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Comment by pdog | Hacker News Reader

> Various ideas are being kicked around to explain why natural selection promoted milk-drinking, but evolutionary biologists are still puzzled. [...] Those who couldn't drink milk were apt to die before they could reproduce.

The success of the lactose tolerance mutation may be partly due to sexual selection[1]. It's been proposed that neoteny[2] is a key feature of human evolution. The ability to drink milk as an adult is a neotenous trait, and it may have been "accidentally" selected for when other beautiful features were sexually selected.

David Rothenberg's book, Survival of the Beautiful[3], argues that biologists are sometimes "blinded" by natural selection and ignore sexual selection.

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[1] - http://en.wikipedia.org/wiki/Sexual_selection

[2] - http://en.wikipedia.org/wiki/Neoteny

[3] - http://blogs.scientificamerican.com/thoughtomics/2012/10/25/...

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> The success of the lactose tolerance mutation may be partly due to sexual selection...The ability to drink milk as an adult is a neotenous trait, and it may have been "accidentally" selected for when other beautiful features were sexually selected.

Accidentally? Try getting laid when your GI tract is in rebellion. An accident my ass! (Or some ancestor's lactose-intolerant exploding one.)

When we see a trait that has no obvious fitness benefit, then it makes sense to ask if it might just be a consequence of genes that encode for other, more useful traits. But when a trait such as lactose tolerance has clear benefits for the organism there's no need to complicate the hypothesis without other evidence pointing that way.

I agree, but the mutation has a ridiculously high selection differential, one that probably can't be explained by a single factor.

How, specifically, can sexual selection operate in this case? It would seem necessary that the lactase-production gene, which the article mentions, must have some external effect that's peculiarly attractive. But I don't think it's possible to tell apart lactose-tolerant people by their appearance.

I think he's saying that the lactose gene would perhaps be somehow connected with other genes that might have more direct implications to sexual fitness. E.g. the lactose gene would somehow piggy-back other youth-continuing genetic traits.

Conversely lactose intolerance could also be unattractive. As a person who is mildly lactose intolerant I can confirm that if I were to drink a tall glass of whole milk I would be a very unattractive person to be around, at worst you'd simply pass out from the assault on your sense of smell.

Side note: a possible attractive quality of lactose tolerance is an increase in calcium intake which can have multiple positive effects on the overal health of the individual including some outward indicators, stronger teeth for example.

Sexual selection would operate on a different (neotenous) trait, and the lactase production gene would come along for the ride.

Genes and organsisms are complex beasts, so traits will often be clustered together. An interesting related experiment is one performed on foxes, where they were bred for 'tameness' and gained a whole raft of other dog-like traits: http://www.freerepublic.com/focus/news/807641/posts

But to prove that you need to show that there exists a neotenous trait that improves sexual selection.

Then you need to prove that the lactate gene is in linkage disequilibrium with that gene (thus capable of being "coming along for the ride".

Given the impact of a genetic mutation that improves metabolism, which has its own evolutionary benefits (improved ability to access energy) there seems little reason to invoke this third trait.

We're not really talking about 'proving' things, particularly on an internet forum, but you could start with a relative lack of facial and body hair, larger heads and bigger eyes, as well as more curiosity, less aggression and a more plastic brain. That do?

Just appearing young is an evolutionary advantageous trait if men pursue what they perceive as younger women to breed with (for increased fertility, less chance of stillbirth and defects, etc. - they wouldn't have to know this, the ones that like young looking ones would just have to reproduce more successfully and the trait of looking for it would be bred in without men actually knowing that preferring young women is advantageous in terms of reproduction).

Milk consumption makes males taller and broader [1]. So the sheilas want to breed with the tall milk-fed hottie and not his short, weedy and flatulent lactose-intolerant friend.

[1] http://www.annualreviews.org/doi/abs/10.1146/annurev.nutr.26...

This is incorrect in its context. While milk consumption does aid growth, lactose intolerance in most cases develops during late adolescence and early adulthood. By that time, most of the growth in height has stopped.

Taking your account as fact (I have no data), this does not entirely refute nikatwork. It may be that in past generations lactose intolerance generally developed earlier than it does now. Today's later intolerance developers are the stubborn residue in the gene pool after the tolerance gene became dominant amongst the children of earlier intolerance developers. The assumption that intolerance was not switched off, but slid back to ever later years could be tested in families of multiple living generations whose intolerance is in flux.

You're going to need a citation for "lactose intolerance in most cases develops during late adolescence and early adulthood".

According to this indirect citation [1], lactose intolerance varies by age and race, eg 85% of Chinese are lactose intolerant by age 10.

If a significant percentage of a genetic group develops lactose intolerance by age 10, then yes - it would most definitely affect growth.

[1] http://boards.straightdope.com/sdmb/showthread.php?t=485313

Citation: http://en.wikipedia.org/wiki/Lactose_intolerance#Terminology

Relevant quote - emphasis mine: "Primary lactase deficiency is genetic, only affects adults and is caused by the absence of a lactase persistence allele.[9][10] It is the most common cause of lactose intolerance as a majority of the world's population lacks these alleles.[11]"

edit: Also, I can't believe you linked to some forum called "The Straight Dope" as your citation.

Did you even bother to follow the link? Here, I extracted the reference for you [1].

"Chinese and Japanese populations typically lose between 80 and 90 percent of their ability to digest lactose within three to four years of weaning." (Swagerty et al, 2002)

Ergo, lactose intolerance is a factor in children in some genetic groups. Your citation (sourced from the same wikipedia page) does not refute that.

[1] http://en.wikipedia.org/w/index.php?title=Lactose_intoleranc...

Briefly, one or a few mutations are responsible for both the child-like traits that are selected for sexually as well as other child-like traits such as lactose tolerance. So selecting for the former also results in the latter, because the traits are genetically linked.

Typically neoteny is achieved by mutations that alter developmental timing, and it's easy to imagine that such a mutation could also result in sustained lactase production past the "normal" age.

As for the question of why all lactose-tolerant people aren't visually identifiable, keep in mind that there may not be just one variant of the lactose-tolerance mutation, and perhaps not every variant has complete penetrance. For example, perhaps one gene variant confers a 50% probability of lactose tolerance, but always confers a particular attractive trait.

Milk drinkers smell different than non.

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