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I posted a question some time ago about how could genetic engineering design people that could adapt to changing conditions beyond what simple acclimatization or activation/deactivation of genes in a fixed genome could enable, and I don't think it sprouted a really satisfying answer but I ended up thinking about a possible solution recently that I want to share and see if it makes any sense at all.

So, hear me out, what if, hypothetically, it happens to exist genetic variability among the cells of a single individual (kinda like chymeras), and if for example, the skin cells get exposed to more sunlinght than the body can handle through acclimatization or activation/deactivation of genes alone, the cells that reproduce before dying get naturally selected in an increasingly competitive environment, so a person with a very ligh skin can become a person with very dark skin after some time.

This is the most basic example that I can think of, and I am not that good at biology or anything like that so please don't get too hard on me, I just want to know if that could work, and if not, how could it work then, thank you.

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    $\begingroup$ A non-hypothetical consequence of this type of activity is the various forms of cancer. $\endgroup$ Commented Nov 14, 2022 at 22:46
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    $\begingroup$ The cells on the outside of the skin are already dead. You cannot operate natural selection on dead cells. And anyway, the cells which make up a healthy multicellular organism do not "compete"; their multiplication is regulated for the benefit of the multicellular organism as a whole. When some of the cells of a multicellular organism become competitive that is called cancer. $\endgroup$
    – AlexP
    Commented Nov 14, 2022 at 22:49

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You have invented T cell selection.

  1. T cells scramble the genes for their T cell receptor. Here are your genetically variable cells in a single individual.

  2. T cells are pruned back if their novel receptor does not work or reacts to self antigens.

  3. A T cell clone is amplified if there is an antigen corresponding to that cell's receptor, suggesting a pathogen which that T cell variety can attack.

  4. Each individual contains a T cell repertoire it has evolved itself according to its own internal conditions.

Marvel, reader! Genetically variable cells that through natural selection achieves evolution in a single individual

It is very cool stuff, T cell biology. Here is a little whiff from wikipeda.

https://en.wikipedia.org/wiki/T_cell#Origin,_early_development_and_migration_to_the_thymus

TCR development Main article: T-cell receptor A critical step in T cell maturation is making a functional T cell receptor (TCR). Each mature T cell will ultimately contain a unique TCR that reacts to a random pattern, allowing the immune system to recognize many different types of pathogens. This process is essential in developing immunity to threats that the immune system has not encountered before, since due to random variation there will always be at least one TCR to match any new pathogen.

A thymocyte can only become an active T cell when it survives the process of developing a functional TCR. The TCR consists of two major components, the alpha and beta chains. These both contain random elements designed to produce a wide variety of different TCRs, but due to this huge variety they must be tested to make sure they work at all. First, the thymocytes attempt to create a functional beta chain, testing it against a 'mock' alpha chain. Then they attempt to create a functional alpha chain. Once a working TCR has been produced, the cells then must test if their TCR will identify threats correctly, and to do this it is required to recognize the body’s major histocompatibility complex (MHC) in a process known as positive selection. The thymocyte must also ensure that it does not react adversely to "self" antigens, called negative selection. If both positive and negative selection are successful, the TCR becomes fully operational and the thymocyte becomes a T cell.

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