Selection/Drift

Natural selection is a simple theory because it can be understood by anybody; to misunderstand it requires special training.

Graham Bell, The Masterpiece of Nature

Interest has been expressed in a thread on selection and drift, so I thought I’d start one, and offer my own 2-cent summary of the concepts.

Evolution, as commonly understood in biology, simply involves change in a lineage. Through mechanisms of change – principally, mutation and the insertion of ‘foreign’ DNA sequence – offspring frequently contain DNA sequences that do not derive by simple copying from their parent(s). This change is inevitable and, iterated, inexorable. There is no memory, no externally-stored blueprint for organisms; the specification of a species ‘floats on the breath of the population’, as Doctor Johnson wrote of the unwritten Gaelic language. Unless there is some kind of boundary blocking all possible avenues, a continuing source of variation is sufficient to keep a lineage exploring never-before-seen genetic space ad infinitum.

Among close ecological competitors (among, for example, the genetically similar members of a species), at a given locus in a finite world one individual ancestor’s genetic sequence is headed towards being the ancestor of every instance of that locus in a future population, and all the others with which it shared a population are heading for extinction. This derives from two facts: samples are more likely to deviate from the frequencies in the wider group than match them, and the probability of fixation of an allele is equal to its current frequency. The distortions on generational sampling tend to reinforce, through to extinction of all but one variant. This same tendency underlies the ecological principle of competitive exclusion between non-interbreeding competing species.

If a particular locus is invariant in a population, fixation has already happened. An original mutation, occurring in a single ancestor, has been passed to every member of the current population. Looking forwards, the mechanism of this concentration continues to operate, and so one particular individual from the present population will become the ancestor at that locus of all members of a future population. From any given starting point, a population of N diploid individuals will take a mean 4N generations to achieve fixation of one ancestor’s copy, and the probability for any diploid locus of being that copy is 1 in 2N. This doesn’t mean that large populations cannot fix neutral alleles, however – the number of mutations occurring scales with population size, so mutations will be fixed at the same rate they are generated, completely irrespective of population size. Doubling the population gives twice as many mutations taking twice as long to fix – the result is the same number of mutations being fixed per generation.

At the point of fixation, all instances of that locus descend from the same ancestor – they coalesce upon that ancestor. The case described – where there is no variation at all at the locus, ie there is just one allele – is the baseline process, the neutral case. If there is no variation, there is nothing for Natural Selection to ‘see’. The only process in operation is random genetic Drift – even though in this instance, it effects no evolutionary change because there are no variant alleles. The change occurred with the original mutation. This latter fact leads me to prefer the view of ‘descent with modification’ over the population geneticist’s ‘change in allele frequency’. It is true that allele frequency change is also evolution, the only part over which selection and drift have a role, but as far as each lineage is concerned, the change occured at the moment of mutation. The lineage changed at that point; the population changed somewhat later, when this mutation became the norm.

Suppose we could uniquely label the locus for every member of the population, in a heritable manner. Now, we have essentially created 2N alleles. If we allow them to operate neutrally, just as when there was no variation, evolution will now occur because allele frequencies must change in the population. Because our labelling has had no effect on the neutral ancestry-fixation process, the label itself will surf to fixation on this process, while all others become extinct.

If, instead of labelling every instance, we simply labelled one, we would find that it still had the same 1 in 2N chance of becoming fixed. And this is the situation for any neutral mutation: 1 in 2N neutral mutations will become fixed; the neutral mutation simply functions as a label.

So now, having laboured the neutral case, where all is Drift, we can look to introduce a differential between alleles. If a new allele consistently performs better or worse than the existing one – meaning that it enhances or hinders the survival and/or reproduction of its bearers – then Natural Selection has come into play. It is a simple and obvious and non-tautological!) truth that a consistent increase in survival/reproduction – in fitness – will tend to favour such alleles over the purely neutral case, and render fixation more likely and speedy, while a reduction will increase the likelihood and speed of elimination.

Unlike the purely neutral case, in which population size is cancelled out, the behaviour of selectable alleles is affected by population size. In smaller populations, random factors have a greater influence than in larger ones, and hence alleles may behave as effectively neutral despite possessing an advantage which would see them selected in a larger population.

Drift does not simply disappear when you start to turn up the selective ‘heat’. Drift essentially derives from random sampling, the tendency of subsets to deviate from the distribution of the complete set, and such sampling is in effect almost all the way along the continuum of selective advantage (apart from alleles that are so strongly detrimental that they never gain a foothold). Even a favourable allele can disappear through Drift, likewise a deleterious allele can become fixed through the same mechanism. But more often, progress will go with the expectation, not against it. The large-number tendency is for genomes to become enriched in advantageous alleles and impoverished in detrimental ones. Because this process is environmentally conditioned, it allows populations to adapt to their circumstances, by purging the traits that do worst in the recent environment.

There continues to be a debate about the relative importance of Selection and Drift in evolution generally, and in driving speciation among sexual forms. Only selection can be adaptive, because it is the only component that is responsive to the environment. But they both have significant contributions to make, and cannot readily be teased apart. Both tend to reduce the variation in a population, which variation is only restored by mutation, recombination or immigration.

243 thoughts on “Selection/Drift

  1. Allan Miller:
    Robert Byers,

    So you’re saying evolution is true, but mutation itself provides a force that must not be underestimated, and without which evolution cannot proceed? I wouldn’t disagree!

    Just the way I argue.
    Yes change occurred like crazy. people changed from the eight on the ark as we see today.
    I am saying Darwin misunderstood how important mutations was not needed. He presumed minor trait differences would explain biological glory.
    I think it was another sloppy presumption.
    to change biology and its innate DNA is a big deal. It was unwelcome to evolution to find such a complex control on genetics as found when finding DNA structures.
    In fact mutations are impossible, we say, to explain biology’s story.
    Mutations are however the engine for evolution if evolution is true. not selection. Selection is a minor later afterthought even if it explains end results between species.
    Biology is a complicated thing and not a simple thi ng like physics. Relative.
    One can’t get away with saying aw shucks everything is just a improvement on ones parents on some minor detail.
    biology demands mechanism for its nature and so its evolution.
    Selection is chump change for evolution.
    i think this is why the public doesn’t understand evolution.
    Natural selection misses the actual engine.
    its really biology equals mutationism directed by natural selection to results at any point in time.
    Of coarse its all mutant error!!!

  2. phoodoo:
    Guillermoe,

    How can you know that not singing is an advantage if you don’t know the reason for singing in the first place?

    So is singing an advantage for crickets?

    Let,s see: you are stating the stupid idea that we cannot know if running away from a predator and staying alive is an advantage for an animal if we don’t know why the predator is trying to eat it? Or way this animal started running? Ain’t it a little beat stupid?

    Singing allows males and females to locate each other and mate. I would say that is advantage.

    Now please, answer the question: environmant changed. A new natural enemy is present: a parasitic wasp. The population of crickets is becoming silent (the proportion of silent males is increasing).

    How do you explain this? How can THIS be a tautology?

    phoodoo: because there are again just so many unexplainable aspects to another evolution story

    No, there aren’t. These are interesting questions, but they are not necessary to explain this case through evolution.

    phoodoo: How often are there males born without the ability to sing in places where singing is not a problem?

    This affects the speed of the process, not the process talking or not taking place.

    phoodoo: How did the males learn a mating strategy to overcome their lack of a call?

    They didn’t learn anything. They don’t know things.

    It’s easy: have you got any other explanation? No? Then evolution is the best. The concept here is “survival of the fittest”. Silent males are fitter than singing ones, so the trait that makes them fitter spreads thorugh the population. No tautology.

    phoodoo: If the evidence is such that evolution can not possibly explain something, that is reason enough to doubt it

    What evidence evolution doesn’t explain it? It does perfectly. I did it in the previous paragraph.

  3. Robert Byers: Your saying selection is more defining of what evolutionary biology is then mutations.

    No, I’m not. I am saying that without natural selection, a new trait appearing only on some individuals won’t increase its frequency in the population.

    Robert Byers:The mutation is the great thing. in one organism its the only important thing. later if selection moves it into the creation of a new population is a minor point to biology. It would AGAIN require another IMPORTANT mutation to move create yet a new population again.

    This is subjective. You can consider GREAT whatever you want to consider great. Most of the time, there is no evolution without some kind of selection. And there can be evolution without mutations.

  4. Guillermoe: No, I’m not. I am saying that without natural selection, a new trait appearing only on some individuals won’t increase its frequency in the population.

    This is subjective. You can consider GREAT whatever you want to consider great. Most of the time, there is no evolution without some kind of selection. And there can be evolution without mutations.

    Hmm. you still seem to be saying selection trumps mutations.
    Evolution without mutation is trivial as explained by evolutionists.
    One must have a sexy mutation to do a important step that then is selected leading to the new population.
    Evolutionary biology is 98% mutation driven. Selection is trivial however much its claimed to be the origin of results.
    Biology is very complex and demands a complex change to some trait or the stuff of a trait. Mutations must be top dog.

  5. Robert Byers: Evolution without mutation is trivial as explained by evolutionists.

    No, it’s not. There are other sources of genetic variability. And mutations without selection won’t have any effect on the population.

    Robert Byers: One must have a sexy mutation to do a important step that then is selected leading to the new population.

    But evolution must not occur always from “important steps”.

    Robert Byers: Evolutionary biology is 98% mutation driven

    Where does 98% come from?

    Robert Byers: Biology is very complex and demands a complex change to some trait or the stuff of a trait.

    I agree. Natural selection is a much more complex process than mutations.

    Robert Byers: Selection is trivial however much its claimed to be the origin of results.

    Ok. Show me how any trait spreads thorugh the whole population without any kind of selection.

  6. Allan Miller:

    Neutral drift.

    I see how neutral drift would maintain genetic variation, but I don’t see how neutral drift would make one trait replace all others (i.e. one trait spreading through a whole population). I know it could happen occasionally, but, as your link says, it’s a misinterpretaion to consider that neutral drift implies that “natural selection is unimportant in shaping genomes” as Robert Byers is claiming.

  7. Guillermoe: I see how neutral drift would maintain genetic variation, but I don’t see how neutral drift would make one trait replace all others (i.e. one trait spreading through a whole population). I know it could happen occasionally, but, as your link says, it’s a misinterpretaion to consider that neutral drift implies that “natural selection is unimportant in shaping genomes” as Robert Byers is claiming.

    I asked for a thread on this and got it. Now, unless I’ve misunderstood everything, Joe F has declared that even a tiny selection advantage will move a variant toward fixation much faster than neutrality. Also, it’s terribly difficult to determine that something is neutral, outside of simulations.

    But Lenski demonstrated that nearly neutral mutations can fix in fast breeding populations, and that mutations neutral to specific current need can not only fix; they can be crucial to “progress’ toward adaptation.

  8. petrushka: But Lenski demonstrated that nearly neutral mutations can fix in fast breeding populations

    Yes, I know it can happen in some cases. I don’t think these cases allow us to say the natural selection is trivial.

    Still, drift is not mutation. I don’t agree with Robert Byers that mutation is the most crucial part of evolution and natural selection and drift are trivial.

  9. Guillermoe,

    I see how neutral drift would maintain genetic variation, but I don’t see how neutral drift would make one trait replace all others (i.e. one trait spreading through a whole population).

    It’s actually the basic process, and it reduces variation, in exactly the same way that selection does, albeit more sloooowly. It’s a surprising and counter-intuitive result, that pure unbiased randomness can effect elimination of variation***, and leave one allele standing, in a manner which is guaranteed at one level (almost certain, in the mathematical sense), if unpredictable at another.

    *** Of course, multiple mutations, themselves drifting, can restore that variation.

    See here for a rough sketch of the idea, and here for a nice graphic illustration by OMagain of the process at work eliminating variation. Both threads contain some of phoodoo’s earliest work!

  10. Guillermoe: Still, drift is not mutation. I don’t agree with Robert Byers that mutation is the most crucial part of evolution and natural selection and drift are trivial.

    Uh, well, without change, selection would soon dry up the possibilities for adaptation..

    Also, many highly visible attributes of living things could well be hitchhikers. They have not been selected, but are genetically co-mingled with something that has been selected.

    The selectionist fallacy is to assume that every attribute has been selected.

  11. petrushka,

    Strictly speaking, Lenski’s method involves a significant contribution from population bottlenecking, too. This increases the role of drift at those population sizes.

  12. Allan Miller:
    petrushka,
    Strictly speaking, Lenski’s method involves a significant contribution from population bottlenecking, too. This increases the role of drift at those population sizes.

    A population of bacteria in a person being treated with antibiotics is bottlenecked. I would guess that populations of single celled organisms are frequently bottlenecked.

  13. petrushka: Uh, well, without change, selection would soon dry up the possibilities for adaptation

    Sure, but mutations are no the only source of variability. Anyway, what I am saying is that with mutations taking place only you can’t account for the change in the population. You need something more to happen for the mutation to spread in the population, wether it is selection or drift.

  14. Guillermoe: Sure, but mutations are no the only source of variability.

    There are a couple dozen types of mutations. What other sources of variation are there?

  15. Allan Miller: See here for a rough sketch of the idea, and here for a nice graphic illustration by OMagain of the process at work eliminating variation. Both threads contain some of phoodoo’s earliest work!

    I’ve just seen this is broken. I’ve had a better version in the pipeline since I wrote that, but have not had a chance to finish it. I’ll fix what was originally there and try to find some time to release v2.

  16. petrushka: There are a couple dozen types of mutations. What other sources of variation are there?

    Chromosomal crossover, sexual reproduction, migration, polyploidy, transposed genetic elements,

  17. Guillermoe: Chromosomal crossover, sexual reproduction, migration, polyploidy, transposed genetic elements,

    I think, perhaps, you have chosen to define some kinds of mutation as not mutation. And you are attributing new variation to sexual sorting of alleles.

    Which of these processes provides novel sequences? And which are the shuffling of existing cards?

  18. petrushka: Which of these processes provides novel sequences?

    Chromosomal crossover and transposed genetic elements.

    Yet, not every trait is determined by one gene; so, sometimes, shuffling of existing cards might give natural selection material to select from.

    That’s my point. Selection does not depend a mutation happenning to have where to select from.

  19. phoodoo,

    It is rather a semantic point, if gene duplication is classed as mutation but genome duplication is not, or where mutation is random (in whatever sense of the word you wish to argue) and genome duplication is not.

    Polyploidy has long been recognised as a source of variation, and a source of vigour. I know you’ll move into mocking mode at this point – “Allan says nothing to see here, move along” … but nothing to see here, I’m afraid.

  20. petrushka,

    Which of these processes provides novel sequences? And which are the shuffling of existing cards?

    It’s an interesting point. If you see genes in the molecular biologist’s way, as discrete sequences producing a protein or RNA product, then one could see crossover as shuffling cards, but it still creates chimeras within such units, if only occasionally. But one also has to consider the interplay of sequence and phenotype.

    The evolutionary gene is more just ‘a stretch of DNA’ than a string of beads. The potentiating mutation in the Cit+ strain in the Lenski experiment simply involved ‘shuffling cards’, by placing the promoter of one gene in front of another. But it created novel sequence, by that juxtaposition (and new information, and a new phenotype).

    Similar arguments can be made relating to the transposition of intra-protein sequence. Put a chunk of one protein inside another, you have both ‘shuffled cards’ and created novel sequence – especially so if you have also induced a frame shift.

  21. phoodoo: More evidence for survival directed evolution, and more bad news for the (imaginary) ToE

    I think it’s more bad news for your (lack of) understanding. Response of plants to grazing is as bad news for evolution as hearts beating is.

    Even further, it’s an inherited trait. Natural selection CAN WORK ON IT.

  22. petrushka: Mutation is a heritable change in the sequence of a gene or a chromosome

    There are still other phenomena that produce genetic variation that are not mutations. And even leaving that aside, once a mutation happens, you still need another processes taking place for that mutation to fix. Those processes are no trivial as Robert Byers suggests.

  23. petrushka,

    It is again a semantic point, but sexual recombination is a change in the sequence of a chromosome, and hence – according to your definition – mutation!

    Of course, it matters less what you call it than what it is. The fuel of evolution is sequence change, however caused:

    Mistakes
    – erroneous copy,
    – erroneous repair,
    – erroneous shuffling,
    – unrepaired damage

    Normal processing involving subgenome elements:
    – ‘deliberate’ shuffling by transposons,
    – incorporation of foreign sequence,

    Or

    normal processing by the cell itself
    – a byproduct of normal meiotic disjunction.
    correct DSB repair from the diploid copy

  24. So I fail at getting the terminology right, but appear to be correct in thinking that evolution requires novel sequences. I suspect that while normal shuffling may produce variation visible to selection, it may not be permanent or heritable.

  25. petrushka,

    No, I think it will all be heritable and permanent in that lineage, pending another change. Some shuffling takes place at a lower level – RNA transcript editing – but if the change occurs to the ‘master’ copy, DNA, it must be heritable (ignoring the bizarre genomic world of the ciliates!).

    The reciprocal exchange of sexual recombination is a bit of a special case. In the presence of recombination, sequences have a ‘half-life’, a time of persistence until another recombination event changes things again, so it might depend on what one meant by ‘permanent’. The effect of that subsequent exchange varies depending upon the degree of relatedness of the diploid pairs. ‘Permanence’ is only secured at fixation, for a given sequence, as exchange between homozygotes makes no difference.

  26. phoodoo,

    Apologies, I missed a nuance in my earlier reply – this isn’t heritable polyploidy, but a heritable capacity to generate polypoloidy. But still, if plants breed true for this ‘adaptive somatic polyploidy’, then it would appear to be under genetic control, and hence fall within the same evolutionary paradigm as any other evolutionary adaptation (note the distinction between evolutionary adaptation and that of an individual).

    If you want to make a case for active response – generating appropriate heritable change due to environmental influence, rather than generating random heritable change which is then filtered by the environment – you’d have to devise a means of distinguishing the two. On the surface, any adaptive response can in principle be explained either teleologically or non-teleologically. It’s not enough just to point to an adaptive response and claim it for one’s pet theory. Again, Lenski is relevant to this, because he can go back in time. The evidence is with filtration of random change, and against directed change.

    Funnily enough, Darwin thought along similar lines to you re:variation. He was wrong.

  27. Allan Miller: No, I think it will all be heritable and permanent in that lineage,

    I don’t want to argue biology from definitions, but I also want to be clear that the online biology dictionaries define mutation as a heritable sequence change in either genes or chromosomes.

    Now unless some of these changes are part of a deterministic process, they are “random with respect to fitness” and if they produce changes in the organism, they are subject to selection.

    So my reading of Guillermoe’s argument is that not all change is point mutation.

  28. phoodoo,

    No, it has been commonly agreed among biologists that Darwin was wrong on that particular issue based upon the work of Gregor Mendel et seq.

  29. Allan Miller:

    *** Of course, multiple mutations, themselves drifting, can restore that variation.

    Here is where I have my major disagreement with the claim that drift reduces variation. Only if you assume that a single mutation goes to fixation through drift will it reduce variation in a population. But only with respect to that particular trait. In the real world, there are multiple mutations popping up, each one which may be spending part of the time drifting, and possibly part of the time being subject to selection. Due to environmental change, a mutation may shift from being neutral, to disadvantageous, to advantageous, and back again, all without ever achieving fixation.

    There are drifting mutations floating about that won’t ever get to fixation, but will not ever get eliminated either, just due to the random dynamics at play in the population.

    So saying that drift reduces variation only applies when you’re following a single mutation, but in the actual world of populations of many individuals living with many mutations, drift writ large is one of the things that keeps variation going.

  30. llanitedave,

    It reduces variation more slowly than selection, but it does reduce variation. If there are multiple alleles at a locus, one of them will become the sole survivor.

    The fact that, in a recombining population, there are multiple segregating loci, and multiple mutations feeding in at the top, is ‘really’ what is causing the population variation, not drift iself. At each locus, one allele is heading towards fixation, by drift or by drift + selection. Absent novel variation, heterozygosity is always on a downward trend, barring selective opposition.

  31. phoodoo:
    Allan Miller,

    So do you think blue eyes are a deleterious mutation, and will eventually be eliminated from the population?

    Sexual selection could be part of the equation for eye pigmentation.

  32. phoodoo,

    So do you think blue eyes are a deleterious mutation, and will eventually be eliminated from the population?

    I have no idea. They’ve certainly never done me any harm! But note that a central theme of this thread is that it is not necessary to be a deleterious mutation to be eliminated from the population.

  33. phoodoo:

    So do you think blue eyes are a deleterious mutation, and will eventually be eliminated from the population?

    So, do you think you can explain the change in the population of field crickets without evolution?

    You seem to have conviniently forgotten about it…

  34. Allan Miller,

    No the central theme of the thread is that the definition of a deleterious mutation is that it is more likely to be eliminated from the population. That is all deleterious means according to you and your cronies. Beneficial simply means more likely to fix.

    Since you just said heterozygosity is always on a downward trend, wouldn’t that include blue eyes?

  35. phoodoo: you and your cronies

    You could at least show the same level of respect you are given here to others.

    What a piece of work you are.

  36. phoodoo: Since you just said heterozygosity is always on a downward trend, wouldn’t that include blue eyes?

    Sexual selection. Also, if you read and understood anything, you would already know that drift is more significant in SMALL populations.

    Now, can you quit trolling trying to find the corners of a circle and try to make a point? Drift vs Selection: have you anything to say about them?

    phoodoo: Why would sexual selection favor a deleterious mutation you think?

    The only person who said blue eyes is a deleterious mutation IS YOU. Perhaps you could justify your claim or at least stop ATTRIBUTING IT TO OTHERS.

  37. Guillermoe: Perhaps you could justify your claim or at least stop ATTRIBUTING IT TO OTHERS.

    yes, reading the comments on this page alone it’s phoo is obviously putting words into people’s mouths then claiming victory when they don’t answer his entirely made up points. So while I’m prepared to believe phoo is arguing in good faith that only goes so far when the evidence says otherwise.

  38. Allan Miller:
    llanitedave,

    It reduces variation more slowly than selection, but it does reduce variation. If there are multiple alleles at a locus, one of them will become the sole survivor.

    The fact that, in a recombining population, there are multiple segregating loci, and multiple mutations feeding in at the top, is ‘really’ what is causing the population variation, not drift iself. At each locus, one allele is heading towards fixation, by drift or by drift + selection. Absent novel variation, heterozygosity is always on a downward trend, barring selective opposition.

    Sorry not to have intervened earlier in this discussion — I’ve been busy.

    If you have neutral alleles, heterozygosity declines on average. But it can increase. For example, if we have two alleles and their frequencies are 0.2 and 0.8, and genetic drift happens to move the gene frequencies to 0.22 and 0.78, the population will have higher heterozygosity.

    But on average the heterozygosity of neutral alleles in a population of size N will be multiplied by (1 – 1/(2N)) each generation.

    Hoever I do note that Allan said “is always on a downward trend” rather than that it always goes downward.

  39. phoodoo,

    No the central theme of the thread is that the definition of a deleterious mutation is that it is more likely to be eliminated from the population. That is all deleterious means according to you and your cronies. Beneficial simply means more likely to fix.

    Since when? That’s your definition, which I have attempted to correct about a dozen times now. Altogether now … a beneficial allele increases the mean offspring of carriers vs non-carriers. Naturally, a consequence of this is that it is more likely to fix. But if an allele is fixed, or conversely it is eliminated, this does not of itself mean that it is beneficial in the former case, nor that it is detrimental in the latter. Neutral alleles fix too. Fixation doesn’t mean they weren’t neutral after all.

    Taller people are more likely to hit their heads on low beams. Is that the definition of ‘tall’?

    Since you just said heterozygosity is always on a downward trend, wouldn’t that include blue eyes?

    Included in what – being on a downward trend? If something is on a downward trend, something else is on an upward one. Which could be blue eyes. Frequencies, like probabilities, add to 100%.

  40. phoodoo: Why would heterozygote blue eyed people be on an upward trend?

    Is blue eyes deleterious, neutral or beneficial?

    Just as an intellectual exercise, why don’t you try and answer your own question first? Then your answers can be corrected, if necessary.

    It’s a good way to learn. Except, of course, that’s not why you are asking is it?

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