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My biggest points are lower muscle mass because of the lower g (gravity) in an artificial habitat and fragile, less dense bones. They would also get sunburns quicker, because most of the time they are being covered from regular sunlight by gigantic shields, thus relying on supplemented vitamine D. These humans would be physically handicapped in an Earth like planet setting, although more trained, because they'd have a strict training rigour to keep themselves relatively healthy.

Is this idea justified by what we already know about space? Is there a way to make it more realistic? What other side effects can you think of? And please give me links to all of the sources you're using.

Mushrooom
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    (1) Allow me to warn against trying to create fiction that absolutely reflects Real Life. We know next to squat about the universe. We're regularly surprised by what we find. You're limiting your ability to dream with that kind of restriction. (2) Noticeable physical changes are a regular trope of SciFi (Orion slave girls come to mind…). (3) You already have an answer to your question. You're asking us for permission to use the answer. That's not what we do. (4) Don't be worried that your readers might judge your creation "unrealistic." Most aren't qualified judges anyway. – JBH Jul 11 '22 at 21:58
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    Upvote for freeing the Orion slave girls. – Willk Jul 11 '22 at 22:22
  • This won’t be physically obvious to look at, but your space dwellers would probably have poor vision. https://www.google.com/amp/s/scitechdaily.com/zero-gravity-causes-flattening-of-the-eyeball-new-high-tech-sleeping-bag-could-solve-vision-issues-in-space/amp/ – Neutralmouse01 Jul 11 '22 at 22:23
  • I'll try to be more precise. I'm not talking about weightlessness. It's around 60% g. The main issue is that whole generations are bred like that. Some of them haven't even set a foot on a home planet. – Mushrooom Jul 11 '22 at 22:38
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    Pleas [edit] this to ask one and only one question. Currently you're asking 3 "is it justified?" , "is it realistic?" and "What else can you think of?". – sphennings Jul 12 '22 at 02:36
  • (a) Exactly how much time are we talking about? Be specific. "Generations" isn't specific. "250 years..." that's specific. No significant observable changes would occur in mere decades. Possibly not even in mere centuries. (b) Further, exactly how many people are we talking about at the beginning of the adventure? Tens? Hundreds? Thousands? The more there are, the longer it will take for changes to become hereditary. (c) See What could a self-sustaining lunar colony slowly lose that would ultimately prove fatal? – JBH Jul 12 '22 at 02:47
  • @JBH generations is far more relevant and 'specific' for the purpose of anything to do with evolutionary changes than the number of years, and also eliminates any potential misunderstandings caused by the asker using a non standard lifespan or age of reproductive maturity .. given years you have to work out the generations that equals for any answer and may require other details, gven generations you don't .. maybe the asker isn't being clear elsewhere but that example of yours of the asker's lack of clarity is wrong. – Pelinore Jul 12 '22 at 10:41
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    @Pelinore "Generations" can refer to two generations or 2,000. I've personally seen a "generation" defined as 20 years (genealogists' favorite number, when you can get them to stop arguing about it) and 100. *It's anything but specific,* But I would have been content if the specific number of generations had been mentioned-but that wasn't given, either. (How long can that space station remain in operation, anyway?) So VTC for needing details. – JBH Jul 12 '22 at 13:58
  • @JBH Yes very much the point, you do not need to know how many years .. the number of years per generation is *entirely* irrelevant to most questions and answers in this area and any recipient of such an answer can do the very simple multiplication math needed for themselves to arrive at a year count if they want or need to .. plus answers in generations can often be applied to other species or a significant variants of one .. for evolution *years are not relevant* generations *are*. – Pelinore Jul 12 '22 at 15:27
  • "They would also get sunburns quicker" because apparently only white folk made it into space? – Starfish Prime Jul 13 '22 at 12:35

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For changes in generation 1 there are observed changes in real astronauts you can read about.

For changes in generation 2 then there is exogenetics where some genes can turn on or off if it is beneficial.

For later generations you would expect the changes to slow down because exogentics I am told is not cumulative. Once you turn a gene off you cannot turn it off a second time.

One reason the space dwellers might not change too much after generation 1 is a lack of natural selection. Yes inability to walk under gravity is not selected for. But it is not selected against either.

Other genetic changes due to life experiences are called Lamarckism. I believe no one has bothered to disprove Lamarckism since we invented the DNA theory of genetics. So MAYBE your colonists can look different after a thousand years. Who knows?

Daron
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