How is lactase an example of fitness in humans?

Evolutionary advantages. Lactase persistence is a text-book example of natural selection in humans: it has been reported to present stronger selection pressure than any other known human gene. However, the specific reasons as to why lactase persistence confers a selective advantage "remain open to speculation".

Lactase breaks down the lactose in food so your body can absorb it. People who are lactose intolerant have unpleasant symptoms after eating or drinking milk or milk products. These symptoms include bloating, diarrhea, and gas. Lactose intolerance is not the same thing as having a food allergy to milk.

In pastoralist populations, individuals with the lactase-persistence mutations were more likely to survive, reproduce, and pass on their alleles than people without the mutations, especially in times of famine.

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