On Monday, October 9, 2017 at 4:00:03 AM UTC-7, Bill Rogers wrote:
> On Monday, October 9, 2017 at 4:30:03 AM UTC-4, Alan Kleinman MD PhD wrote:
> > On Saturday, October 7, 2017 at 5:40:02 AM UTC-7, Bill Rogers wrote:
> > > Lots of people have pointed out obvious problems with Alan's model...
> > >
> > > 1. It doesn't model selection
> > It's pretty sad when you adopt John's argument that my model doesn't model selection. After all, John's awareness of probability theory is limited to the addition theorm and he regularly applies that theorem incorrectly. Absolute fitness to reproduced is measured by the number of replications and I assure you, that variable is in my equations. Your problem is that you only can see selection in terms of relative fitness to reproduce when it is the absolute fitness to reproduce which determines the probability of a beneficial mutation occurring.
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> But you don't model selection. There is no parameter in your model for the fitness of the wild type or the mutants. You simply say "amplification occurs". That's not a model of selection. As I've said before, you could prove that your model incorporated selection simply by setting the mutation rate to zero. Start with a mix of genotypes with different fitness and model the change in their populations over time.
Like I say, you are fixated on the notion of relative fitness to reproduce when rmns is dependent on the absolute fitness to reproduce. Multiple different variants in your mix of genotypes can evolve to a particular selection pressure on their own particular evolutionary trajectory as long as they have sufficient absolute fitness to reproduce. It is the number of replications of a particular variant which determines the probability of a beneficial mutation occurring.
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> > > 2. It applies only to situations in which multiple, lethal selection pressures are applied suddenly to a population
> > That explains why you didn't see emergence of resistance in your Malaria study and why combination therapy drives HIV to extinction. You are a very smart cookie Bill to use multiple, lethal selection pressures and apply them suddenly for your treatment of Malaria, you don't see emergence of resistance. On the other hand, perhaps you don't quite see that when Malaria populations can achieve absolute numbers of e12 and that there is a reasonable probability of a double beneficial mutation occurring.
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> The selection pressures you are imagining are indeed lethal. You simply calculate that in a large enough population there would be a reasonable probability for the occurrence of double mutants resistant to both lethal selections. The failures, according to your model, are not due to non-lethality of the selections, only to the size of the starting population and the chance that it will contain a double mutant. So, in spite of your gripes, you are still modelling lethal selections.
Your use of language is nothing short of bizarre. If you use two "lethal" selection pressures are you killing them twice? It should be obvious to you that if a selection pressure is 100% lethal, your population should go extinction (and isn't that your goal when treating malaria?), therefore your selection pressures are not 100% lethal when you see the emergence of resistance.
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> > > but there's another problem that seemed unimportant, but perhaps is not.
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> > > Alan is convinced he's had an important insight showing what's wrong with standard Haldane or Kimura style models of population genetics.
> > I don't say that Haldane's and Kimura's model are necessarily wrong, it is that they are inaccurate and incomplete in describing the physics of rmns. What Haldane and Kimura are attempting to describe is half of the cycle which constitute the rmns phenomenon. The half of the cycle they are attempting to describe is the amplification phase of rmns.
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> Yeah, the "amplification phase of rmns" that's what other people here call selection.
But fixation is not necessarily amplification. That's why Haldane's and Kimura's models are not correct physically to describe rmns.
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> >Haldane's concept of the substitution of a more beneficial allele for a less beneficial allele in a population is neither necessary nor sufficient for amplification to occur. This is well demonstrated in the Kishony experiment where multiple different colonies are evolving resistance to the antibiotic without the other variants disappearing. Kimura uses the same conceptual approach as Haldane but instead of modeling fixation as a substitution process, he models it as a diffusion process. On the other hand, you might try to apply the Haldane and Kimura models to the Lenski experiment because in this case you have different variants competing for a fixed amount of resources. In this case, the more fit variants will drive the less fit variants to extinction potentially causing “fixation”. But it doesn't matter, even if the particular variant now has a relative frequency of 1, this variant must still have sufficient absolute fitness (do enough replications) to have a reasonable probability of another beneficial mutation occurring on this particular variant (thus creating a new variant). Neither Haldane nor Kimura correctly take into account the multiplication rule of probabilities for rmns in their models.
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> When you say "the variant most still have sufficient absolute fitness," all you mean is that the variant must be able to reproduce to fill the carrying capacity of the environment. That's no problem in most real settings; it is a problem if you are treating an infection with two lethal drugs and the variant you are talking about has resistance to only one of the drugs.
Would you do me a favor and stop putting phrases in quotes which I have not made and then attributing them to me? Perhaps you were trying to quote this phrase that I did make "this variant must still have sufficient absolute fitness (do enough replications) to have a reasonable probability of another beneficial mutation occurring on this particular variant (thus creating a new variant)". It is not necessarily true that the variant must fill the environment to its carrying capacity in order to amplify a particular mutation. The Kishony experiment plainly demonstrates this fact. Now if you want to get the Kishony experiment to work with two drugs, you are going to need a much larger petri dish which will support much larger colonies to improve the probability of double beneficial mutations occurring.
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> > > And that is that the important thing to follow is the absolute number of geneotypes rather than the relative frequency of genotypes. That's because, per Alan, what matters to the probability of getting a second beneficial mutation is the number of organisms carrying the first mutation - the number of wild type organisms is irrelevant because they are on their own trajectory at that point and don't affect what happens to the single mutants, who, likewise, are on their own, independent trajectory. On this view, relative frequency doesn't matter.
> > You are restating my argument correctly. Another way of saying this is that a mutation which might be beneficial for one variant may very well be detrimental to a different variant on a different evolutionary trajectory.
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> > > At first I thought this was just a trivial error, since relative frequency and absolute numbers are just related by the population size (even if you have to make population size a function of time). But it's not trivial; it's at the heart of what Alan doesn't understand about evolution. When you imagine independent trajectories for different genotypes you are ignoring competition. And, if you ignore competition, then, indeed, the "multiplication rule of probabilities" means that it will take a phenomenally long time to get some target set of mutations in one organism.
> > I don't ignore competition at all. In fact, all that competition for resources in an environment does is slow down the rmns process.
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> Competition in the environment speeds up evolution; it does not slow it down. If there were infinite resources, infinite space, and infinite opportunities to reproduce, evolution barely proceed at all.
Perhaps this happens in your dreams after you ate a really spicy pizza but in reality, your own empirical examples which you recommended to us shows that you are wrong. Recommend to Lenski to run his experiment with both e coli and staph together instead of e coli alone or for Kishony to use both e coli and staph together in this experiment instead of e coli alone. The competition for the resources in the environment alone is enough selection pressure to drive one or the other bacterial populations to extinction. I do hope you enjoyed that spicy pizza.
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> >The empirical evidence clearly demonstrates this.
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> No; the empirical evidence demonstrates no such thing.
Really? Do you believe that competition from different variants for resources in the environment helps the absolute reproductive fitness of each of the variants? Where is your empirical evidence?
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> >The population attempting to adapt to a set of selection pressures which also must compete against different variants in the same environment for the same resources will not be able to reproduce as efficiently as if they didn't have this competition.
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> The one that wins the competition will be the one the reproduces more efficiently than the others.
Until the winner of the competition drives the other variants to extinction, the less fit variants will be depriving the more fit variant resources necessary to reproduce. This is a simple conservation of energy and conservation of mass problem. Why are you having trouble seeing this?
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> >This is why the Lenski experiment takes so many generations for each beneficial mutation. And what do you think would happen in the Kishony experiment if he grew not only e coli but also staphlococcus on the same plate? The competition between the two bacterial populations would limit the growth of either colonies reducing the absolute fitness of both reducing the ability of either population to adapt to the antibiotic selection pressure.
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> No. Whichever species grew faster on his medium would take over the experiment in short order and would develop antibacterial resistance.
Let me see if I get your argument correct. Are you really saying that a bacterial lineage will evolve more quickly against an antimicrobial agent if the particular lineage must also compete against other lineages at the same time for resources of the environment than if the lineage only had to evolve against the antimicrobial agent alone? If that's your argument, I think you had a little too much beer with your pizza.
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> > > But competition is pretty much the rule, not the exception. Darwin's basic insight was that far more young of a given species are produced than can be supported by the environment and that competition will lead to gradual selection for the more fit - those more able to use food sources efficiently, withstand environmental stresses, avoid predation, find mates, etc. If you pretend that there are infinite resources and infinite space and infinite opportunities to reproduce, well, then, indeed every genotype will go off on a trajectory of its own, the odds of accumulating a specific collection of beneficial mutations will be infinitesimal, and nothing much will happen in the way of evolution.
> > Certainly the limitation of resources constitutes selection pressures but what is the resource limitation in your malaria study? Not much because this parasite can achieve populations of e12 in a single sufferer. And with populations this size, two drug therapy does not necessarily constitute sudden, lethal selection pressures as your study demonstrated. Competition between variants for resources of the environment only slows the rmns process against any other selection conditions the environment is offering.
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> > > I thought that Alan's insistence on absolute numbers versus relative frequencies was just a quirk, but it's actually essential for excluding competition from his model, and thus making evolution impossible except in cases where the applied selections are so strong that they make competition moot anyway.
> > If you think about this a little more carefully Bill, you may come to the correct conclusion that my model works correctly whether there is competition between variants or no competition between variants. Competition between variants only slows the ability of the particular variants to achieve the number of replications necessary give a reasonable probability of a beneficial mutation occurring. rmns works much more efficiently when variants do not have to compete with each other. But the number of replications required to give a reasonable probability of a beneficial mutation occurring on a particular variant remains the same whether there is competition from other variants or not. Competition between variants reduces the absolute reproductive fitness of all variants in the environment.
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> Someone needs to think a bit more carefully, that's for sure. Consider a population which, like most wild populations of anything, is limited by the carrying capacity of the environment. The population size is approximately constant from generation to generation. That means that in each generation, there is an equal chance of a beneficial mutation occurring. When one such mutation occurs, it takes relatively few generations for that mutation to go to fixation (you'd know this if your model actually modeled selection). Once that happens, every new beneficial mutation that occurs will be occurring on a genetic background that already contains the first beneficial mutation.
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You are not doing your math correctly. Each generation a fixed sized population reproduces improves the probability of a beneficial mutation occurring. Each time any member of a variant reproduces improves the probability of a beneficial mutation occurring. A simple analogy is the more times you roll a die, the higher the probability that you will roll a one.
> So, here's a crude calculation (it's crude, but it won't differ by your full model by more than an order of magnitude or two, and as you'll see, that sort of difference won't change the conclusion).
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> Imagine a population of 10^5 organisms. Imagine that they each produce around 10 offspring of which, on average, only one survives to reproduce in the next generation. Imagine that by "beneficial mutation" we mean one that increases the chance that an organism bearing that mutation will be one of the approximately 1 in 10 lucky offspring that gets to survive to reproduce. And imagine that beneficial mutations happen at a rate of 10^-8 per replication.
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> So you have 10^6 replications per generation. You therefore expect one beneficial mutation in 100 generations. By the multiplication rule of probabilities you expect that double beneficial mutations will happen at a rate of 10^-16 per replication. So, roughly, you expect that it should take 10^10 generations for you to get a double beneficial mutant. This is based on using your model, without considering competition or selection, just the plain "physics of the rmns phenomenon" as you describe it. I can't really quantify how your model would incorporate "amplification" in such a system, because you never say. In any case, for a population of 10^5 organisms, you're looking at 10^10 generations for a time frame to get to two beneficial mutations in the same organism.
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> But realistically, what happens when there is selection and competition? You expect that within 100 generations there will appear a beneficial mutant. It takes a relatively marginal fitness difference between the mutant and the wild type for the mutant to become fixed within a few hundred generations, but let's say 1000 generations.
Here is where you go off track. That mutant is the progenitor of a new subpopulation which must now amplify in order to improve the probability of the next beneficial mutation. If that new variant must compete for the resources of the environment with other variants as well as continue to evolve against the selection pressure, the other variants will deprive the slightly more fit variant of the resources necessary to reproduce, slowing the evolutionary process. As the more fit variant drives the less fit variant to extinction, the more fit variant will have more resources necessary to reproduce.
> Then you have a population that consists entirely of those with the first beneficial mutation. Now we expect another 100 generations to get a second beneficial mutation. So within 1200 generations, rather than 10^10 generations you have a double mutant.
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> The difference between 1200 and 10^10 is why people have told you many times in the past that once you really consider selection in your model, the probability of a double mutant no longer is correctly described by the multiplication rule of probabilities. Before you cluck and tell me I need to take a basic probability course along with Harshman, just go ahead and show what you model predicts for the situation I described here. Don't just wag your head and say "the physics of rmns is governed by a nested series of binomial distributions"; go ahead and show how your model would describe the relatively commonplace scenario described here.
Bill, a population e5 replicating for e10 generations gives the exact same sample space as a population of e10 replicating for e5 generations. This is why with the huge populations' malaria can attain, it doesn't take very many generations for a double beneficial mutation to occur. And as long as mutations are random independent events, the multiplication rule will apply when computing the joint probability of two or more particular mutations occurring. Natural selection through absolute reproductive fitness can improve this probability by increasing the number of replications of the particular variants. That is the physics of rmns.