Genetic Drift for dummies

How, when and where does drift happen?

I don’t know much about the Kahn Academy but their Wikipedia entry suggests they don’t have any political or religious axe to grind. Here is their description of genetic drift:

Genetic drift is change in allele frequencies in a population from generation to generation that occurs due to chance events. To be more exact, genetic drift is change due to “sampling error” in selecting the alleles for the next generation from the gene pool of the current generation. Although genetic drift happens in populations of all sizes, its effects tend to be stronger in small populations.

So drift happens all the time but its effects are more noticeable in small populations and two ways that populations become small have their own names: bottleneck effect and founder effect. Bottlenecks occur when some catastrophe reduces a population to a small number of individuals, a partial extinction. The result is that individuals carrying alleles (alternative copies of genes) occurring at low frequency may be eliminated altogether or so reduced in number that those alleles are lost for good in the next generation or two and thus genetic diversity is lost. Founder effect is where a small sample of a large population (or the extreme example of a single individual) becomes permanently isolated from the main population, such as by being transported to a new environment devoid of the main population. Again, alleles present at a low level can be lost completely in the isolated small population.

I don’t think there is controversy that genetic drift is a real phenomenon. What I have had difficulty in seeing is how genetic drift contributes to evolution as a whole. Being, by definition, a random process and one that, in small populations, reduces diversity, I think I’m correct that drift does not contribute to selective adaptation. But perhaps, where small subsets of populations go through a bottleneck, it creates a consequent springboard for subsequent adaptation in the surviving population.

But I place myself in the dummy category regarding drift and appreciate any corrections as necessary in my text above. This isn’t the first time I’ve expressed skepticism as to the importance that drift should be accorded so my apologies for being a slow learner and I look forward to input from those more informed than me.

I should credit Berkeley University University of California, Berkeley for the diagram above.

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165 thoughts on “Genetic Drift for dummies

  1. Allan Miller:
    Alan Fox, What I was trying to emphasise is that drift takes longer to fix an allele than selection, and so more variants can be sustained under a drift scenario than under selection.

    I’m re-confused. I thought the idea was the tendency to fixation of one allele (and loss of others) by drift raised the visibility to later selection. Less variants but more visibility.

    If the ancestral sequence AAAAAA was advantageous, the comparative speed of selection means that it becomes fixed as an ancestor more rapidly. ‘Fixed as an ancestor’ meaning that the entire population descends from that sequence at that locus, even if they vary in detail – a handful of AAAAAB’s, AAAAAC’s etc (assuming they are equally as fit as AAAAAA) may be found; AAAAAA has not yet been fixed as a sequence.
    But with drift’s more leisurely progress, by the time AAAAAA has become ‘fixed as an ancestor’, it has had time to spawn more variants than in the ‘selective sweep’ scenario. Descendants of AAAAAA will be ‘more polymorphic’.

    I’m missing something. In the absence of selection, mutations can tick over without being eliminated by purifying selection. What does drift add to the mix?

    Therefore drift has the capacity to increase a population’s standing variation over a selection-only scenario.

    Not sure what I can say here. I’ll sleep on it and read your comment again.

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  2. Alan Fox: I’m re-confused. I thought the idea was the tendency to fixation of one allele (and loss of others) by drift raised the visibility to later selection. Less variants but more visibility.

    Nah. Forget fixation! Drift can make rare alleles less rare, giving selection a clearer look at them (e.g. a recessive allele, which selection can’t see at all until it meets itself in a homozygote).

    I’m missing something. In the absence of selection, mutations can tick over without being eliminated by purifying selection. What does drift add to the mix?

    OK, having said ‘forget about fixation’… 🤣

    It’s not about purifying selection, but about positive selection – a beneficial allele arising and then being swept to fixation by its advantage compared to other alleles. This happens more quickly than if the allele were neutral, meaning there is less time for mutation, so less variation in the selected case.

    Of course mutations mean that the original allele isn’t strictly ‘fixed’, since the locus would be polymorphic. Nonetheless, there is a point at which all loci in the population are direct template descendants of an original copy, even if certain instances are mutated versions of that progenitor sequence.

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  3. Corneel,

    You saying that someone else claiming they can differentiate between neutral and positively selected alleles doesn’t make it so for crying out loud.

    Oh, I have discovered a statistical way to prove God exists. Hey, God exists!

    But back to your claim first, how about the eyebrows, can you plug it into their formula for me?

    Zebra strips-would it be convenient to plug that one in as well, a lot of people have wondered…

    They should have a website with a free calculator on it (get a good atheist sponsor to pay for it) where you can type in the feature you want and it will go zip, zip zip and spit out the answer-positive, negative, neutral feature. That would be cool! Maybe they could even convert people subtly with this little moneymaker.

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  4. phoodoo: Maybe they could even convert people subtly with this little moneymaker.

    How much were you paid for your conversion into a collaborator?

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  5. Alan Fox: Who took the photo?

    I don’t know, it seems impossible to track down the original source it’s been used so many times. I am aware however that drone footage has been taken of the camps without the permission of the government.

    Based on this: https://www.theguardian.com/world/2019/jan/11/if-you-enter-a-camp-you-never-come-out-inside-chinas-war-on-islam

    I would hazard a guess this group, if not the government itself for whatever reason.

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  6. phoodoo:
    Corneel,

    They should have a website with a free calculator on it (get a good atheist sponsor to pay for it) where you can type in the feature you want and it will go zip, zip zip and spit out the answer-positive, negative, neutral feature.That would be cool!Maybe they could even convert people subtly with this little moneymaker.

    How could it compete with the stunning success of the Explanatory Filter? What’s the Specified Complexity of an eyebrow? You can’t see me, but one of mine is currently slightly higher than the other.

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  7. phoodoo: You saying that someone else claiming they can differentiate between neutral and positively selected alleles doesn’t make it so for crying out loud.

    True, but then you’d better give some reasons for why you think the existing methods fail, rather than denying that those methods even exist. The former would be more persuasive (and fun) than tedious repetition of outright denial.

    phoodoo: But back to your claim first, how about the eyebrows, can you plug it into their formula for me?

    No, because the McDonald–Kreitman test works on sequence information, not phenotypes. But let me ask you about something peculiar. When did you, someone who is constantly marveling at the wondrous complexity of the eye, suddenly start taking an interest in eyebrows?

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  8. Corneel,

    When did you, someone who is constantly marveling at the wondrous complexity of the eye, suddenly start taking an interest in eyebrows?

    Eyebrows are the pelmet of the soul.

    +2

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