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Comment by anthonyb | Hacker News Reader

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

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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).

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