Natural Limitation to Evolutionary Processes (2/14/05)

Genetic drift (aka the Neutral Theory) was originally proposed to explain why there is more genetic change than could be reasonably explained by selection.Genetic drift relies on the chance spread of neutral mutations. Statistically it is inevitable that some will become fixed in the population, explaining the observed genetic differences.However it implies an overall increase in variation compared to a scenario without Genetic drift. The process is as follows:1) A neutral mutation appears, entering the gene pool
2) Some of these will be eliminated quickly, but others will spread to a significant proportion of the population
3) A proportion of these will eventually become fixed.So at any one time there will be a number of neutral mutations present in a significant portion of the population. Some of these will eventually become fixed and others will not. However their very presence itself represents a pool of variation. And since the whole theory requires that this pool of variation is being constantly topped up it directly contradicts your idea that the net rate of change must be an inexorable decrease in variation.

PaulWould it not also be true that a neutral or even harmful mutation might be beneficial under different circumstances?

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Aslan is not a Tame Lion

Yes. The evaluation of a mutation as beneficial, neutral or deleterious is relative to the environment. I did not raise the issue mainly because it was an unnecessary complication. Also some mutations are almost always neutral (e.g. silent mutations) and some are almost always detrimental (e.g. cystic fibrosis is detrimental because it is often fatal at a young age - and it is only because it is recessive that it is not severely detrimental).

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[About genetic drift]
Funny it's categorized on so many biology sites as a "process of evolution" then. I myself am pointing out that all those processes (with the exception of immigration and mutation) are processes of selection. For heaven's sake it's obvious. It selected out the cheetah.

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There is a problem here with communication, caused by your lack of familiarity with scientific definitions. "Selection" means natural selection in biology, and applies only to alleles with positive or negative effects. Neutral alleles are not selected by drift or by anything else, by definition. (You had a similar confusion earlier when you said that PBS meant "mutation" when it said evolution. No, it meant evolution, which as defined by science includes frequency changes in existing alleles.)On one point you are correct: genetic drift does reduce genetic variation. Depending on how you choose to measure variation, drift either always decreases variation or both increases and decreases it.As far as I can tell, however, that is one of the few true statements you have made about genetics here. Scientists actually do know quite a lot about mutation and drift, and they are not all extremely stupid, which they would have to be not to have noticed that evolution doesn't make any sense. We can measure the mutation rate in a wide variety of organisms. We can calculate how much variation there should be in a given size population, based on the mutation rate (and assuming most mutations are neutral). We can measure the amount of variation actually present in populations, and when we do we find that everything makes good sense. (Not perfect sense -- the calculations are approximations, and there are plenty of details to worry about, but the big picture is clear enough.)Let me be concrete. Based on the measured mutation rate, it is likely that every single base in the human genome has mutated at least once just in the current generation. That represents an enormous input of variation, in just a single generation. Much of that variation will disappear immediately: roughly a quarter of those mutations will disappear (by drift) in the next generation, and more in the generation after that. But each of those generations will introduce its own large load of new variation. In smallish population, the forces increasing and decreasing variation are often in balance, so that the total amount of variation stays about the same. Humans, as it happens, have too large a population, and too long a generation time, for the forces ever to come into balance, barring a massive population collapse. So for us, genetic variation is actually increasing.

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Excuse me??? It sounds like YOU are denying that a genetically stable population happens at all!!

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It depends on what you mean by genetically stable. There are populations in which the total amount of variation stays about the same. There are no populations in which the actual variants present in the population stay the same over any significant length of time.

I have to take a break from this exciting discussion for a while, but I do eventually want to get back to posters I haven't yet answered.It seems that so far the general tendency has been to contradict my assertion that the processes of evolution all involve a reduction in genetic variability by insisting that there are balancing or even greater processes of increasing variability going on.I would really appreciate it if at least the fact that the processes I am talking about DO entail an inevitable reduction in genetic variability were acknowledged as this part is fundamental.I guess I'm repeating, but since it doesn't seem I've been understood I think I need to repeat: My general answer to the claim of increasing variability is that -- assuming that such processes exist, and I doubt that they do; most of them can be accounted for by the misnaming of pre-existing potentials as "mutations" -- the overall effect of speciation is a reduction in genetic potentials in any case whatever. That's what speciation/evolution IS -- a selection of specific characteristics from a larger roster of possibilities, and the genetic variability of the new "species" of NECESSITY is reduced in order for the new type to appear phenotypically. All the supposed processes of increasing variability would do is provide a larger roster of possibilities from which to select, but the selection process always produces reduced variability and theoretically at least always has an end point beyond which evolution/speciation simply cannot continue.I understand this to follow from what EVOLUTIONISTS say. Or do you all think it is brought about by mutations in the sense of NON-pre-existing accidents, or outside intervention as it were? If so, somebody should tell all the biology sites that say evolution is change in gene or allele frequencies over time (they fail to note, however, that the change in question is always a reduction except in the case of mutation and immigration. This is my own observation).This message has been edited by Faith, 02-15-2005 10:44 AM

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I may not be following you at this point, but my observation is that the processes that SELECT the variations REDUCE genetic variability and that this is the overall tendency in ALL processes of speciation or "evolution."

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This is true, more or less. What it neglects is the process of mutation, which is constantly adding more variation to the population, both during speciation and afterwards. Suppose a small subpopulation get separated from the larger group, perhaps in a different environment. Natural selection in the new environment and drift in the small group will indeed reduce genetic variation, and may lead to a new species. If the new species is successful in this environment, however, its population will expand, and it will immediately start accumulating more genetic variation. Small populations have little variation because there simply aren't enough individuals to carry many variants. With more individuals, more of the new mutations (which occur every generation) will persist in the population.

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I believe that much of what is called "mutation" is in fact merely pre-existing allelic potentials that come to the fore as combinations typical of the parent population are eliminated from the new population.

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Beliefs are better when they correspond to reality; your belief here doesn't. "Mutation" is defined as a change in genetic material; if it already existed, it isn't a mutation. When biologists study mutation, they're studying changes to genetic material, and it is those mutations that we are talking about. It's already been explained to you how you can observe new mutations occurring in bacteria. Perhaps you should go back and think about that example some more, and see if you can formulate a response to it (other than simply dismissing it).

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Or do you all think it is brought about by mutations in the sense of NON-pre-existing accidents

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Yes. As far as we can tell, all genetic variation comes from mutations, in the sense of non-pre-existing accidents.

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If so, somebody should tell all the biology sites that say evolution is change in gene or allele frequencies over time

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Why? Evolution is change in allele frequencies over time. Mutation introduces new alleles, and the processes of evolution change the frequencies of those alleles. It seems pretty straightforward to me. What's wrong with it?

And, of course, a mutation is also a change in allele frequency -- an allele that previously existed with 0% frequency now appears with non-zero frequency.

I don't think that anyone has denied that selective processes remove variants from the population. That's what "selection" is, after all - differential survival/reproduction among varying individuals. You've yet to show even one instance where this is the case; nor have you rebutted the evidence that proves that new variants in populations arise through mutation.Of course the variants are already present when selection occurs; they have to be. The variants arise through random mutations. We call them "random" because they do not occur in response to environmental pressures, and because the outcome of a mutation is non-deterministic. You're going to have to show the math on that one. You could just as easily claim that the variability caused by mutation will always outweigh the variability removed by selection, and that therefore variability will always increase. (Which appears to have been the case during the history of life on Earth.) Why on Earth do you feel that's an "observation" worth mentioning? It's like saying "except for the color blue, there are no blue colors."

Well you started in post 1 by stating that you intended to SHOW that there was a definite limit to evolution.We now find that the heart of your argument is the assumption that relevant mutations do not happen. But as yet we see no support for such a claim - and your call of a "time out" suggests that at present you don't have a case on this point.On the other hand evolution has always assumed a continual supply of new variations (Chapter V of Darwin's Origin deals with this issue). So your claim to the contrary is simply untrue.So ultimately all you ahve done is assume that the new variations required by evolutionary theory do not appear and your whole argument essentially boils down to this assumption.

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You're going to have to show the math on that one. You could just as easily claim that the variability caused by mutation will always outweigh the variability removed by selection, and that therefore variability will always increase. (Which appears to have been the case during the history of life on Earth.)

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Actually we CAN argue that given a stable situation the degree of variation is likely to remain constant on average.Assuming a fixed population size, and ignoring alleles that are not passed on even once (for BOTH rates) :1) The number of new variations entering the population will be constant on average.2) The number of variations removed from the population will depend on the number of variations within the population - the lower the number of alleles the lower the rate of removal (drift should be directly proportional, while selection can only work against alleles that are present in the population)3) Therefore the situation will tend to reach a dynamic equilibrium because a net removal of alleles produces negative feedback on the rate of removal, while the rate of increase is unaffected.The only way to refute this argument is to show that the rate of new variations entering the population is too low for variation to recover from selective events.

I'm not sure Faith but it appears to me that you may have a slightly off picture of what is going on.If a selective "event" -- the death of an individual carrying a gene producing a less fit phenotype than those around it occurs it removes that particular copy of that gene from the gene pool. It may remove at the same time other unique alleles of course.However, the gene pool as a whole may well contain almost all the alleles of that individual. The reduction in variability may not be as drastic as you seem to think.In addition, the differential breeding and survival of those carrying another gene such that the gene spreads to the entire population may only supress other alleles at the site of that particular gene not all the other 1,000's of genes. The selection process may be more subtle than you are imagining.Since all individuals of an animal species carry several new mutations the selecting out of a few isn't likely to keep up with the creation of new ones.When an event occurs that is "catastrophic" and overwhelms the fitness differences of individuals the survival becomes "random" and then the variability may drop dramatically -- e.g. the Cheetah.If the event is dramtically selective that too, of course, my reduce variability dramatically.If there is a separation of a founder population the very occurance of the separation has produced a population of limited variability. If the population is small enough chance events my reduce variability further.In the more "normal" case the rate of mutation is pretty darn high. You carry several mutations yourself. This rate in a large population makes it hard to see how your idea of continued reduction of variability makes any sense.You have yet to do more than say it happens. I've seen no support for that.

So basically your entire argument boils down to one of a few things:1. Mutations do not happen at all. All genetic variation is pre-programmed.or2. That beneficial or neutral mutations do not occur with any significant frequency.or3. That mutations in general are not any source of genetic variation.If one of these is true then you have your limit to evolution. You have given quite a bit of your hypothesis but have yet to produce any evidence that any of those things are actually true. Moreover, please produce the evidence that contradicts the mainstream evidence that:1. Mutations do occur and are observed.2. Benefits derived from genetic mutation have fixed within polulations of living creatures with significant frequency, also observed.3. That mutations are the primary source of genetic variability withing a population, also observed.When given the example of bacteria resistence arising from a monoculture you claimed that said resistence could not have formed from mutation. You need back up this claim with evidence or retract it as mere speculation based on personal ignorance.I eagerly await your reply.This message has been edited by Jazzns, 02-15-2005 10:36 AM

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By the way, for a fun second-term drinking game, chug a beer every time you hear the phrase, "...contentious but futile protest vote by democrats." By the time Jeb Bush is elected president you will be so wasted you wont even notice the war in Syria.
-- Jon Stewart, The Daily Show

Thanks for that tidy boiling down Ned, I got a lot from it.I was doing some reading on creationist sites and read about the older earth creationist's Hugh Ross and his idea that large creatures periodically die out and have to be restocked by subsequent creations. To him this explains transitional forms ( horses and whales used as examples), but I could not find out what mechanism he thought brought this about. Faith's idea of continually lessening genetic variability sounds compatible with this view, as far as it goes.And before the alarm bells ring, I believe neither... just thought that this lessening variability idea represents an appealing concept for some branches of Christian apologetics.ABB

A general reply again. The reason I took a time out was that I had work to do, and there's more of it to be done today; but I'm up in the middle of the night and did read through the answers, although I will no doubt need to read them again.It is apparently true that I don't understand genetic drift. I have found it presented with bottleneck and founder effect in discussions of evolution as if it were the same kind of process though not as extreme; that is, as if it involves the destruction of some number of a population, only fewer than are destroyed in bottleneck and founder effect. It seems to me that if it refers to a reduction in a population that it would lead to a reduction in genetic variability. I suppose if the reduction in population isn't dramatic there isn't necessarily a reduction in genetic variability, or at least not a noticeable one, but in any case there wouldn't be an increase.Now you are all introducing mutations into the mix, which, if they occur at the great rate you claim, would provide the great increases in variability you all seem to be assuming, to such an extent that they overcome the processes that reduce variability -- if the population is large enough to begin with, if I'm following correctly. If such mutations are always part of the picture in genetic drift (or any of the processes if the population is large enough to be affected by them), it seems to me mutations ought to be assumed in connection with ALL the "processes of evolution," but in fact mutation is only treated as ONE of the processes of evolution in the usual presentation of evolution. That is, from what you all seem to be saying, evolution should be defined as Mutation Plus all the other processes, and should be assumed prior to each of the others.In any case I'm still not sure what genetic drift is. Is it a reduction in population or an increase in variation through mutations or what?My point was always that overall the tendency of all these processes including genetic drift (as it seems to be defined but I perhaps have misunderstood it), is reduction in variability and that this seems to conflict with the assumptions of evolution which these processes define. Yes, it does seem odd that scientists wouldn't have noticed this, to answer the person who said scientists aren't stupid, but I figure scientists are assuming evolution to such an extent that they think evolution into all of it. One suggestion of such a bias is that I simply don't find these processes DEFINED as a decrease but merely as "change" as if they sometimes lead to increase. Hence the necessity, so it seemed to me, to point out that they all lead to decrease -- except mutation and immigration of course. Yes, again, it does seem odd to me that the scientists who inspire these lists define them as change in frequencies rather than decrease in frequencies. Somebody said pointing this out was pointless as it's obvious. If so, then why isn't it already pointed out on these lists? I can give some links but they aren't hard to find. I find versions of them whenever I google something related to evolution.Again, overall the answer to me from others here appears to be that mutations overcome all the processes that reduce population and therefore reduce genetic variability. Again, a redefinition of the "processes of evolution" seems to me to be in order: Mutations PLUS, as the entire edifice of evolution must rest entirely on mutations, as mutations are obviously the ONLY counterforce to all the forces of genetic reduction that I have been focusing on.If such tremendous numbers of mutations do indeed occur such that they contribute an overall adaptive or beneficial effect that overcomes all the processes that reduce genetic variability, then my argument certainly is defeated.==============
Somebody also said that I misuse the term "selection." I am aware that it is normally used in a specific sense, but I was applying it logically to all the processes that reduce genetic variability as it certainly seems to me to apply to all of them. Didn't Darwin apply it to the Galapagos turtles which were developed not from natural selection proper which involves the death of most of a population, but from migration, or possibly even a bottleneck? Any process which eliminates some alleles from a population while permitting the phenotypic expression of others previously latent seems to me is reasonably described as selection.As usual I'm leaving out the enormous input of mutations which seem to make the whole idea of the normal shuffling of given allelic variations obsolete, though this wouldn't affect the definition of selection. Or perhaps I should ask: Is there any room any more for the idea of normal allelic variation or is everything mutation? Actually somebody pretty much already answered that everything is mutation.Again, if so, my argument is certainly defeated.But I hope to have some time to think about this more later.This message has been edited by Faith, 02-16-2005 06:42 AMThis message has been edited by Faith, 02-16-2005 06:49 AM