Evolutionary momentum

I was writing a paper on sociobiology, and I included a speculative argument about momentum of evolved traits (think increasing surface area of wings). I'll paste the relevant paragraph, and highlight the particular part I'm curious about. The context is of the predisposition that baby chimps have to acquiring fears of snakes. The study for anyone who is interested:Mineka S. (1987) A primate model of phobic fears. In H.J. Eysenck & I. Martin (Eds) Theoretical foundations of behaviour therapy. New York: Plenum Press.

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Laboratory studies have shown, that while baby chimps are not innately fearful of snakes, they acquire such a fear with remarkable efficiency after seeing even a videotape of another chimp reacting badly to a snake. Yet they do not show a similar adoption when viewing a fearful reaction to a flower. (Mineka 1987) While there are many different studies that attempt to pinpoint which aspects of such stimuli are responsible for the reaction, it is nonetheless a remarkable feat for the brain to be able to recognize such complexities at birth. For to do so, implies that somehow not only is there a genetic basis for this recognition software, but that it arose out of random mutation. Additionally, these genes not only have to be able to code for software that recognizes such stimuli, but also have to be associated with a fear response, or more precisely, a conscious experience that is painful to the creature. This very association must be presumed to be a result of random mutation, although it could be argued that once even a small fear response was in place, momentum took over and allowed for an evolved intensification of this fear. (the mutative mechanisms behind such a magnification effect would have to be entropically viable) Now it could be said that the power of the theory of evolution is such that it provides us with a model whereby such genes could arise, after all what is evolution but based on pure probability? However, because of this very virtue, it could be criticized on the basis that it isn’t very falsifiable.

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Now my question is this:Is is theoretically plausible to posit such a phenomenon (evolutionary "momentum"), given the known mechanisms of mutation? What about any speculative mechanisms?Is there any evidence of such phenomena?Thanks,Marwan

I guess another way of asking the question, or perhaps a related question, is whether there are any "stable" states for genes to be in. I know hardly anything about mutational hotspots, let alone the mechanisms of mutation, so I'm groping in the dark here.

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Originally posted by [xeno]Julios:
I guess another way of asking the question, or perhaps a related question, is whether there are any "stable" states for genes to be in. I know hardly anything about mutational hotspots, let alone the mechanisms of mutation, so I'm groping in the dark here.

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++++++++++++++++Hi XJ,I am not sure what you are trying to get at since your example is a behavioral trait (for which the underlying genetics is currently not known) but your second post is a genetic question. There are different types of stability if I grasp what you are asking in the second post. A specific allele of a gene can become fixed in a population i.e. the sequence is identical in most individuals, by selection or genetic drift. There are also more complicated examples where heterozygosity is maintained or balancing selection occurs which is also a stable state (unless the environment changes and one allele is favored over another or a genetic bottleneck occurs and one allele is lost by chance).An example of a mutational hotspot (or hotspots) is the mitochondrial control region or hypervariable regions I and II. There are a few conserved transcription domains in this segment but the rest is free to vary without being selected against. Coupled with an about 10 fold higher mutation rate in general in the mitochondria due to a relatively crap DNA repair system, some of the sites are mutational hotspots. They are free to vary with little or no selection against them. This is seen by the relatively high amount of variation within a species i.e. you and I are probably different in our hypervariable regions as compared to coding sequences like mitochondrial cytochrome b where you and I would likely be identical.I am not sure if this answers your question(s)As to the specific behavior question...schrafinator is our resident expert I believe.cheers,
M

Thnx for the reply M,I'll try to word my question differently - the question is a genetic one.Let's postulate that the intensity of an associate fear response is characterized by a certain amount of neurotransmitter X at the time of stimulus.Let's assume that there is a system of genes that produces and regulates this neurotransmitter (regulation would be highly indirectly I'm guessing)Let's also assume that the higher the intensity of the fear response, the more likely the organism is to survive.Now, once this system of genes arose that enabled the fear response, is it possible that further mutations could occur that would vary the intensity of the fear response by causing a more sensitive reaction to the stimulus (ie: more neurotransmitter released).And, assuming this is possible, and that it isn't simply an all or none principle, is it possible that there are "basins" of mutational states that exist, and as you go deeper into these basins, the more neurotransmitter is released?If so, then the answer to my question is an affirmative.Basically it's two-fold - can cumulative mutations arise that give rise to gradual changes in the phenotype - for example the surface area of a bird's wings - i remember hearing about how they gradually evolved to be bigger and bigger (the bigger they were the more effective the fanning function would be).The second face of the question is this: if such cumulative mutations can exist, is it possible that there will be a natural tendency for the phenotype to accumulate? (but independent of the selective advantages)From what I understand, mutational hotspots are a matter of chemistry - and certain chemical processes are more likely to occur than others.My understanding of genetics is very very limited - so it's hard for me to form these questions without dealing in broad abstract terms.I suspect that the genetic underpinnings of these phenotypes are far more complex than I am making them out to be - to illustrate:take the shape of a nose - let's say that the bigger a nose is, the better the survival (just for sake of argument). Now from what I have read, we don't really understand the epigenesis of facial structure - so it's hard to understand the mutational processes necessary to facilitate a change in nose shape/size, let alone conceptualize "mutational basins".-
Marwan

Hi Xeno.I'm really unclear what you're asking overall, but I would like to address this:

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The second face of the question is this: if such cumulative mutations can exist, is it possible that there will be a natural tendency for the phenotype to accumulate? (but independent of the selective advantages)

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I really don't know what you mean by "natural tendency". However, it is a well-established phenomenon that phenotypical changes in one area will often "force" or cause phenotypical changes in another. The simplest example I can think of is that you'll never see back legs increasing in length without a corresponding increase in length of the front legs. It's called frequency-dependent selection, and it can be observed at the phenotype, genotype, and molecular level.In addition, you can have non-adaptive selection. Sexual selection and Mullers Ratchet come to mind (c.f., peacock's tail).If you could explain a bit more what you're after, I may be able to come up with some specific references for you.

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Originally posted by Quetzal:
Hi Xeno.

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I'm really unclear what you're asking overall, but I would like to address this:

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The second face of the question is this: if such cumulative mutations can exist, is it possible that there will be a natural tendency for the phenotype to accumulate? (but independent of the selective advantages)

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I really don't know what you mean by "natural tendency".

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What I mean by natural tendency, is simply a natural tendency for the gene to further mutate towards a magnified phenotype without any selective pressures. Obviously, in such a scenario, if the magnification of this phenotype was advantageous, then the survival advantage would help guide the selection.So in a really really simplistic model - let's say that a gene exists like this:A-A-A-A-Anow a mutation occurs:A-A-A-A-Gthis causes a small fear response to be associated with the image of a snake.Let's say that A-A-A-G-G codes for a more intense fearA-A-G-G-G is even more intenseand finally:A-G-G-G-G is really intense, and also poses the best survival advantage for the host organism.Now obviously, left to nature, if any of the subsequent mutations arise, then A-G-G-G-G will be the best genotype and will probably be the end result of the evolutionary pathway.My question, is this:All other things being equal (ie no selective pressures), will there be a tendency for the following mutations:A-A-A-A-G ----> A-A-A-G-GA-A-A-G-G ----> A-A-G-G-GA-A-G-G-G ----> A-G-G-G-Gso that A-G-G-G-G would be a "mutational basin"I'm not asking if it would definitely mutate to that state, but rather if it is possible that mutational pathways exist that are likely to occur irrespective of the survival factor - ie - this tendency would be purely a matter of chemistry on the genomic level.There may indeed be mutational pathways that exist, but my question is even more tantalizing - do such pathways exist that also happen to result in an expression of a gradually magnified phenotype as the pathway is "travelled" along.Hope this makes it a bit clearerMarwanJulios, 12-16-2002

Hi XenoSo in a really really simplistic model - let's say that a gene exists like this:A-A-A-A-Anow a mutation occurs:A-A-A-A-Gthis causes a small fear response to be associated with the image of a snake.Let's say that A-A-A-G-G codes for a more intense fearA-A-G-G-G is even more intenseand finally:A-G-G-G-G is really intense, and also poses the best survival advantage for the host organism.Now obviously, left to nature, if any of the subsequent mutations arise, then A-G-G-G-G will be the best genotype and will probably be the end result of the evolutionary pathway.M: Actually, this is not necessarily the case. Evolution does not always lead to the absolute best outcome. If one phenotype has an increased chance of reproductive success over the others, it will be more successful. Thus in your scenario, if most individuals are A-A-A-A-A and the mutation A-A-A-A-G occurs, the latter could become fixed and you may never reach the optimal A-G-G-G-G.X:
My question, is this:All other things being equal (ie no selective pressures), will there be a tendency for the following mutations:A-A-A-A-G ----> A-A-A-G-GA-A-A-G-G ----> A-A-G-G-GA-A-G-G-G ----> A-G-G-G-Gso that A-G-G-G-G would be a "mutational basin"I'm not asking if it would definitely mutate to that state, but rather if it is possible that mutational pathways exist that are likely to occur irrespective of the survival factor - ie - this tendency would be purely a matter of chemistry on the genomic level.M: I would say the answer is a qualified yes. Qualified because there is no directed pathway that would guarantee that A-G-G-G-G would ever appear in the population. The yes comes from the chemical bias for transitions over transversions i.e. C-T changes are more likely than C-A for chemical reasons...thus, some types of mutations are more likely than others.X:
There may indeed be mutational pathways that exist, but my question is even more tantalizing - do such pathways exist that also happen to result in an expression of a gradually magnified phenotype as the pathway is "travelled" along.M: You seem to be interested in neutral evolution. For example, without selective constraints, some genomes grow and grow in size because of repetitive DNA. So at some level this is your gradually magnified process.cheers,
M

Thanks for the reply again M

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Originally posted by [xeno]Julios:
Now obviously, left to nature, if any of the subsequent mutations arise, then A-G-G-G-G will be the best genotype and will probably be the end result of the evolutionary pathway.

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Originally posted by Mammuthus:
M: Actually, this is not necessarily the case. Evolution does not always lead to the absolute best outcome. If one phenotype has an increased chance of reproductive success over the others, it will be more successful. Thus in your scenario, if most individuals are A-A-A-A-A and the mutation A-A-A-A-G occurs, the latter could become fixed and you may never reach the optimal A-G-G-G-G.

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That's why I said if any of the subsequent mutations arise, they would likely result in a stable A-G-G-G-G state. Anyway, it's just the base model I included to ground the question.

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Originally posted by [xeno]Julios:
I'm not asking if it would definitely mutate to that state, but rather if it is possible that mutational pathways exist that are likely to occur irrespective of the survival factor - ie - this tendency would be purely a matter of chemistry on the genomic level.

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Originally posted by Mammuthus:
I would say the answer is a qualified yes. Qualified because there is no directed pathway that would guarantee that A-G-G-G-G would ever appear in the population. The yes comes from the chemical bias for transitions over transversions i.e. C-T changes are more likely than C-A for chemical reasons...thus, some types of mutations are more likely than others.

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Ok - so implicitly, you are saying that such mutational pathways do exist that are likely to be followed purely on chemical grounds alone. If that is the case, then the first part of my question is answered.

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Originally posted by [xeno]Julios:
There may indeed be mutational pathways that exist, but my question is even more tantalizing - do such pathways exist that also happen to result in an expression of a gradually magnified phenotype as the pathway is "travelled" along.

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Originally posted by Mammuthus:
You seem to be interested in neutral evolution. For example, without selective constraints, some genomes grow and grow in size because of repetitive DNA. So at some level this is your gradually magnified process.

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Yes this is the heart of the question; neutral. But I'm not sure how to interpret your answer. If i understood your comments correctly, you have implied that it is theoretically possible for mutational basins to exist in a neutral context. I'm not convinced you understand the real question I'm asking:is it possible that a gradually magnified phenotype will be expressed by this "neutral" mutational pathway (for example the increasing size of a bird's wings - forget the fact that it serves a survival purpose).Thanks,Marwan

Hi Xeno,
I still don't think I get the question. Increase in size of a bird wing without selective constraints would unlikely reach a steady state. It would increase or decrease randomly within the population without consequence. A specific size might be reached and remain stable for a period of time if there were a genetic bottleneck. But if the population expanded again the trait would also begin to vary.As to mutational load or a mutational basin, I guess you mean heterozygosity? Most diploid species have some level of heterozygostiy including recessive lethal mutations (why inbreeding is such a bad idea). But this may not be what you are getting at either...sorry to be so dense.cheers,
M

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Originally posted by Mammuthus:
Hi Xeno,
I still don't think I get the question. Increase in size of a bird wing without selective constraints would unlikely reach a steady state. It would increase or decrease randomly within the population without consequence. A specific size might be reached and remain stable for a period of time if there were a genetic bottleneck. But if the population expanded again the trait would also begin to vary.

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That's why I'm stressing the factor of the mutational pathway being feasible through chemical processes alone. If anything, the survival consequences of these mutations would reinforce the basin, but alone would not account for the stastical likelihood of these mutations becoming widespread.Do you understand what I mean by a mutative basin?The increase in wing surface area would not spread randomly without selective constraints if there was such a neutral basin underlying the phenotype's progression. That is the whole point of the basin! If such a basin existed, then it would justify the speculation that I posed at the beginning of this thread (in the quote out of the essay I was writing) about "evolutionary momentum" - momentum being the key word here, since chemical processes add directional force to the survival trend.

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Originally posted by Mammuthus:
As to mutational load or a mutational basin, I guess you mean heterozygosity? Most diploid species have some level of heterozygostiy including recessive lethal mutations (why inbreeding is such a bad idea). But this may not be what you are getting at either...sorry to be so dense.

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I don't know what heterozygosity or diploid means - i have a basic understanding of mendelian genetics, so i know what recessive vs dominant means (on basic terms). But I think you're right - probably doesn't address the main issue.Perhaps re read the thread carefully and it'll make sense - if not I'll try rewording it some more. Either way I thank you for taking the effort.peaceMarwan

I'm not sure that this response would necessarily be mutation based.
I believe that, ultimately, everything is genetic, but a particular response need not be. It seems to me that an overall pattern of response would or could be 'hard wired,' but there is so much variation/plasticity in specific responses that it seems to me that this would be a behavioral issue, not a genetic one.

Hi XenoThat's why I'm stressing the factor of the mutational pathway being feasible through chemical processes alone.M: Ok, the mutational process is by chemical processes alone in any event i.e. C to T transitions are a chemical change. There are other types of chemical mutation that do not change the DNA sequence called imprinting.X:
If anything, the survival consequences of these mutations would reinforce the basin, but alone would not account for the stastical likelihood of these mutations becoming widespread.M: However, if it has a survival consequence then it is going to be selected for or against and is neutral.X:
Do you understand what I mean by a mutative basin?M: I am not sure. I have never heard this term before.X:
The increase in wing surface area would not spread randomly without selective constraints if there was such a neutral basin underlying the phenotype's progression.M: However, mutations will occur at random and under neutrality will be lost or fixed in the population at random so in this model, the trait would vary randomly. You would probably end up with a very large distribution of wing surface area since no one phenotype would have an advantage over the others.X:
That is the whole point of the basin! If such a basin existed, then it would justify the speculation that I posed at the beginning of this thread (in the quote out of the essay I was writing) about "evolutionary momentum" - momentum being the key word here, since chemical processes add directional force to the survival trend.M: The chemical processes generate the novel mutations upon which slection will act if it is directional. Otherwise it is neutral and will be more free to vary...of course it could be in a non-recombining part of the genome etc and not mutate very much but there is nothing constraining it. There is also no intrinsic amount of variation one can expect in a given population. This will be determined by environmental stress i.e. radiation, repair enzymes, effective population size and a host of other factors. So I don't really see directional momentum without selection i.e. an intrinsic tendency for a trait to consistently change in one direction without any advantage.cheers,
M

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Originally posted by SLPx:
I'm not sure that this response would necessarily be mutation based.
I believe that, ultimately, everything is genetic, but a particular response need not be. It seems to me that an overall pattern of response would or could be 'hard wired,' but there is so much variation/plasticity in specific responses that it seems to me that this would be a behavioral issue, not a genetic one.

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This is besides the point - the whole purpose of the question is assuming such a genetic based behaviour existed...

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Originally posted by Mammuthus:

X:
If anything, the survival consequences of these mutations would reinforce the basin, but alone would not account for the stastical likelihood of these mutations becoming widespread.

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M: However, if it has a survival consequence then it is going to be selected for or against and is neutral.

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Forget the survival consequences for now - conceptually dissociate survival consequences for the time being.

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Originally posted by Mammuthus:
X:
Do you understand what I mean by a mutative basin?

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M: I am not sure. I have never heard this term before.

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That's coz i just made it up - here's a crude analogy that might help: Think of a bowl or a basin. Now put a marble into it and swirl the bowl for a few seconds. The ball will start to roll around the sides of the basin until it settles in a stable state, most likely at the bottom of the bowl. This is because of the way gravity, and the curve of the inner surface of the basin, interact. Similarly, by mutative basin, I mean a genetic state that will arise because of mutational hotspots (the surface of the bowl), and the laws of physics and chemistry (gravity). It has absolutely nothing to do with survival consequences. Survival consequences will determine which mutations are allowed to persist, while mutative basins will determine which mutations are likely to occur in the first place!!

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Originally posted by Mammuthus:
X:
The increase in wing surface area would not spread randomly without selective constraints if there was such a neutral basin underlying the phenotype's progression.

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M: However, mutations will occur at random and under neutrality will be lost or fixed in the population at random so in this model, the trait would vary randomly. You would probably end up with a very large distribution of wing surface area since no one phenotype would have an advantage over the others.

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No. If there is a mutative basin, then the whole point is that mutations do not occur at random. It would be a tantalizing coincidence if such basins coincided with useful phenotypical progressions!!

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Originally posted by Mammuthus:
X:
That is the whole point of the basin! If such a basin existed, then it would justify the speculation that I posed at the beginning of this thread (in the quote out of the essay I was writing) about "evolutionary momentum" - momentum being the key word here, since chemical processes add directional force to the survival trend.

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M: The chemical processes generate the novel mutations upon which slection will act if it is directional. Otherwise it is neutral and will be more free to vary...of course it could be in a non-recombining part of the genome etc and not mutate very much but there is nothing constraining it. There is also no intrinsic amount of variation one can expect in a given population. This will be determined by environmental stress i.e. radiation, repair enzymes, effective population size and a host of other factors. So I don't really see directional momentum without selection i.e. an intrinsic tendency for a trait to consistently change in one direction without any advantage.

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The direction that the pathway takes will be (in part) determined by the "slope" of the basin. This slope will be determined by the series of mutational hotspots that exist along the pathway. The whole essence of the hotspot is that a mutation is more likely to occur at that site due to chemical tendencies - ie the mutation is not solely random based.thanks for your perserverenceMarwan

XJ:I think I'm beginning to understand what you're trying to get at here. Are you asking whether consecutive mutations at a "mutational hotspot" that might have a neutral phenotypical effect can accumulate phenotypical change over time?If that's the case, then I would say that theoretically there would be no reason to preclude this from occurring. I can't think of any actual example where this might have occurred, however. I would also hasten to add that cumulative change in a trait - whether from mutation at a hotspot or not - will at some point come under selective pressure even if it starts out neutral. Your bird wing example threw me - increase or decrease in the area of a bird's wing will have an adaptive effect (either positive or negative).I also have a problem with this statement:

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If there is a mutative basin, then the whole point is that mutations do not occur at random. It would be a tantalizing coincidence if such basins coincided with useful phenotypical progressions!!

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I thought you were equating your "mutative basin" concept with mutational hotspot. There are three problems here:1. Mutational hotspots are only non-random in a statistical sense. That is, there is an increased probability that the particular nucleotide site will undergo mutation because of various factors. It is still random in the sense that you cannot predict when or even if a mutation will occur at that site except stochastically over many generations. In other words, IF a specific mutagen is introduced somewhere in the genome, there is a slightly higher probability that it will effect the hotspot site than some other location. It doesn't mean it will.2. Once you start bringing in "useful" in relation to a phenotypical trait, natural selection comes into the game, because "useful" is only possible to be understood in the context of the environment. "Useful" indicates that there has been an increase in marginal fitness. This holds true whether you are dealing with the genome or organism level.3. "Tantalizing coincidence" seems to be a bit loaded. It would certainly be a coincidence if a randomly-induced neutral phenotypical trait/change accidently produced an increase in fitness (in which case it would no longer be neutral) OR if an environmental change caused the neutral trait that has been slowly accumulating to suddenly have a positive (or negative) fitness effect. Still, the only part of this that is non-random is the selection pressure when/if it arises.I hope this addresses your question.[This message has been edited by Quetzal, 12-19-2002]