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Evolution of small populations

  1. Nov 17, 2004 #1
    Hey guys

    I have some small questions on evolution. Or what would be the driving factor, or whatever.

    Take a population of individuals, that say lived in the mountains in high altitude for thousands of years, and they tend to be shorter with more barrel-shaped chests so that they are efficient at the high altitude.

    What would be the driving force to select towards that body type? It wouldn't be that a person that didn't look like that wouldn't survive. Would it? I mean people can move to high altitude areas and be fine.

    Or what would cause the change in a persons skin color? I know that it happened to individuals in small groups that were reproductively isolated, but how would the skin color change so much?

    Thank you :)
     
  2. jcsd
  3. Nov 17, 2004 #2

    Phobos

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    General description –
    A population has variations within its gene pool (e.g., some tall & thin, some short & stout). The environment its in helps determine which variation becomes the majority that is expressed in the population. When a fringe group becomes isolated, it’s different environment may bring a different variation that is already present within the main gene pool to the forefront. That is part of evolution (adaptation). Another part is the development of new features. Note that in a fringe environment, the ecosystem may be very different (since it is, by definition, at the fringe of what the main population’s preferred habitat is). That change may accelerate the selection process or may be a stressor that accelerates mutation rates or amplifies the differential success of recombined genes (mixing of genes into new variations via sexual recombination). Also note that the genetic statistics of the small fringe group may be different than the main group (thus giving a certain variation an advantage from the start). In the vast main population, new mutations/recombinations may become diluted out. But in a smaller population, they have a better chance of gaining a foothold to spread in the gene pool. Eventually, as the fringe variation succeeds and spreads, it may once again overlap with the original population. It could merge peacefully, in which case, the new variations/mutations/recombinations would be added to the main population (perhaps being diluted to insignificance). Or it could clash with the main population and enter into more direct competition for the similarly-desired resources (food, shelter, etc.). I’m not sure of the current status of evidence on this, but past studies have indicated that in such a situation, the overlapping populations will diverge more than they would if they were kept separate due to the direct competition (which I suppose leads to more specialization or adaptation into other areas so they don’t clash so much).

    I’d have to check into the literature for explaining specific instances of evolution like skin color, etc. But consider sending a tall/skinny/pale/mostly-vegetarian southern Californian to northern Alaska. That person may survive, but he/she may have a harder time of it than the native shorter/stouter/meat-eating folks that are better adapted for that environment. He/she may dedicate more time trying to stay warm and finding preferred foods than simply going with the flow. The visitor could survive and maybe even reproduce, but odds are, would have fewer kids than the native peoples.
     
  4. Nov 17, 2004 #3
    To answer it very short:
    The driving force for the increase of some feature would be that individuals with that feature produce more offspring (that will also have that feature) than the others who do not have that feature. Over many generations everyone will have this feature. It depends on the environment which feature is beneficial.
     
  5. Nov 18, 2004 #4
    Natural variation within a species provides the raw material for evolution. Natural selection ensures that only the best adapted will reproduce leading to constant change and improvement, this ultimately leads to the formation of new species.
     
  6. Nov 18, 2004 #5
    Hey Phobos

    Thank you for that reply, that is the kind of info I was looking for!
     
  7. Nov 21, 2004 #6
    What would happen if natural selection selected FOR barrel chested people, but sexual selection simultaneously selected AGAINST barrel chested people (because theire ugly?)

    What would be the stronger selection agent? I'm guessing sexual selection.

    K.
     
  8. Nov 21, 2004 #7

    Phobos

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    Not sure. It probably depends on the situation. But certainly there are many examples of where sexual selection wins out over natural selection (e.g., peacock's beautiful, but cumbersome tail).
     
  9. Nov 21, 2004 #8

    Moonbear

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    Sexual selection and natural selection are not two different things. Sexual selection is a type of natural selection pressure. Typically, sexual selection mirrors other measures of fitness. For example, that beautiful peacock's tail suggests that male must be incredibly adept at evading predators in order to be able to display such a large, noticeable tail and still avoid being eaten, and that he isn't infested with parasites that would lead to a raggedy tail. If only barrel-chested people survive, you'll grow up around more of them than people with other body shapes, and will probably begin to associate that body shape with attractiveness.
     
  10. Nov 22, 2004 #9

    Phobos

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    FWIW, I agree that sexual selection, artificial selection, etc. are subsets of natural selection. But sometimes it's easier to discuss them separately.
     
  11. Nov 26, 2004 #10
    I think that in some cases the environment doesn't select the shape of the individuals, but rather the individuals "select" the environment they live according to the shape they already have. I think that could be the case with barrel-shaped persons living in high altitudes... but... this pattern really exists? I've read something recently about human adaptations to higher altitudes, but I don't remember of being related with the lung volume, what I remember was about Andeans having more hemoglobin and Tibetans breathing quicker than people non-natural of higher altitudes.

    But for the skin color I don't know... seems obvious that high melanin is adaptive in sunny zones, selecting negatively individuals with lower melanin, but in the other hand seems perfectly fine to people with high melanin survive in other environments, and at least judging by common sense, the higher melanin concentrations doesn't seems to be a tremendous loss of energy/resources for development, causing to the lack of melanin alone to have selective advantage. I guess that could be that other adaptive traits were linked, hitchhiking, or something, with lower melanin concentrations (or at least that was the case with some founder populations), or they aren't linked but the lower melanin concentrations really save energy to develop these adaptive traits for the new climate. The adaptive traits I can think that could have been "switched" by melanin are more bodily and facial hair...
     
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