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.

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

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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

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

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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

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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

Thanks for the response Quetzal

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Originally posted by Quetzal:
XJ:

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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?

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Think of it in terms of a mutational pathway: each "state" along this pathway would have a different "mutational hotspot" and the series of mutational hotspots would define the "pathway". As you have indicated later on in your post, yes indeed this pathway would only pose a statistical likelihood of being "traversed"Now - think of the entire pathway as a conceptual "basin" - the stable centre of which is simply a point of probability. Thus, there is only a higher probability of the genetic state settling in that basin, as compared to any other arbitrary state. Forget about survival pressures for now; the whole point of this thought experiment is to conceptually dissociate the environmental pressures and the ensuing survival consequences from the mutations that occur - remember, this basin would determine the statistical likelihood of a mutation actually occuring while the environment would influence which mutations "survive" and are propogated.Now, my question in part is whether these pathways could incidentally be expressed by a gradual change in a specific phenotype. By this, let me try to illustrate: imagine a mutational pathway exists, by virtue of the successive hotspots that arise after each mutation. Now, in some cases, these very mutations will be expressed in a certain way in the organism. Sometimes each of these successive mutations will express completely unrelated "traits" - for example the first mutation would code for a protein X, and the second one would code for a completely different enzyme. Or even more abstractly, mutation A could alter the eye colour, while mutation B would influence production of haemoglobin, and mutation C would sprout an extra limb!However, I am asking whether it is feasible to postulate pathways that express a gradual change in a specific phenotype. For example: mutation A would express itself as a longer finger, mutation B would give rise to an even longer finger. And of course if this was the case, then all things being equal (for example say this set of organisms evolved in a completely (hypothetical) neutral environment) then there would be an increased statistical likelihood that over the eons, longer fingers would become the norm.Now back to the real world: let's say that longer fingers actually was a huge survival advantage. In this scenario, we have two independent "pressures" that direct the growth and survival of longer fingers: on the one hand, the survival advantage would facilitate an increased likelihood that longer fingered organisms would survive and reproduce more, etc... on the other hand, there is the mutative basin that guides the necessary mutations. In this case, the mutational basin would give momentum to the evolution of longer fingers.I hope this makes sense, and I appreciate all the responses so far.MarwanJulios, 12-19-2002

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Originally posted by Mammuthus:

However, this is contrary to what is expected. You would expect a normal distribution of mutations among individuals if it is a stochastic and random process and this is what is observed. Thus, you would not expect to settle in any one state.

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In other words, such hypothetical basins probably do not exist, and most mutational pathways are indeed not basins but rather a random accumulation of genetic states.

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Originally posted by Mammuthus:
Unfortunately, almost all of the "hotspots" are in non-coding sequences as those that do have an impact on a trait/protein are immediately selectively advantageous or disadvantegeous.

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Ah - I didn't realize this. However, is it feasible to postulate that what we might consider non-coding sites, are actually crucial in some ways? What we might consider "junk" DNA might actually be an integral part of a bigger equation? Going back to the shape of a nose - the fact that we do not understand the epigenesis of the shape of a nose seems to indicate that we have not cracked the code of DNA. As far as I understand, what we do know about gene expression is simply the manufacturing and regulation of protein production - perhaps there are deeper layers of information hidden inside the structure of DNA, and these apparent junk spots are actually parts of this information structure - pure speculation here, but would like to hear other's thoughts...

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Originally posted by Mammuthus:
I could envisage a trait that is not selected on becoming fixed in a population by chance i.e. by genetic drift you end up with an overall averge small size in the population though larger sizes would be equally supportable. The environment changes suddenly and small size is a huge advantage and maintains the state or causes average size to decrease further. However, this is not a directional mutation. This is plain evolution. Chance mutations that in a given environment are either advantageous, neutral, or detrimental. The larger the population (the population could be seen as your mutation basin) the more possible mutations exist (since a diploid organism only has two copies of any given nuclear gene), and the more possible genotypes available on which selection can work. That is why genetic bottlenecks can really suck for a species.

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So this genetic drift would also be a random process, but is it not interesting that such a drift would be expressed as a gradual phenotypical change? The (non-hotspot) mutations that facilitated this drift may indeed be random but the fact that they resulted in a gradual change in a given phenotype may indicate some deeper mathematical pattern - maybe something to do with the homology of the organism.You ever wonder at the almost supernatural quality of metaphors? The fact that one can take two seemingly independent and unrelated processes and create an analogue with them, shows that there is a mathematical pattern linking the two - for example - we talk about "waves" of fear or emotion - the analogy is between a physical wave, and the dynamics of perceived emotion - a wave starts out gradually, builds up to a crest, and propogates - the mathematical equations that govern the propogation of a wave can be applied to the dynamics of emotion. Perhaps these same sort of universal patterns underly genetic evolution, and thus give the appearance of intelligence or direction. I'm sorta groping in the dark here...

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Originally posted by Mammuthus:

Another scenario is that a highly selected trait is genetically linked to a completely neutral trait. The non-selected trait will get fixed in a particular state because of linkage with the selected trait. But this is not the same thing as what you are proposing because the non-selected trait is hitchhiking to fixation because of selection on a different trait.

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Yes exactly it would be hitchhiking - I guess my biggest question is how to explain gradual phenotypical changes - it doesn't jive well with my instinct to think of it as purely random - random in the sense that the mutations may occur at random (stochastically as you said) yes - but the fact that these random changes give rise to patterned growth may imply that selection alone may not account for this. This is not necessarily to say that there is intelligence governing the principles of evolution - but perhaps that there may be hidden (at least at present) patterns within DNA that allow for interesting and gradual phenotypical progressions even with a random mutative engine.Thanks for the reply - I think you finally understood what I was getting at.Marwan

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Originally posted by Brad McFall:
There is an alternative in the concept of neo-phenogenesis for which I tend to think salamander guilds can be explained. But this idea being against Darwikins more conservative postition is hard to get across. A one, Gottlieb, put the idea out under this rubric.

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Can you elaborate on what exactly this means? Also, I had a lot of difficulty understanding your last two posts before this one - they weren't very coherent

Thanks for the reply Quetzal

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Originally posted by Quetzal:
I think the reason it doesn't jive well is because you're forgetting the key "non-random" part of the equation. Natural selection is the primary engine of evolution. Random mutation simply generates the variation - a nearly continuous process - upon which natural selection operates.

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I haven't forgotten this part at all - the way I see it, the mutations determine which genotypes are "put to the test" or "judged by the context of environment", while natural selection determines which genotypes are more likely to survive.Thus, one could argue that mutation is just as much an engine of evolution as natural selection.Natural selection cannot explain the arising of phenotypes or changes of phenotypes, it can only explain which ones remain, and which changes are allowed to continue.I still find it mind boggling that phenotypes can change gradually with random mutation.

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Originally posted by Brad McFall:
Yes, but I need to "cool" down from the "holland" interaction. By "hot spot" I meant merely a short cut for my understanding of geneic balance and the whole issue of the position effect in general genetics and as for herpetology that will take some telling too. I will try to go back over all this with a fine toothed comb as soon as I dont feel like reading Einstein anymore.

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I have no idea what you're talking about. Please try to structure your posts coherently and logically. Are you sure you're posting in the right thread?