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Thread: Genetic Drift

  1. #1 Genetic Drift 
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    Don't know that much about the concept so I'm looking for opinions and info.

    I'm guessing that the most common occurrence is when an extreme selection pressure is removed, as when something blunders into a totally new environment. Without the constant selection pressure, extreme features drift back to a less extreme state.

    Like if the cheetah became extinct, the Thompson's Gazelle would probably "drift" back to a slower version.
    Or fish with extremely venomous spines might lose them, if they colonised a lake with no large predatory fish.

    Are there any confirmed examples of drift, apart from this kind of thing?


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    WYSIWYG Moderator marnixR's Avatar
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    i think you misunderstand the concept of genetic drift - it's the change in the genetic make-up of a population free of selection pressure
    e.g. the founder effect whereby a small number of individuals populate an island, whose genetic make-up is different from that of the main population where they came from

    any breeding population that splits in two can start drifting apart through random differential mortality, with the surviving populations becoming different from one another over time


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    It's such a sweeping statement, "free of selection pressure".
    Surely there is always some selection pressure in nature.

    If a population split, and the selection pressures were all still there for both populations, but were identical, wouldn't the differences that arose then be down to drift?
    In which case, it would't be a lack of selection pressure, just a lack of difference in the selection pressures?

    Edit : I can see now that in my example of the cheetah and gazelle, this would mean that it's not drift but a drastic change in the balance of selection pressures that would bring about a slower gazelle.
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    WYSIWYG Moderator marnixR's Avatar
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    Quote Originally Posted by mistermack View Post
    It's such a sweeping statement, "free of selection pressure".
    Surely there is always some selection pressure in nature.
    i'm sure you're right that sometimes we haven't yet figured out how natural selection has operated to produce certain changes, but it is equally true that random changes in a population's genetic make-up can become fixed, purely from the available variation in that population - since no 2 populations are identical in genetic variation, there's one possible point of divergence that doesn't require natural selection to start (although obviously subsequent selection can make the divergence permanent)
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    At its most basic (i.e. the statistical definition) Genetic Drift is that change in allele frequencies that is not distinguishable from random (i.e. noise exceeds signal (or signal does not exceed noise)).

    On the other hand, the general tendency for genomes to accumulate variation through mutation is referred to as drift, with a small 'd'. Two populations that do not exchange genetic material much, or at all, will drift apart - will accumulate variations. This tendency is used for the molecular clock process that puts a date on a supposed past divergence event.

    Genetic Drift, with a capital 'G' and a capital 'D', is any change in allele frequency (not just through mutation) that does not present with a noticable signal (and therefore cannot be distinguished from random).
    Nearly all of the above lines of evidence can be questioned, and all have more than one possible cause (although some may have no cause at all).
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    Note that Genetic Drift does not say that there is no selection. It only says that in a particular observation of change in allele frequency no signficant selection could be defined.
    Nearly all of the above lines of evidence can be questioned, and all have more than one possible cause (although some may have no cause at all).
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