Random Genetic Drift: a controversy?

Over my time as a dilettante observer of the science blogging community, I have noticed a certain frisson of controversy over the idea of random genetic drift. Sewall Wright, who with Ronald Fisher and J. B. S. Haldane (Bill Bryson’s observations on Haldane’s research into diving and decompression are entertaining) established the science of population genetics, is credited with coining the phrase in 1929. Thanks to Professor Joe Felsenstein for pointing out his seminal paper.

Will Provine has written a much admired biography of Sewall Wright, Sewall Wright and Evolutionary Biology, and yet has expressed doubt about the significance of genetic drift, culminating in his recent work, The “Random Genetic Drift” Fallacy, available as an E book here. This has caused a storm of comment on the internet. At the ID-friendly blog currently run by lawyer Barry Arrington, there was a portentous post by “News” which generated immense interest and four comments that I failed to notice and it was only when Larry Moran criticized Provine’s book recently at The Sandwalk that I became aware of its existence.

In ignorance of Provine’s doubts, I’ve recently expressed my own inability to grasp the significance of the effect for evolution. Allan Miller has been very gentle and patient with me and OM’s computer simulation was most helpful. In the commments, Joe Felsenstein links to his 1971 paper which should give the mathematically inclined food for thought. It just made my head spin!

So I’m hoping that someone can gently take me by the hand and lead me to the promised land where the effects of random genetic drift are clear to all. I have purchased Provine’s book. At less than three dollars and only 180 pages, why would I not?

172 thoughts on “Random Genetic Drift: a controversy?

  1. Alan:

    I’ve already said this…Again, I’ve already said that…I’ve already said that.

    Saying things isn’t enough. You need to understand them and draw out their implications. Allan and I are explaining how this works, but we can’t simply reach in and implant the ideas in your brain. You’ve got to do some thinking.

  2. Allan Miller:

    But I’m also not quite following this either “or a recombinational event generates a beneficial chimera that might not arise without it”. Could Allan or anyone help me out here?

    Given drift, alleles with selection coefficient below the beneficial threshold (effectively neutral or worse) can increase in the population for a period before disappearing.

    That is where I’m struggling. I thought the process of drift was one of fixation of one allele at the expense of others (unbiased selection).

    I’m sure Joe could provide the equations – though they would probably be of the ‘efficient mixing’ kind, but the idea would be the same.

    It would be great if he felt it worth the effort though I’m a bit math-challenged!

    While such alleles are around, they may recombine, and hit upon a beneficial variant. This is an important factor missed in Behe’s Plasmodium argument. If the double mutant ‘needs to’ occur, his calculations had the two mutations A and B only occurring in the same lineage. But in fact A and B subpopulations can arise through drift, even if A and B are detrimental, and AB arise by recombination. So you need to add the probability that AB will occur in that way, and because each added member of A has a chance with each member of B (and vice versa) these probabilities escalate significantly as the subpopulations grow.

    Ah, so drift fixes mutation 1 in population A and drift fixes mutation 2 in population B. Subsequent recombination and selection et voilà. So the point about sub-populations is that these sub-populations can fix different neutral alleles that can later recombine via out-breeding to produce a new advantageous allele set.

  3. Allan Miller:

    Yet the sub-population gets less diverse, unless sufficient new variation replaces the losses. What rôle is drift playing here?

    Because it causes allele progress to be more leisurely than selection, it allows alleles to stick around longer than selection would. Fancifiully, it’s chucking novel variants up before the court of selection, and saying “how about this one? No? OK, what about …”.

    Are you saying where mutations arrive quicker than drift can fix them, there is more opportunity for selection to find adaptively advantageous allele sets?

    Or trying a visualisation of a mental map, if all was selection, this would colour the entire population red. But drift plus mutation adds pockets of varying shades of pink, constantly shifting in and out unless one of them gets grabbed by selection and promoted to fixation by that.

    Subpopulations get less diverse, but the overall population is more varied, with drift. I wish I had the time (and the skillz!) to code a visualisation.

    Are you saying that sub-populations, though getting less diverse through drift, get less diverse differently so the overall effect is increased diversity?

  4. Alan Fox,

    That is where I’m struggling. I thought the process of drift was one of fixation of one allele at the expense of others (unbiased selection).

    Not just fixation, is the important point. In fact fixation is the hard part. But your club can get a few dozen members, no trouble! It’s the ‘law of small numbers’ at work.

    It would be great if he felt it worth the effort though I’m a bit math-challenged!

    You’re not the only one!

    Ah, so drift fixes mutation 1 in population A and drift fixes mutation 2 in population B. Subsequent recombination and selection et voilà. So the point about sub-populations is that these sub-populations can fix different neutral alleles that can later recombine via out-breeding to produce a new advantageous allele set.

    That’s not the only point about sub-populations, but as far as recombination is concerned that’s it.

  5. Allan Miller: Not just fixation, is the important point. In fact fixation is the hard part. But your club can get a few dozen members, no trouble! It’s the ‘law of small numbers’ at work.

    Just as I though I was getting it, you hit me with this! So genetic drift leading to random fixation of an allele is not the whole point? I’m trying to avoid thinking of quasi-latching!

    So drift will eventually lead to fixation, but on the way some alleles fluctuate in frequency, as per OM’s visualisation, and this is important and results in more variation available for selection?

  6. Alan Fox,

    Are you saying where mutations arrive quicker than drift can fix them, there is more opportunity for selection to find adaptively advantageous allele sets?

    No. I think you might be a bit fixated on fixation! It’s just that there is a constant shift of variants being thrown up, and drift increases their residence time over mutation alone.

    Are you saying that sub-populations, though getting less diverse through drift, get less diverse differently so the overall effect is increased diversity?

    One way of putting it, I suppose! ‘Getting less diverse’ must mean that an allele is increasing in frequency, at the expense of rivals. It’s that composite set of increases in frequency, however temporary, that generates the population’s overall variation.

  7. Allan Miller:…drift increases their residence time over mutation alone.

    I sense this is key to my inability to see the benefit of drift. I can see the longer an allele remains in the gene pool, the more opportunity for it to be spotted by selection. I’m just missing how drift, in the process of randomly eliminating alleles, achieves this.

  8. “..The influence of this process (random genetic drift) on important evolutionary change, though, is probably minor, because it does not have the moulding power of natural selection. Natural selection remains the only process that can produce adaptation…”

    “…the mechanism of most of evolutionary change is natural selection…”
    Jerry A. Coyne -Why Evolution is True

    Since evolutionists can’t even agree on their own speculations they often and so freely make about the mechanism of evolution, I propose a new mechanism:

    Random Genetic Fart

    …since we already have a genetic explosion in relation to miraclevolution in the Cambrian…lol

    I’m sure many will prove me wrong 😉
    Speculations are not accepted… lol

  9. J-Mac:
    Since evolutionists can’t even agree on their own speculations they often and so freely make about the mechanism of evolution, I propose a new mechanism:

    Random Genetic Fart

    …since we already have a genetic explosion in relation to miraclevolution in the Cambrian…lol

    Finally someone proposes an ID mechanism. Illustra Media’s answer to Darwinism: https://www.youtube.com/watch?v=RmD9ZWDUsNY

  10. Alan Fox,

    I’m just missing how drift, in the process of randomly eliminating alleles, achieves this.

    Perhaps think about the complementarity of this process. You can’t randomly eliminate alleles without, at the same time, randomly increasing others. Yin and yang.

    Consider a slightly detrimental allele A that arises in single copy. The frequency of A has gone from 0 to 1/N; simultaneously the frequency of not-A has gone from 100% to a shade under. If there was no drift, A would only stick around for a very short while. You’d only get more A’s by repeat mutation.

    You wouldn’t see the process of mutation as ‘eliminating alleles’, but it lowers the frequency of alternatives all the same. But with Added Drift, A can also gain a few offspring randomly. It’s unlikely to buck the trend indefinitiely, and go to fixation, because of the Law of Large Numbers. But it can do so for a while. As A increases in the population, not-A decreases. Say we now have 3% A and 97% not-A. I’d argue that the variation of the population has gone up, and would keep going up, through drift, until the proportions were 50% each. If it passes 50%, the variation of the population then starts going down again, exactly mirroring the increase as it climbed towards 50%. In the unlikely event (A being detrimental) of not-A getting down to 1 copy, the variation would again be the same as it was when A was in single copy. So … drift doesn’t just eliminate variation. While an allele is rare, drift acts like repeat mutation would, and increases variation.

    Then – to complicate matters – there is ‘meta-variation’. There is a dynamic, shifting equilibrium of old and new alleles being promoted and lost. While all this A/not-A stuff is going on, mutation has not just stopped. New instances are popping up, being promoted for a time, popping out again. It’s a quantum foam of alleles – of sorts.

  11. Alan Fox:
    J-Mac,
    Thanks for providing the Jerry Coyne quote. That found me a Larry Moran blogpost regarding Jerry Coyne’s views on genetic driftas expressed in his Why Evolution is True, written in 2008.

    I guess Larry Moran used the same book or was it a different book?
    Please advise…so I can obtain the updated copy…

  12. J-Mac: Why is my proposition of the mechanism for evolution any different than say Larry Moran’s?

    Are you saying you’re proposing the same mechanism as Larry Moran? Why are you stealing his idea and sell it as your own?

  13. J-Mac: I do…Quantum Transition of Energy Into Matter similar to the idea that physicist propose to the origins of the universe…
    I’m sure you have heard of this…

    Ah yes, the Farttoman hypothesis.

  14. J-Mac,

    Here is in the context:

    That’s just a link to an entire book. Still a quote mine. And for today’s thread, I shall be donning the glove puppet of … ta-dah! … Jerry Coyne. Don’t agree with a single word he says, but he says something different to someone else, so he’s my man.

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