Lactose intolerance is a common gastrointestinal complaint in adults

Lactose intolerance is a common gastrointestinal complaint in adults

Lactose intolerance is a common gastrointestinal complaint in adults. As children are weaned, the gene for lactase, an intestinal enzyme that breaks down lactose, the main sugar in milk into digestible sugars disappears which gives rise to gastrointestinal symptoms. As a result, the majority of adult human find lactose indigestible and avoid drinking milk. However, present findings from Sarah Tishkoff’s study suggests that in parts of Africa there is a positive selection for lactase persistence that evolved and spreads rapidly. Lactase persistence that was discovered illustrates the effects of evolution through mutation and natural selection that was previously thought to have proceeded gradually. These findings are important to the field of human evolutionary biology as it offers a new conceptual understanding to the process of evolution.
In an early analysis, milk consumption in adult humans is widely seen in pastoralist or semi-pastoralist groups such as the Dinka, Maasai, Zulu, Xhosa, and Swazi, that rely on herding for survival. Later, anthropologist noticed that individuals that raise cattle and whose ancestors had herded tended to be the same ones who could drink milk as adults (Check, 2006). This exclusivity of milk drinking found in only some human population illustrates the effect of a genetic generational transmission that traces back to the living conditions during their ancestral times in which animals’ milk was valuable. As a result, lactase persistence that arises as an adaptive trait in the population was maintained and passed down successively as it continued to be well suited to the population’s domestic lifestyle. Through each successive generation, evolutionary roles maintain this variation that was present in early ancestors and parents allowing for a subset of the adult human population to continue being able to tolerate milk (Caillaud, 2018).
The genetic changes that allow for milk tolerance in adult humans can be measured through the Hardy-Weinberg principle. If random mating occurs in a large population with no disruptive circumstances, the principle predicts that both genotype and allele frequencies will remain constant because they are in equilibrium (Boyd and Silk, 2015). This equilibrium can be disturbed by the occurrence of a mutation that pulls the balance in another direction through the introduction of new alleles into a population. As a result, this changes the genotype and allele frequency in which certain gene variants become prevalent that caused lactase production within and between generations (Caillaud, 2018). However, the Hardy-Weinberg equilibrium is quite robust and it demonstrates that variation is maintained indefinitely with random mating in a large population (Caillaud, 2018). Subsequently, genetic variation that allows for milk tolerance in adult humans will be inherited through successive generations. This was evident in the gene sequencing done during Tishkoff’s study which showed the mutations seen in the Tanzanians had evolved recently spread rapidly (Check, 2006).
These genetic changes throughout the evolutionary history of milk tolerance in human population illustrate that there is a trade-off between survival and reproduction that is strongly biased against characteristics that prolong life, which leads to the favoring of mutated alleles that increase individual fitness (Boyd and Silk, 2015). This persistence of the lactase mutation is positively selected as it’s nutritional value would have been advantageous to individuals with the lactase mutation in times of famine when those benefiting from the mutation would have been able to drink without the risk of diarrhea, which exacerbates dehydration (Check, 2006). Additionally, the cost of the mutation is less severe than compared to the production of sickle cells to protect against malaria. These genetic changes provide evidence that evolution is quick to adaptation in increasing species fitness in a population.
On the contrary, if a percentage of the population who could tolerate milk stopped drinking milk due to the increased availability of other foods then it would be predicted to have a decrease in the appearance of the mutation for lactase persistence gene. Having alternative food sources indicates that the absence of milk was not vital to the sustenance of life. Based on the principle of Hardy-Weinberg, natural selection would cause changes in gene frequencies in organisms so that they are better adapted to their current environment (Boyd and Silk, 2015). As the demand for lactase persistence decrease, selection may track these fluctuations and lower gene frequencies in the population as they are not heavily reliant on milk. As a result, the decrease in milk consumption would affect the gene frequencies of the population to eliminate certain alleles that do not contribute to the survival and reproductive success of the organisms that carry them.
Neandertals that lived in Europe and western Asia between about 200,000 and 40,000 years ago, subsisting exclusively on hunted and gathered foods would not be able to tolerate milk as adults. Their foraging culture that depends primarily on wild food for subsistence allows them to consume a variety of foods to reap the nutritional benefits. Natural selection or mutation that generates new alleles and changes genotype frequencies to adapt to milk consumption would not be seen as it would not contribute to the survival of the population. Rather, a balancing selection that maintains genetic variation by favoring multiple alleles at a locus would appear (Caillaud, 2018). By favoring multiple alleles, Neandertals gain other characteristics that increase benefits their foraging lifestyle such as height and agility that allows them to run faster.
The development of lactase persistence that is seen in parts of African illustrates a fascinating case of genetic diversity and the importance of evolution. Through evolutionary roles such as genetic mutations and natural selection that favors individual fitness in a population, more adult human would develop tolerance to milk and reap its nutritional benefits. These discoveries are significant to the human evolutionary biology as it provides a better understanding to the origin and evolutionary history in adult human lactase persistence. Additionally, it can assist other fields of biological study in providing future advances that contribute to the relief of gastrointestinal complaint in adults.


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