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A comment on The heritability of fertility makes world population stabilization unlikely. Parent comment

Re:The heritability of fertility makes world population stab... by barry-cotter on Friday July 05, 2019 @05:31AM (#20361098)
If a trait is not strongly selected for but variation exists it will be maintained in the population. See height, hair colour, propensity to gain weight, etc.

Then you change the environment so that the trait is under strong selection. The people living in Scandinavia 3,000 years ago couldn’t digest lactose. Now ethnic Scandinavians who are lactose intolerant are basically nonexistent. It’s at above 99% homozygotes.

Imagine taking a population and killing everyone over 18 under 1.8m and doing this for four generations. Assuming anyone was left after the first generation the fourth generation will be substantially taller than the first.

Desire for children was never under strong selection pressure before because desire to have sex led to children pretty reliably up until the birth of the demographic transition in France in the ~1700s. Now it is under selection pressure, everywhere with reliable access to contraception so the people who like children are going to have more children, and their share of the population will grow.

> Large families simply happen due to poverty. When poor, you want more children because there's strength in numbers. Plus the women are staying home and can afford to have children.

This will be news to the Mormons, Amish, Hutterites and the Hasidim among a number of Orthodox sects of Jews. Large families do not just happen due to poverty. Some people want more children and arrange their lives around that.

[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by bin0 on Friday July 05, 2019 @08:18AM (#20361931)
> Large families do not just happen due to poverty

You're right, they happen due to culture. However, the cultures you listed are notably insular and traditional, meaning they do not have large families for the same reason much of the world does (high infant mortality). I doubt these will ever change, but it is very possible the rest of the world will.

[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by barry-cotter on Friday July 05, 2019 @08:23AM (#20361962)
Though I’m sure the Mormons would cop to traditionalism calling a religion where the median man spent two years of his life in a foreign country insular is slightly odd, considering most Americans never leave their own country. Even aside from that cosmopolitanism Mormons have been state governors and been contenders for the presidency. Insular they are not.
[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by pharke on Friday July 05, 2019 @08:46AM (#20362101)
Insular was probably the wrong term to use, culturally rigid or conservative might be better. The point is they maintain a high fidelity to the cultural and behavioural patterns of their group. One of which is to eschew birth control and purposefully create large families.
[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by jjeaff on Friday July 05, 2019 @03:44PM (#20365259)
Mormon's do have and encourage large families. But there is no modern prohibition or even suggestion regarding birth control.
[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by knicholes on Friday July 05, 2019 @10:08AM (#20362603)
They're commanded to "multiply and replenish the Earth."
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Re:The heritability of fertility makes world population stab... by pas on Friday July 05, 2019 @05:53AM (#20361206)
> It’s at above 99% homozygotes.

Meanin 99% of people who have this allele (trait) have two set of the corresponding gene/genes.

Okay, but is lactose tolerance a dominant allele?

[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by qualmer on Friday July 05, 2019 @06:37AM (#20361429)
Yes, lactose tolerance means you have functioning enzyme. That makes it a dominant allele.
[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by lotsofpulp on Friday July 05, 2019 @10:11AM (#20362621)
That's not how it works.

https://en.wikipedia.org/wiki/Dominance_(genetics)

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Re:The heritability of fertility makes world population stab... by maratd on Friday July 05, 2019 @08:15AM (#20361918)
If it was advantageous for us to have lots of children, then women would have a dozen at a time, rather than one. There are plenty of mammals that do that. There’s clearly a tradeoff there and the selective pressures for humans tend to lean toward fewer. Not sure where we’ll end up, but time will tell.
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Re:The heritability of fertility makes world population stab... by bin0 on Friday July 05, 2019 @08:19AM (#20361940)
I think there was such a selection pressure when we were nomadic hunter-gatherers. Agriculture is a very recent development anthropologically, so we were still in the mode of one-kid-per-pregnancy, though multiple might have been advantageous. It's possible we developed higher fertility due to that, but we didn't have enough time for that new pressure to kick in.
[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by barry-cotter on Friday July 05, 2019 @08:29AM (#20362005)
It’s been 5,000 years since the dawn of agriculture. If multiple births were fertility increasing they’d be, if not normal, at least common in some groups. You don’t even get 1 in 3. Single births are clearly not being found wanting by the Blind Idiot God.
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Re:The heritability of fertility makes world population stab... by bin0 on Friday July 05, 2019 @08:50AM (#20362117)
Not necessarily. Mortality of mothers is higher in multiple births; maybe that's a counter-pressure (fewer children). Children do worse as well ([0], [1], [2]). Ladies are not built to carry more than one child, especially considering the size of our children, and agriculture has been such a blip on the evolutionary/anthropological timescale that we have not had time to evolve so significantly as to change our basic reproduction.

[0]: https://watermark.silverchair.com/151663.pdf?token=AQECAHi20...

[1] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5541191/

[2] https://bmjopen.bmj.com/content/4/4/e004514

[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by malvosenior on Friday July 05, 2019 @12:10PM (#20363599)
The size of the human brain makes this physically impossible.
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Re:The heritability of fertility makes world population stab... by maratd on Friday July 05, 2019 @04:26PM (#20365584)
Ok, explain triplets. https://en.wikipedia.org/wiki/Multiple_birth
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Re:The heritability of fertility makes world population stab... by malvosenior on Friday July 05, 2019 @06:58PM (#20366569)
Three and twelve are very different from each other. Even three is very rare and often comes with complications.
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Re:The heritability of fertility makes world population stab... by option_greek on Friday July 05, 2019 @11:59AM (#20363514)
Minor correction: Advantageous to the children (conferring evolutionary advantage). Not to us.
[ Parent | on HN ]
Re:The heritability of fertility makes world population stab... by maratd on Friday July 05, 2019 @04:13PM (#20365481)
I'm not sure I understand your distinction. From a gene-based perspective, the gene, whether in the child or in the parent, is the same. The selection pressure is on a gene level, not on a species or individual level.
[ Parent | on HN ]