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I have little knowledge of evolution theory, can you please help me understand this? How can one force evolution by invoking a fitness cost? Wouldn't that actu

by warent 6y ago

I have little knowledge of evolution theory, can you please help me understand this? How can one force evolution by invoking a fitness cost? Wouldn't that actually just drive extinction? I would have thought that if producing lactase when consuming milk has a cost, then over time, milk-consuming populations would suffer and disappear from lower fitness, and the ones who aren't drinking milk would survive and win the genetic race.

TinkersW 6y ago
You are forgetting that drinking milk was beneficial(lots of extra calories for example), this obviously outweighed the downside of lactose intolerance.
- asymptosis 6y ago
  Yes. And it didn't have to be an all-or-nothing thing. It's also plausible that cow milk was used for babies who lost their mother. So to create an evolutionarily significant effect, it was sufficient to simply select for babies who could keep drinking cow's milk for longer and longer, on average.
  - thaumasiotes 6y ago
    You might be interested to know that there are -- broadly -- three lactase production strategies that humans exhibit. Everyone produces lactase as an infant, for obvious reasons. Some people cease producing lactase at some point in childhood, so that they are unable to digest lactose as adults. Some people continue producing lactase throughout their lives, so that they are always able to digest lactose. But some people continue producing lactase as long as they maintain regular dietary lactose intake, even if that circumstance lasts well into adulthood. However, if they ever go for an extended period without consuming lactose, they permanently cease lactase production.
enkid 6y ago
The fitness of the group drinking the milk was higher than group not drinking the milk (even with an intolerance). After drinking started, the individuals with a tolerance had higher fitness than the group without. This is super simplified.
SamBam 6y ago
The fitness "cost" isn't the "net cost." Pretty much everything our bodies do has a cost. Our organs are all consuming a tremendous amount of energy. Our guts harbor bacteria that can absolutely overwhelm the body if they get out. It takes us more than a decade of growth to reach sexual maturity. You can only determine the value by taking into account the benefit. Populations that could consume milk, an easy and plentiful supply of calories and calcium, did better off than some of their local neighbors who didn't.
thaumasiotes 6y ago
> I would have thought that if producing lactase when consuming milk has a cost, then over time, milk-consuming populations would suffer and disappear from lower fitness, and the ones who aren't drinking milk would survive and win the genetic race. Your focus is oddly misplaced. There is a cost of producing lactase, regardless of whether you do anything with it. This cost indeed caused those who continued to produce lactase after weaning to suffer and disappear from lower fitness. That is why, from a world perspective, almost nobody today can digest lactose in adulthood. If you drink milk as an adult, you still have to pay the normal cost of producing lactase. But you get the benefit of being able to metabolize the lactose you're already consuming. This makes the production of lactase profitable. Note that this relies on you drinking milk. If adults are not drinking milk, the ordinary dynamic of lactase production being a waste of energy that lowers your fitness will prevail.

by warent 6y ago

TinkersW 6y ago

You are forgetting that drinking milk was beneficial(lots of extra calories for example), this obviously outweighed the downside of lactose intolerance.

asymptosis 6y ago
Yes. And it didn't have to be an all-or-nothing thing. It's also plausible that cow milk was used for babies who lost their mother. So to create an evolutionarily significant effect, it was sufficient to simply select for babies who could keep drinking cow's milk for longer and longer, on average.
- thaumasiotes 6y ago
  You might be interested to know that there are -- broadly -- three lactase production strategies that humans exhibit. Everyone produces lactase as an infant, for obvious reasons. Some people cease producing lactase at some point in childhood, so that they are unable to digest lactose as adults. Some people continue producing lactase throughout their lives, so that they are always able to digest lactose. But some people continue producing lactase as long as they maintain regular dietary lactose intake, even if that circumstance lasts well into adulthood. However, if they ever go for an extended period without consuming lactose, they permanently cease lactase production.

enkid 6y ago

The fitness of the group drinking the milk was higher than group not drinking the milk (even with an intolerance). After drinking started, the individuals with a tolerance had higher fitness than the group without. This is super simplified.

SamBam 6y ago

The fitness "cost" isn't the "net cost." Pretty much everything our bodies do has a cost. Our organs are all consuming a tremendous amount of energy. Our guts harbor bacteria that can absolutely overwhelm the body if they get out. It takes us more than a decade of growth to reach sexual maturity. You can only determine the value by taking into account the benefit. Populations that could consume milk, an easy and plentiful supply of calories and calcium, did better off than some of their local neighbors who didn't.

thaumasiotes 6y ago

> I would have thought that if producing lactase when consuming milk has a cost, then over time, milk-consuming populations would suffer and disappear from lower fitness, and the ones who aren't drinking milk would survive and win the genetic race. Your focus is oddly misplaced. There is a cost of producing lactase, regardless of whether you do anything with it. This cost indeed caused those who continued to produce lactase after weaning to suffer and disappear from lower fitness. That is why, from a world perspective, almost nobody today can digest lactose in adulthood. If you drink milk as an adult, you still have to pay the normal cost of producing lactase. But you get the benefit of being able to metabolize the lactose you're already consuming. This makes the production of lactase profitable. Note that this relies on you drinking milk. If adults are not drinking milk, the ordinary dynamic of lactase production being a waste of energy that lowers your fitness will prevail.