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I'm developing a story with 4 distinct group of humans who inhabit 4 geographically separated and ecologically distinct islands (Iceland to Greenland sized). These groups descend from a single diverse group of people on a terraforming expedition who were forced to settle on the planet and were separated in the process. Assume that the original groups were large enough to avoid inbreeding or genetic drift. Due to circumstances on the planet, the groups were totally separated for roughly 10-15,000 years.

My question is: how much phenotypic change is plausible over that period? How far could the differences be pushed?

I was thinking about traits like blue eyes, which are possibly as recent as 10,000 years, but which now dominate some parts of the globe (89% of Finland, for instance).

Some considerations:

  • could there be an "island effect" of some kind, accelerating rates of phenotypic change, especially if the environment on each island is vastly different?
  • Are certain kinds of changes more likely than others? For instance, changes in hair,skin, eye color vs more physically involved changes like height, limb length, etc.? For the latter, I was thinking about some physiological changes among different populations that have been argued to serve an evolutionary purpose, such as nose shape.
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    $\begingroup$ If you Google up "out of Africa" and do some reading you should be able to quickly find several features that diverged in the last 60K years. Some are known to be more recent, some have less precise data. $\endgroup$
    – Boba Fit
    Commented Mar 21, 2023 at 19:29
  • $\begingroup$ More details are needed-what are the ecological differences? Phenotypic change will arise from selection pressures, which would range from almost nothing(Only sexual selection) to "Big"(E.g. increased melanin production in higher sun areas) $\endgroup$ Commented Mar 23, 2023 at 16:43

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If there was strong or especially intentional selection (for instance, on the basis of tall or short) due to, for instance, one island having abundant fruit that's never close to the ground while another has decided that small partners are preferred over tall ones, to the point the taller folk aren't permitted to breed, one could see significant changes in a heritable characteristic like height over that kind of time frame -- perhaps even less time than that.

They key is that the selection needs to be for a heritable characteristic (male pattern baldness, blue eyes, polydactyly, etc.) and needs to very strongly affect the ability of carriers vs. non-carriers to breed.

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