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So, in my world (or more rather the region in which my story takes place) when the nuclear war happened the majority of people survived in small, survivalist compounds, remote wilderness shelters, and underground bunkers. All in all, 20,000 people survived the war.

I wanted to use the Founder’s Effect, that genetic thing, to make the majority of people in the surviving communities colorblind. My question is, can the founders Effect make this happen?

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    $\begingroup$ It would be a good idea to tell us why you think that it could happen, along with a link to some articles on this effect. Frankly, right now my suspension of disbelief isn't strong enough for this. The idea that there are multiple communities that display this is hard to believe. $\endgroup$ – NomadMaker Aug 2 at 22:18
  • $\begingroup$ @NomadMaker: I just wanted it to happen in one particularly large community, and then for that communities people to slowly spread it out $\endgroup$ – DT Cooper Aug 2 at 22:29
  • $\begingroup$ That makes sense, but it would probably not have effected other communities, though they may have other genetic problems. $\endgroup$ – NomadMaker Aug 2 at 22:30
  • $\begingroup$ @NomadMaker: What do you mean? $\endgroup$ – DT Cooper Aug 2 at 22:31
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    $\begingroup$ @JBH: Even if we invoke magic to destroy everyone's cones (and damage by light is not specific to the receptors for that frequency) that effect would not carry over into the next generation. $\endgroup$ – Beta Aug 3 at 12:52
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Yes. This happened on a small Pacific island with a population bottleneck after a typhoon.

https://en.wikipedia.org/wiki/Pingelap

Complete achromatopsia is normally a very rare condition, and its prevalence on the island has been traced back to a population bottleneck in 1775 after a catastrophic typhoon swept through the island, leaving only about 20 survivors. One of these, Doahkaesa Mwanenihsed (the ruler at that time), is now believed to have been a carrier for the underlying genetic condition, but the achromatopsia disorder did not appear until the fourth generation after the typhoon, by which time 2.7% of the Pingelapese were affected. Since achromatopsia is an autosomal recessive disorder, inbreeding between the descendants of Doahkaesa Mwanenised would result in an increased recessive allele frequency.[8] By generation six, the incidence rose to approximately 4.9%,[7] due to the founder effect and inbreeding, with all achromats on the island nowadays tracing their ancestry to Doahkaesa Mwanenihsed.

Today the atoll is still of particular interest to geneticists; due to the small gene pool and rapid population growth, the disorder is now prevalent in almost 10% of the population...

The same thing can happen in your post apocalyptic world. After a population crash (bottleneck) the founder effect leads to this and possibly other genes being represented at a markedly higher frequency. If I recall, you considered having reduced fertility among survivors. If there were a man back in the day who had a reputation as a baby maker, most or all of the survivors might trace their ancestry to him.


Comments by @Mary is right. 20K is too many for the founder effect. I thought that 20K was the population rebounded some generations after the apocalypse and bottleneck. But you could have the founder effect responsible for colorblindness going into the apocalypse. Maybe there is a Doomsday cult that started with a charismatic leader back in the 1800s. Many of the current cult members are descended from that leader and his many wives. It turns out their preparation served them well, because the cult represents a large proportion of those surviving the apocalypse.

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    $\begingroup$ Twenty thousand people is waaaay too many for something like this to happen. $\endgroup$ – AlexP Aug 3 at 10:49
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    $\begingroup$ As revealed in the documentary on Pingelap by Oliver Sacks, achromatopes have amazing night vision. In a carefully arranged post-apocalyptic world, this could be strongly selected for. $\endgroup$ – Beta Aug 3 at 13:06
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    $\begingroup$ @Beta If the trait is selected for, it's not the Founder Effect that's responsible for the increase in prevalence. You might get a head start if a population bottleneck happens to exhibit the selected-for quality, but it won't change the population characteristics in the end. If colorblindness is naturally selected for, you'll see that trait increase in prevalence regardless of any bottleneck. There's a few ways to have the trait increase in prevalence, but a Founder Effect with 20k individuals won't do it. $\endgroup$ – Nuclear Wang Aug 3 at 14:49
  • $\begingroup$ @NuclearWang: True, which is why I made it a mere Comment. After all, the correct answer to the question is "No, because..." $\endgroup$ – Beta Aug 3 at 15:44
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Twenty thousand people is an enormously large population to have a big Founder Effect. To produce this you would need:

  • Some reason why this population is disproportionately color-blind, or if females carriers of it
  • Some reason why it is selected for and not against.

One possibility would be that for some reason, ability to see through camouflage is immensely important; colorblindness helps with that.

Still, the sort of flukes that would get it started are much more likely to occur in maybe one or two small communities, and its spreading would turn on other communities be wiped out for the lack.

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    $\begingroup$ You do not need natural selection for a founder effect. Usually the founder effect is invoked to explain prevalence of some gene with no apparent selective benefit, or even a detriment to survival (e.g. Huntingtons) $\endgroup$ – Willk Aug 2 at 22:39
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    $\begingroup$ When you have a population of twenty thousand, you do. $\endgroup$ – Mary Aug 2 at 22:39
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    $\begingroup$ I don't understand what population size has to do with the founder effect. If you lose genetic diversity to a bottleneck and have limited # of founders reconstitute the population, what does it matter what eventual population size you achieve? $\endgroup$ – Willk Aug 3 at 2:34
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    $\begingroup$ Yes. The population size at the time of the founder event, not the 20,000 size of the re-expanded population after the population bottleneck at the time of the apocalypse. Elephant seals are instructive in this regard. There are hundreds of thousands now, genetically very similar to one another after almost going extinct in the 1800s. There were only a few dozen founders of todays population. . $\endgroup$ – Willk Aug 3 at 3:11
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    $\begingroup$ Except that "20,000 people survived the war." -- that was the bottleneck. $\endgroup$ – Mary Aug 3 at 3:39
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Your problem for limiting genetic diversity is that you have too many "breeding pairs" to get any significant genetic diversity. You need some way to limit that. The obvious way is to limit which men breed., or that you simply (for whatever reason) don't have that many men.

19,999 women, one man

Your survivors were a girl guide jamboree. The vast majority naturally were female, with one or two male site assistants. I have trouble imagining another context for the survivors being overwhelmingly female though, but this is kind of vaguely possible.

Sterilising effect on male population

Your survivors were fairly evenly gender-balanced, but the effects of near-extinction killed male fertility. After 10 years of no pregnancies, the doctor screened sperm samples for all men, and found only one guy with properly viable sperm. He became the sperm donor for refounding the world. They could easily manage artificial insemination to increase reproduction - that's low tech with a turkey baster - but they wouldn't have the ability to gene-edit to fix anything wrong with his chromosomes.

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    $\begingroup$ You could certainly make some arguments that "Most of the men were on the front-lines and were vaporised". Perhaps colour blindness could even be a reason some men were kept away from the front lines, and bump up the chances of it being passed on. $\endgroup$ – DBS Aug 3 at 11:01
  • $\begingroup$ @DBS I like the idea that colour-blindness would be a reason to not be on the front line; although in practise it doesn't really affect your abilities as an infantryman. More practically though, even in wars of existential survival like Germany's invasion of the USSR and then the USSR's retaliation into Germany, there were substantial numbers of men still back home. Many would be disabled, injured or whatever, but they'd still be fertile. For sure there'd be a large gender imbalance, but to get such a significant effect as the OP has described seems hard to achieve. $\endgroup$ – Graham Aug 3 at 12:43
  • $\begingroup$ @Graham " I like the idea that colour-blindness would be a reason to not be on the front line; although in practise it doesn't really affect your abilities as an infantryman. " Colorblindness affects one's ability to be an infantryman enough that the US military will disqualify people with red/green colorblindness from every combat MOS. The people at military entry processing will test for this several times and failure means being shuffled off into something like clerical, legal, medical, or other noncombat positions. If someone wants to join the Army and play a bugle then they don't care $\endgroup$ – MacGuffin Aug 4 at 19:56
  • $\begingroup$ @MacGuffin I guess that's important in more recent times, with things being a lot more technical. It doesn't seem to have always been the case though, nor to have been screened for. See bbc.co.uk/history/ww2peopleswar/stories/99/a2107199.shtml And there's a fairly well-known ability of red/green colourblind people to see through camouflage better, which is a positive benefit. $\endgroup$ – Graham Aug 4 at 22:12
  • $\begingroup$ @Graham " And there's a fairly well-known ability of red/green colourblind people to see through camouflage better, which is a positive benefit. " It's this ability to defeat some forms of camouflage is speculated on why color blindness is as prevalent as it is in males, as it can be helpful when hunting for meat. In modern warfare this benefit is overwhelmed by dealing with color coded signals and controls on weapons. Modern optics will allow for some compensation for red/green colorblindness but that's just something to get lost or break in battle. $\endgroup$ – MacGuffin Aug 4 at 23:04
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I suggest going with the founder effect, as described in Willk's answer, combined with a political effect.

At first the colour blindness is just present in the leader and their family. They tend to get more children than other people making the gene more common.

Later anybody who claim to see these "colours" will be executed as this is an implication that the First Family is less then perfect.

Eventually the few remaining people who are not colour blind have learned to shut up about it.

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  • $\begingroup$ A political / social effect is a great idea. $\endgroup$ – Fattie Aug 3 at 11:34
  • $\begingroup$ Colour-sighted people who grow up in a society that doesn't talk about colour may not even be aware that they see something different from what others see. We now know that there are human tetrachromats, people with a rare, recessive gene for a fourth type of cone; some of them can perceive colours that most human beings can't, and they aren't aware of this until scientists tell them. $\endgroup$ – Beta Aug 3 at 13:01
  • $\begingroup$ Uh, actually, according to the Wikipedia article you linked to, it could be up to 50% of human females who are tetrachromats? (that seems like a lot... but stranger things have happened, when it comes to how poorly understood female biology seems to be) So only a rare condition if we forget about half of the human population (otherwise, it seems remarkably common!) ... I guess women truly ARE invisible (and have long learned to shut up about their lived experience) $\endgroup$ – WanderingClown Aug 3 at 13:53
  • $\begingroup$ Actually, true human tetrachromancy is extremely rare. In fact despite decades of searching only one has every been positively identified, and indeed, she is the only proof that they exist at all. (This is also documented in the Wikipedia article) $\endgroup$ – RBarryYoung Aug 3 at 16:08
  • $\begingroup$ @WanderingClown: I can't tell whether you're serious, but either way I'm going to save your comment as a beautiful example of the rule that feminist language can turn anything into oppression of women. $\endgroup$ – Beta Aug 3 at 20:52
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Color vision is so vital to human survival that it would be difficult to explain this with only genetics. Red-green colorblindness is sex linked so it may be possible to explain a prevalence of red-green colorblindness after a war by explaining that men with good color vision went off to war and died, leaving noncombat military and those unfit for service behind to have children.

Blue-yellow colorblindness is rare as there are redundant genes for this, it would take quite a bit more effort to remove the blue receptor gene from the gene pool.

Blue-yellow colorblindness can be acquired from overexposure to UV light. Even cheap sunglasses will have a UV protective coating, and many kinds of glass (like the thick glass used on high rise buildings and in car windshields) naturally filter out UV. In a society that has had scarcity caused by war there may be people using lighting that produces a lot of UV, and people in dimly lit places will not be wearing sunglasses. This can have a genetic component in that some people have more natural protection against UV than others. Lights that produce UV are fluorescent lights, arc lamps, and various gas/vapor lamps. Mass produced lights of this kind will have UV filters but those improvised after a war might not.

Diseases like diabetes can cause colorblindness if not managed well. This has a genetic component as people can present inherited diabetes at a young age, or acquire it from a combination of bad diet and genetics for poor metabolism over time.

Vitamin A deficiency can cause colorblindness if allowed to last long enough. This is unfortunately quite common with children receiving poor diets. This can have a genetic component as some people absorb and process vitamin A more readily.

Some drugs can cause colorblindness. In a war torn society it's easy to choose a near certainty of color blindness over near certainty of death from some disease. A common tuberculosis treatment was known to cause colorblindness as an example.

I'd "stack" the causes of colorblindness to explain it's prevalence. Bad diet, bad lighting, and less than desirable treatments for disease leaves everyone colorblind before they become an adult. Add in genetics from many healthier people in the population going off to die in war makes them more vulnerable to such causes.

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A different kind of founder effect - disease:

There are a multitude of diseases in the world, and viruses can cause some pretty unique ones. In conditions with poor healthcare, a minor virus no one ever noticed can become a huge public health problem. Polio was a significant virus mostly because changes in health standards meant people started catching it during a time in life where its side effects were most severe.

If ONE of your survivors had a virus that, in poor health conditions and sanitation, could infect the majority of your survivors and circulate in the population, you have a vector to cause any disability you want. Let's say your virus infected small infants during eye development and caused a defect in cones. No one noticed the virus before because it was rare, and good healthcare meant children rarely caught it. In an adult, it would be like a cold. Well-meaning survivors giving vital supplies to fellow survivors unwittingly have caused a new epidemic.

Your population wouldn't probably even understand why so many people were colorblind - the disease might not even cause any symptoms detectable in a baby. The great thing about this is that you can have it be as mild or severe as you want - some people might only have partial effects, others miss only certain colors, and a few rare people would have destroyed foveas and serious vision problems. The details are up to you, and wouldn't need to run in families (although they could, there can be sensitivities, or the disease can be hard to transmit without close contact).

The virus could also be neurological, affecting the parts of the brain that interpret color. For this, the virus could even affect adults - one day you're fine, then you catch a cold, and a week later you have red-green color blindness.

If you want to be poetic, make it a prion passed in tears - the sorrows of your survivors caused its spread.

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