[help] Question about human evolution

Is 25-50 generations sufficient elapsed time for a human population under significant selection pressure to exhibit significant phenotypical variations?

I’m assuming something like a lost colony scenario, and/or multigenerational space farers. I’m also not looking for 10 foot tall green men or anything like that, but more along the lines of meaningful changes in height, build, coloration, body mass, musculature etc. such that current extreme expressions might become more common or even typical.

13 thoughts on “[help] Question about human evolution

  1. Fellshot says:

    I would say that 25 generations is easily enough for there to be both phenotype and genotype variations. What really needs to be taken into consideration is how big the population is (inbreeding risks), how high the mortality rates are for those with less desirable genes and how the community socially adapts to protect its surviving members (medical technology does have an effect on who survives to reproductive age).

    You might want to throw in a few random mutations (because they do happen) that are found to be useful and thus passed on the next generation as well.

    In a low gravity environment, your subjects might have a lower bone mass and a smaller heart and a lower pressure circulatory system since those systems are designed to deal with a specific force acting on them and that would change with prolonged exposure to such an environment.

    A common thin air adaptation on Earth is an larger ribcage and lungs to get the most oxygen one can.

    Overall general appearance would vary according to the genotypes of your starting group. If most of your colonists have medium skin, dark to mid brown hair and a mix between brown eyes and green eyes, one could infer that that particular color palette will most likely stay the same. If the group is more varied (some have Zulu features some have Han Chinese features and still others have Polynesian features) then over several generations (assuming society doesn’t mind intermarrying between different phenotypes) most likely the later generations would have features that look like a blend from all three different groups.

  2. Postulate series of recessive genes.
    Postulate an unusual level of unrecognized/non-lethal radiation.

    Add the stress of things like high light/low light, extreme cold/unusual heat, etc.

    Those elements, if carefully placed in the story structure, and you could justify some expressions of changed physical structure for humans.

    Good luck.

  3. Are there additional environmental pressures present? A situation with a high mortality rate would certainly accelerate normal rates of change.

  4. C.E. Petit says:

    I suspect that it will, in the end, depend at least as much on how heterogeneous the initial gene pool is as the amount of time that passes. As an example, consider the lactose-intolerance problem in historically homogeneous southwestern China, compared to the lactose-worship (in the form of cheeses and lassis) of the more heterogeneous populations less than 200km farther southwest.

    The key point is that the broader the selection of nonmutation-requiring recessive traits in the initial population, the more rapidly any environmental factors will begin to select for advantageous pheno- and genotypes… that is, alter the apparent phenotype of the population, with all of the usual unanticipated side effects (consider how selection for broader, flatter molars required by a higher-cellulose diet will begin to influence facial expressions and lingual characteristics).

  5. Yes. Current research indicates that in humans phenotype changes can occur in less than 20 generations. Specifically in the study case, changes in base melanin levels were set that quickly (ie. how quickly white people became white).

    It would really depend on the change (if it was in the realm of existing variability), the selection pressures, and other mutation factors. So, within the basic possible patterns (height, color, distribution, etc)? Yes. If you’re thinking of extra arms, or new processes and systems, probably not.

  6. Greg Morrow says:

    Anti-malarial traits have evolved several times since the exodus from Africa, and similarly melanin levels have gone up and down rapidly as migrants move into and out of tropical zones.

    An anti-black death trait (that coincidentally helps protect against HIV) has reached significant levels in northern European populations in under a millennium (so < 50 generations).

  7. The result depends on the initial diversity of the gene pool and the degree of selective pressure (i.e. how many people die or otherwise fail to reproduce).

    As long as there’s some initial range, and what you’re selecting against is a substantial (but not overwhelming) fraction of that range, then you can see selection pressure right away. If, for example, you started killing or sterilizing anyone who grew to more than 5’8″, you’d probably start seeing shorter stature in just a couple of generations. Set the cut-off at 5’4″ and you’d get an even strong effect, although you run the risk of killing off your whole population.

    Charlie’s lactose-tolerance example is a good one. Alcohol metabolization is another: There was probably considerable diversity among humans as to how well they could metabolize alcohol. In Europe, where there was extensive use of fermentation to make safe beverages, there was no doubt considerable selective pressure to metabolize alcohol effectively. In Asia, where tea was the more common safe beverage, there was much less pressure. The observed differences in the ability to tolerate alcohol between Europeans and Asians probably result from this differential pressure.

  8. Rick York says:

    Given all of the comments above, wouldn’t the number and strength of mutagens be equally important? The ability of these mutagens to interact with the human genome would be an important factor in any change in phenotype or, for that matter, genotype.

  9. In fact, mutagens probably don’t matter.

    I base that on the fact that we don’t see much spread of phenotype, even though people live in areas with widely different levels of mutagens.

    For example, solar and cosmic radiation is much higher at very high altitudes. And yet, although there are some adaptations to altitude, they’re really quite subtle—nothing that you’re going to notice without a blood test (or a brisk hike uphill).

    Similarly, there are plenty of places with high levels of chemicals that are probably mutagenic occurring naturally in the soil and water. But none of those places stand out as places where the people are somehow “different” from other people.

    The fact is, mutagenic changes largely have no effect at all—and the ones that do have an effect are overwhelming more likely to be harmful than beneficial. So, preexisting variation (together with the effects of mixing and matching such variation) ends up being much more significant.

  10. Lise says:

    Sexual selection is going to be a huge part of how much variation you get and how fast.

    If there’s a good reason for people to pick partners based on traits they did not in the past you should have strong variation relatively quickly.

    For an example check out the silver fox domestication project. http://cbsu.tc.cornell.edu/ccgr/behaviour/Index.htm You should be able to find more information but that was the first good site I pulled up. They had significant changes in very few generations.

  11. Shlomi says:

    Equivocation: the lubrication of writers.

    Some phenotypes will change between a generation or two: race blends, especially over 10-25 generations, will gradually even out, q.v. above answers.

    Some phenotypes are environmental (see others for zero-gravity presentation of circulatory system).

    Just like there are natural height limitations in an expressed racial type (see studies on maximum heights given optimal nutrition amongst Scandinavian peoples), similar limits exist, without mutation, around circulatory and other issues.

    Genotypes are trickier, owing to natural selection and social choices. Will lower body odor make for a better mate? How about lowest oxygen consumption (efficient lung action)? Darwin’s premise is that evolution might start with a mutation, but it’s got to be helpful to be spread. And given how we have socialized logical evolution to counter it (see # of folks with glasses, bad genes breeding), you’ll need to come up with environmental AND social conditions that will predicate the ‘evolution’ of your space-based folks.

    Yes, I’m always this helpful. 🙂

  12. John Gibbons says:

    To expand on a key point in Fellshot’s original comment (about community attitudes and medical technology): for modern much less star-faring humans, a huge factor will be what level of medical technology they have and how they choose to use it. I see 3 major scenarios. One is that the “lost colony” has also lost most of their technoly, living on a perhaps barely habitible world. Most of the comments above clearly apply to that scenario. If the colony has retained their technology level, the environmental pressure would have to be pretty severe to force genetic drift in the face of conscious and determined effort to prevent it. Such pressures are of course possible, equivalent to the black death / malaria adaptation, although severe allergens might be more plausible; not clear if any of these would yield interesting phenotype changes. Hard to believe, though, that environmental factors would be sufficient to drive (for instance) melanin levels on a technically sophisticated population. I haven’t seen mention of the third scenario, however: consciously managed genetic selection / manipulation to accomodate environmental factors. Even without any gene-tweaking, it would be possible for a technical population to select for traits among the gene pool of a particular couple, cf. Heinlein’s _Beyond This Horizon_. Active genetic manipulation might become socially acceptable very quickly in response to severe enough pressures…

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