Lactase persistence and natural selection

Here is an interesting post on John Hawks’s blog: Selection and lactase persistence.

After reading an article by John Hawks in Scientific American (No, Humans Have Not Stopped Evolving), a reader reached out to him, asking for clarification about a point that left him puzzled: why would a genetic mutation that allows lactase persistence after weaning age (meaning the lactase enzyme keeps being produced in adults so that they can still efficiently digest the lactose in milk) stick around in a population when it (apparently) does not confer a direct advantage in survival/reproduction?

In other words: it’s easy to conceive that a genetic mutation allowing individuals to be more resistant to malaria will help them survive and reproduce, therefore passing on the beneficial allele to their offspring and allowing that allele to expand in the population ; by contrast, being unable to digest lactose past infancy/childhood will certainly makes an individual uncomfortable when consuming milk, but as it is not life-threatening, how does that affect one’s survival and reproduction chances?

For a short and easy to understand answer, it’s here: Selection and lactase persistence.

Spoiler: it has something to do with nutrition, energy availability, and its effect on reproductive function.

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One thought on “Lactase persistence and natural selection

  1. Jack Cameron February 27, 2015 / 9:08 pm

    The mutations which enable adults to drink milk are under the strongest selection of any in the human genome which suggests that the ability to digest lactose confers a direct biological advantage in survival/reproduction. Archaeologists and geneticists have been puzzling over what biological advantage triggered the ability to digest milk beyond early childhood but have yet to come up with a widely accepted theory.

    Many theories have been proposed regarding the biological advantage conferred by the ability to digest lactose in adulthood but to date none have been widely accepted. John Hawks hypothesized that the biological advantage conferred by lactase persistence was an increase in available calories of about 50% that resulted because fermentation of yogurt and cheese was no longer necessary to make the dairy digestible. His theory assumes that the calories of lactose disappear when lactose is fermented which is simply not true. In fermentation of yogurt and cheese, lactose is broken down to form lactic acid and glucose and no significant reduction of calories occurs. The caloric content of yogurt is practically the same as the milk from which it is made and only part of the lactose content is degraded. The caloric content of cheese is reduced by the removal of whey, but in a society where calories are valuable the whey would either be consumed by humans or fed to animals, not wasted. Note that the whey can be fermented.

    Many lines of evidence from recent studies suggest that the biological advantage that stimulated lactase persistence was not provided by non-fermented milk but from lactose fermented yogurt and cheese. Fermented dairy provides many health benefits, but the primary advantage is the content of a form of vitamin K2 (MK-9) that is more effective than the MK-4 from animal fats in activating the vitamin K dependent proteins that are essential for reducing risk of heart disease, cancer and diabetes All lactose fermenting bacteria utilize vitamin K2 as an electron carrier in their respiratory system so they must synthesize vitamin K2 to survive. The “long chain” vitamin K2 subtype MK-9 synthesized by fermented dairy is more lipid soluble and more readily absorbed than “short chain MK-4 from animal fats. It has been found that the “long chain” MK-9 from dairy fermentation is a better suppliers of MK-4 than MK-4 itself. (PMID 23140417) Thus fermented dairy provided a superior form of vitamin K2 which confers a very significant biological advantage which may have brought about lactase persistence.

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