct fitness advantage (Burger et. al 2006). This is evident in examining the mammalian lactase gene (LCT), which decreases in expression after the weaning stage, resulting in a lowered production of lactase enzymes (Burger et. al 2006, Gerbault et. al 2011). When these enzymes are produced in low quantities, lactose nonpersistence (LNP) results (Enattah, et. al 2007). Lactose nonpersistence may be referred to as lactase deficiency and can also result in lactose malabsorption; a state in which unprocessed lactose is moved to the colon where it is further broken down by bacteria, rather than lactase enzymes. Lactose intolerance is specifically considered a digestive stress reaction as a result of such (National Institute of Health 2014). Though lactose nonpersistence is considered the natural state for mammals, and thus, humans, there are populations in which lactose digestion is persistent in to adulthood. Specifically in humans, this persistence has been well documented in European and African groups, as well as some Middle Eastern populations (Gerbault, et al. 2009). The genetic basis for the persistence of lactase production in to adulthood is a result of allelic variants of the LCT gene, which appear to be mutations. There are several allelic mutations which have given rise to lactase persistence, making the trait polymorphic (Gerbault et al. 2009). The variants are passed on as dominant traits, which exhibit Mendellian inheritance patterns. When introduced to populations in which they may provide a fitness advantage, their frequency increases with each generation (Gerbault et al. 2009). These alleles represent the LP genotype, while the lactase persistence phenotype is expressed by the ability to digest milk sugars. This phenotype can be evidenced through lactasetests, which include: intestinal biopsies, measuring hydrogen levels in the breath. After individuals’ exposure blood glucose levels are examined (Swallow 2003). Lactase persistent alleles vary in their geographic distributions. Lactase persistence within European and descendent populations is almost entirely correlated with the presence of the 13,910 C/T mutation of the lactase gene (LCT). This differs from LP allelic distributions in East African and Middle Eastern, as well as Northern African populations. Amongst in East African and Middle Eastern groups, researchers have found the 13915 T/G mutation to be the most prominent allelic contributor to lactase persistence. In Northern Africa, the 14010 G/C allele variant is most closely correlated to the trait’s expression (Gerbault et al 2009). Though lactase persistence is prominent in each of these populations, the difference in genetic basis suggests convergent evolution. This indicates that there must have been a commonality amongst these groups which created selective pressures for these LCT variants (Bersaglieri, et. al 2004). This connection may be evidenced through the variation in LP frequencies amongst different geographic and cultural groups. Lactase persistence is highest in northern European populations and African populations for whom animal husbandry, or pastoralism, is the main way of life. In these groups, LP is seen at rates of around 90%. The prevalence of lactase persistence drops to about 50% amongst southern European, Spanish, French, and Middle Eastern populations. In Asian and African populations, in which livestock rearing is uncommon, frequencies of lactase persistence alleles reach as low as 1% and 5%, respectively (Tishkoff et al 2006) These frequencies indicate a correlation between cultural schema and genetic inheritance, which appears to support the major hypothesis proposed for this convergent adaptation: the frequency of lactase persistence has coevolved with cultural practices of the domestication of milk producing animals (Gerbault et al. 2007, Gerbault et al. 2011). This hypothesis suggests that as populations began to rear livestock for purposes of meat consumption, the use of milk as a secondary nutritional source was also discovered. The ability to digest fresh milk through adulthood is genetically coded for by LCT variants, which differed amongst populations. Individuals, who expressed lactase persistent phenotypes, would have had a significant advantage in nutritional acquisition (Enattah et al. 2003, Gerbault 2011). In a way, the combination of pastoralism and LP genes would have allowed these individuals the benefit of “niche construction,” meaning, they would have had less competition by deriving a “new” food source (Gerbault et al. 2011). In addition, milk as a nutrition source may have been more advantageous than meat, as it was renewable. Rather than having to raise and slaughter animals, one cow or goat could repeatedly serve as a resource with fewer time and energy constraints. The competitive advantage conferred on lactose tolerant individuals would have given rise to strong selective pressures for this genotype, especially in times of starvation and famine, which in turn gave rise to higher frequencies in lactase persistence within the populations. In contrast, societies which did not engage in pastoral behaviors, no selective advantage exists for lactase persistence. Mutations which may have developed allelic variations which code for lactase production into adulthood are simply neutral mutations. They seemingly confer no fitness benefit to individuals. As a result, no selection has perpetuated the spread of these allelic variants, and the LP genotype and phenotype remains rare (Swallow 2003, Tishkoff et al 2006). DNA evidence supporting this hypothesis in the case of Europeans was reported in 2006 by Burger, Kirchner, Bramanti, Haak, and Thomas. Their study focused on genetic testing of ancient skeletal remains. By replicating DNA extracted from human remains they secured the “genotypes from eight Neolithic skeletons of central, northeast, and southeast Europe ranging in age between 5800 and 5000 B.C” (Burger et al 2006). They compared these DNA sequences to that of a Mesolithic sample, which predated the samples, and a medieval skeleton, which postdated the samples. The sequences were all tested for allelic variations on the −13.910C/ T locus of the LCT gene, the known region for LP alleles amongst modern Europeans. Of the samples, only the medieval skeleton showed heterozygosity at this locus, indicating that the earlier DNA sequences would not have produced a lactase persistent phenotype (Burger et al 2006). These results, though they represent a small fraction of the populations, at the time, can were compared to the hypothesized timeline for pastoralism within European cultures. It is hypothesized that pastoralism was introduced between 8500 and 6500 years ago (Burger et al 2006, Gerbault et al 2011). When compared to the prevelance of lactase persistant alleles within these populations, this timeline supports the culturehistorical hypothesis, which suggests that LP increased in frequency as a result of higher levels of pastoralism (Gerbault et al. 2011). Studies of the distribution of the C14010 allele variant, which is highly correlated to LP within Northern African populations, have also shown correlation between the influx of pastoral practices and the increased frequency of the allele within the population (Tishkoff et al 2006). The development of lactase persistence in humans, is a result of convergent evolution amongst subpopulations. These groups, having been exposed to similar conditions, instituted by cultural construction, have repeatedly given rise to higher frequencies of mutant alleles which support lactose digestion through adulthood. Frequency differences amongst pastoral and nonpastoral groups, as well as ancestral DNA evidence, serve to support this hypothesis. The widespread of few alleles within large geographic regions may suggest that each allele rose to fixation and was passed through interbreeding between nearby subpopulations, with geographic and social barriers barring the intermingling of mutant alleles (Gerbault 2009, Tishkoff et al. 2006).