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. There are several simulation tools that model drift. I just downloaded this one from University of Washington. It didn’t work straight out of the box as I didn’t realise I no longer had Java on my laptop. Once I downloaded and installed it, PopGen works fine.

    Thanks to Professor Felsentstein for making it available to all.

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  2. OMagain,

    Yes, my first thought was to revisit that! It is hair-tearing how something which demonstrably occurs, can be modelled computationally or physically, and has practical applications and implications, is nonetheless deemed wrong because it contradicts some book.

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  3. Prove it. evidence please. if drift happens all the time or sometimes or five times since tuesday then where are the new populations with new bodyplans requiring a ne species scientific name?? the reality is no evolution happens today because it never happened yesterday. very unlikely is evolutionism was a living breathing mechanism.

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  4. Robert Byers: if drift happens all the time or sometimes or five times since tuesday then where are the new populations with new bodyplans requiring a ne species scientific name??

    OK. I don’t believe typographical errors occur. If you (Byers) think that they do occur, show me the entire new novels that result, requiring a new ISBN number! Otherwise I will not believe you when you say that typographical errors do occur.

    +3
  5. Please Alan, University of California, Berkeley (or “UCB” or “Cal”) but not “Berkeley University”. Europeans tend to assume that the “University of Washington” is the same place as “Washington University”, for example. I once had a guy send me a reprint and say that he had lost the address of Alan Templeton, and would I please pass that on to Templeton. Who was then at Washington University, about 2000 miles from me.

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  6. Joe Felsenstein: Please Alan, University of California, Berkeley (or “UCB” or “Cal”) but not “Berkeley University”.

    Thanks for the correction, Joe. Edited.

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  7. Joe Felsenstein: Europeans tend to assume that the “University of Washington” is the same place as “Washington University”…

    Phew, I got that one right.

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  8. How do you decide that a trait exists from genetic drift rather than natural selection?

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  9. phoodoo: How do you decide that a trait exists from genetic drift rather than natural selection?

    Quite a change from your original objections, such as they were:

    phoodoo: Great.

    But when do you give the explanation of how a great great great great great great great grandmother is giving birth at the same time as a newborn infant?

    Or nevermind, just use Joe’s line. If the numbers support your idea, then the analogy to biology is sound. If the numbers don’t work the way you want, well, its because the logic was flawed.

    Never fails! Science without contradiction!

    So, is the analogy to biology sound or not? By asking the question you are asking it seems that in the last 5 years you have actually accepted the concept as valid.

    I guess that’s progress.

    In fact, that thread seems mostly to be people attempting to teach you. It seems that by asking the question you have just asked they have succeeded. So, I guess only another 100-200 years and you’ll be at freshman level level.

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  10. Joe Felsenstein: Europeans tend to assume that the “University of Washington” is the same place as “Washington University”, for example.

    In the interests of staying married, I must point out that there is also [clears throat] Washington State University. Go Cougars! 🙂

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  11. phoodoo:
    How do you decide that a trait exists from genetic drift rather than natural selection?

    Not sure what “exists from” means, nor what you mean by a trait. A trait in biology is usually taken to mean some quantifiable characteristic which could be body mass, limb length, eye colour etc. Eye colour in humans shows variation and pattern of heritability. There is correlation between eye colour and levels of melanin that equate with light levels and the production of vitamin D. So I’d reckon eye colour is/was under environmental selection. I wouldn’t rule out an element of sexual selection either. But drift could play some part. I don’t think it is easy to establish precise weight in complex examples. Others with professional expertise are welcome to correct me.

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  12. Alan Fox,

    So you can’t then.

    You can of course make up just so stories about any trait and say why it has some advantage-but its just a game of who can tell the best story.

    Thus natural selection as a concept is based solely on the ability of the storyteller to make up a scenario they find compelling.

    Evolution-From Darwin to Kipling…

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  13. phoodoo: Thus natural selection as a concept is based solely on the ability of the storyteller to make up a scenario they find compelling.

    And yet the purported replacement, Intelligent Design, is so disastrous that 97% of actual scientists don’t support it.

    So natural selection as a concept, whatever it may be based on is still more convincing a just so story they what Intelligent Design is selling.

    That’s because Intelligent Design is only being sold now by people whose most compelling argument for it is to never actually mention it at all and just denigrate something they don’t actually understand at all instead of making a case themselves for anything at all.

    What does phoodoo believe the origin of species is? We could start a thread and not find out, of course but of course we all know it ultimately comes down to :poof: something somewhere magicked some thing up and that’s good enough for his incurious mind.

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  14. phoodoo:
    Thus natural selection as a concept is based solely on the ability of the storyteller to make up a scenario they find compelling.

    But I suppose you’re aware that the breeder’s art had been practiced for a very long time before Darwin. From one perspective, you can argue that Darwin added nothing to what breeders had been doing, except to suggest that other aspects of an organism’s environment, in addition to people, could be doing de facto selection. After all, from a dog’s perspective (or corn, etc.) human selectors ARE the environment.

    Selective breeding, by means of whatever influence, is a compelling scenario because it works. It produces hunting dogs, trees that bear more and better fruit, even goldfish. The story of producing such things is compelling because the story is not fiction.

    However, I’ll admit that human breeders produce stunning successes within reasonable time frames because the selection process is draconian – ONLY the “fittest” (in the opinion of the breeder) survive, rather than nature’s very slight bias. So human selection can do in a couple of centuries what nature needs millions of years to do, in terms of divergence from some starting point.

    The human breeder, however, is limited to the genetic variation already present. I don’t think we’ve reached the point where humans can generate “designer mutations”.

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  15. Joe Felsenstein: OK.I don’t believe typographical errors occur.If you (Byers) think that they do occur, show me the entire new novels that result, requiring a new ISBN number!Otherwise I will not believe you when you say that typographical errors do occur.

    All that yopu show is that mutations happen. Yet drift is about them happening , selected on, and making new populations with new bodyplans.
    yet in a probability curve its very unlikely drift mutations ever are johnny on the spot fir selection with a important goal. Again Drift does not make new species in the modern world or none known. this because its so unlikely this series of actions. Further they do seem to be saying drift just creates new species with hardly any need to have been selected on. Its like thety are suggesting speciation taking place without a reason in nature. Just drifting into evolution of new species. where? Anywhere or rather evidence? indeed if true evidence should be beyond common. Yet its non existent as if it never does or did take place.
    yes specication happened but not from mutations drifting about or any mutations just when needed. I suggest mutations are only the end of a spectrum of the bodies ability, genetuic ability, to create genetic change or bodyplans and what we call mutations is only a hair trigger result of this system. i listened to IN OUR TIME. a bRit show on genetic mutations and they said some biology. ,under stress, increases mutations. The opposite of drift. Then i expand the spectrum to a natural reaction of biology to change its bodyplans as needed and mutations aree not really mutations at all. just a system that in a spectrum has wayward “mutations”. like in dog and cal coat colours.

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  16. Flint: The human breeder, however, is limited to the genetic variation already present. I don’t think we’ve reached the point where humans can generate “designer mutations”.

    True that we don’t generate designer mutations (until recently, anyway). But it is not true that all selection response in artificial selection uses existing variation. Additional mutations can and do occur in selection lines, particularly when the population size is large (see, for example the Illinois Long-Term Selection Experiment in corn — rather like Rich Lenski’s experiments, and you’ll find that Rich does reference them in his papers).

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  17. phoodoo:
    Flint,
    Are you saying that Jews during WWII were less fit, and that is why their numbers dwindled?

    I doubt you want a serious answer but both drift and selection change the mix of alleles (different versions of a gene found at a particular locus) in a population of organisms within a species. Biological fitness is a measure of the success of an individual organism at passing their alleles to the next generation.

    I imagine you can work out why your question is so disingenuous in this context, or maybe not.

    +1
  18. Alan Fox,

    From one perspective, you can argue that Darwin added nothing to what breeders had been doing, except to suggest that other aspects of an organism’s environment, in addition to people, could be doing de facto selection. After all, from a dog’s perspective (or corn, etc.) human selectors ARE the environment.

    Wrong Alan, according to Flint’s logic, human’s changing genetic frequencies is just more natural selection. So its not disingenuous. Not if that is what Flint thinks.

    And again, according to you, there really is no way to differentiate drift from selection-other than being able to come up with a believable story. So all of life’s variety could be selection or all of life could be drift, and you can’t prove either.

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  19. phoodoo: So all of life’s variety could be selection or all of life could be drift, and you can’t prove either.

    Science works with evidence, not proof. Drift in the absence of selection results in random change. Selection results in adaptive change due to feedback from the niche.

    Remember the niche, phoodoo?

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  20. phoodoo: Wrong Alan, according to Flint’s logic, human’s changing genetic frequencies is just more natural selection. So its not disingenuous. Not if that is what Flint thinks.

    Flint will surely speak for himself but human plant and animal breeders are the most important selecting element of in the niches occupied by plants and animals which are undergoing artificial selection.

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  21. Alan Fox: Drift in the absence of selection results in random change.

    Apparently science doesn’t even need evidence-you can just say whatever nonsense you want and claim it is science.

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  22. phoodoo, Well, not really. The scientific method involves constructing hypotheses (story telling, if you like) and then testing those hypotheses against reality. Hypotheses that don’t model reality accurately get discarded or modified as new evidence emerges.

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  23. Selection and drift aren’t opposites. When there’s no selection, it’s all drift. But even with selection, there’s still some drift.

    Drift happens inevitably in a population. If organisms produce variable numbers of offspring, and half of parental alleles are withheld each time a gamete is formed, then it is inevitable that gene frequencies will change due to chance effects alone. Selection is a bias on this inexorable background process, often varying in strength both temporally and geographically.

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  24. Joe Felsenstein: True that we don’t generate designer mutations (until recently, anyway).But it is not true that all selection response in artificial selection uses existing variation.Additional mutations can and do occur in selection lines, particularly when the population size is large (see, for example the Illinois Long-Term Selection Experiment in corn — rather like Rich Lenski’s experiments, and you’ll find that Rich does reference them in his papers).

    Ah, thanks. So new mutations can arise and be selected for during the breeding process itself. But I guess breeders don’t control what new mutations do.

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  25. phoodoo:

    Wrong Alan, according to Flint’s logic, human’s changing genetic frequencies is just more natural selection.So its not disingenuous.Not if that is what Flint thinks.

    As I understand it, “natural” selection means the environment is doing the selecting without human influence, while “artificial” selection means humans are primarily responsible for the selections. Selection is the common factor.

    Are you saying that Jews during WWII were less fit, and that is why their numbers dwindled?

    Certainly Jews were regarded as “less fit” in the eyes of the Nazis. Which raises an interesting point: when humans do selective breeding (“artificial selection”), traits being emphasized are often chosen for somewhat whimsical reasons — goldfish because they look pretty, chihuahuas because, well, beats me, but some folks like them.

    There have been historical periods when eugenics was regarded as desirable – that we could selectively breed ourselves to be superior – healthier, smarter, longer lived, stronger, etc. But eugenics fell out of favor when it turned out that what people wanted to breed for is people more like themselves, and less like “the other”. Maybe slave owners breeding slaves to be more profitable workers left a sour legacy. I’m not entirely sure what factors led the Nazis to deselect Jews, but my reading is that prejudice, resentment, and social cohesion were important.

    And again, according to you, there really is no way to differentiate drift from selection-other than being able to come up with abelievable story.So all of life’s variety could be selection or all of life could be drift, and you can’t prove either.

    I don’t understand your focus on “story” as opposed to theory combined with observation. But I return to the coin-flipping analogy. If no selection is done, nonetheless the total count difference between heads and tails continues to increase. Think of that as drift. Now, if the coin flipper is deliberately tossing out a few tails from time to time, this is selection and is going to increase the rate at which the total heads count increases. So both drift and selection are happening concurrently.

    And no, I don’t think there is any “proof” here in a mathematical sense, there is only a model which seems to fit the observations. Science doesn’t ever prove anything, it only constructs increasingly predictive models. Underlying causes are proposed, and I supposed these proposals can be called “stories”, but they are grounded in observation rather than entertainment.

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  26. Flint: I don’t understand your focus on “story” as opposed to theory combined with observation.

    I suspect it’s an attempt at a false equivalence.

    If your explanation and my explanation are both just stories then it’s just a matter of personal preference which I prefer. All phoodoo has is a literal story, written by herders of various animals, and so as that cannot be compared to theory combined with observation he starts the conversation by converting science into story and then continues along those lines. He does indeed have his own ‘facts’.

    999/1000 of the words he types are about the thing he knows is incorrect and does not believe in. But very little about the positive case for whatever it is he does believe. As, deep down, he knows that science has triumphed over stories every single time so far so it’s just not worth talking about. Better to have that lack observed occasionally then be demolished at every turn by reality itself.

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  27. OMagain,

    If a scientist can’t tell if a given allele is present due to selection or drift, it’s neither! And a wizard did it.

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  28. OMagain: I suspect it’s an attempt at a false equivalence.

    If your explanation and my explanation are both just stories then it’s just a matter of personal preference which I prefer.

    I hadn’t looked at it that way, but I believe you’re right. Phoodoo knows he has a story (and he knows who told it, and when, and what it’s based on). So to defend his story, he has to label any other possibility as a story so all stories are on equal footing. Once he’s established this, he can attribute his story to a higher power which cannot be disputed, and his story comes out as the winner.

    He’s always reminding me of Mark Twain saying “faith is believing something you know ain’t so.” I’m always impressed at the mental agility necessary to do this. I would personally love to believe things I know ain’t so, but I just can’t do it.

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  29. Flint: Certainly Jews were regarded as “less fit” in the eyes of the Nazis.

    No, no, no, they are regarded by YOU as less fit! Not by the Nazis. And by Alan and by Allan and Joe,…Because you have decided the meaning of fit and less fit just comes down to which has more and which has less. That’s how YOU measure fitness.

    If you are going to tell your story make sure you know what your story means.

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  30. phoodoo: Are you saying that Jews during WWII were less fit, and that is why their numbers dwindled?

    Jews were less fit in an environment where they were being selectively killed off. Always remember that fitness depends on the environment.

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  31. phoodoo: No, no, no, they are regarded by YOU as less fit! Not by the Nazis.And by Alan and by Allan and Joe,…Because you have decided the meaning of fit and less fit just comes down to which has more and which has less.That’s how YOU measure fitness.

    If you are going to tell your story make sure you know what your story means.

    This is not what I said, and not what I meant. Selection MEANS bias according to some criterion. So if selection is happening, it’s necessary to identify the criteria being used, and by whom (or by what). Personally I think the eugenics movement (and the Nazis exterminating Jews was part of it) is misguided and wrongheaded. But I also tried to say that I wonder at the sanity of those breeding chihuahuas. NOT how I would try to shape a wolf, but YMMV.

    I know what my story means, I don’t need you to try to distort it for your own purposes.

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  32. phoodoo:
    Neil Rickert,

    Or maybe it was drift!

    If you were freshly arrived from Mars, and completely ignorant of the fact that humans have motives, you might have to use statistical means to see whether selection or drift was more strongly supported. But us humans understand that humans have motives, and we also understand the mechanism that was used, so we have more clues.

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  33. Neil Rickert: Jews were less fit in an environment where they were being selectively killed off.Always remember that fitness depends on the environment.

    Kind of morbid to wonder whether we’d be better or worse off if such selection were being done by Nazis, or by the religiously blinkered and fanatical.

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  34. Flint,

    So new mutations can arise and be selected for during the breeding process itself. But I guess breeders don’t control what new mutations do.

    In the usual kind of artificial selection the breeder knows nothing of the genomics of the character. So there can be no designing at the level of genes. The breeder in effect sets up the fitnesses of different phenotypes and then the exact genetic changes depend on things they don’t know.

    So it is much more analogous to natural selection than to genetic engineering.

    +1
  35. phoodoo: No, no, no, they are regarded by YOU as less fit! Not by the Nazis.And by Alan and by Allan and Joe,…Because you have decided the meaning of fit and less fit just comes down to which has more and which has less.That’s how YOU measure fitness.

    If you are going to tell your story make sure you know what your story means.

    Indeed.

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  36. Flint: Personally I think the eugenics movement (and the Nazis exterminating Jews was part of it) is misguided and wrongheaded.

    Really? Huh.

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  37. Flint: But I guess breeders don’t control what new mutations do.

    I think the technology to synthesize new DNA sequences exists already to some extent. The ability to predict what effect a novel DNA sequence might have when inserted or substituted in an organism genome is way off in the future. Considering the consequences of such an ability is a bit scary, I find.

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  38. phoodoo,

    Eugenics (forced selective breeding in humans) does not have anything to do with genetic drift. If you want to discuss eugenics, why not open a new thread?

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  39. phoodoo: What does “forced” selective breeding mean?

    I’m sure you are not as thick as you pretend to be. Humans can (in modern Western society at least) choose who to procreate with. The selection of mate is made by the individuals themselves. Eugenics would involve forced matings or inseminations, depending on the whims of those doing the selecting.

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  40. phoodoo: You mean evolution?

    FFS, evolution is change in allele frequency over time. Mate choice, whoever makes that choice – mates, peer pressure, family, culture or eugenecists – will have an affect (maybe vanishingly small) on the change in allele frequency that happens over time in the general population.

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