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Author | Topic: The End of Evolution By Means of Natural Selection | |||||||||||||||||||||||||||||||||||||||||||||||||||||
Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
I've been wanting to see if I can do a better job on my original topic here: Natural Limitation to Evolutionary Processes . As I've reread that old thread I see I got overwhelmed and defensive and didn't do a very good job of thinking through my answers. In some cases I never got back to answering some people at all, Pink Sasquatch in particular, and I'd like to try again.
In a nutshell: Evolution is said to be powered by natural selection, and some say other processes contribute as well, such as genetic drift. You will also find in definitions of evolution on some websites the statement that variability is necessary for evolution to occur. My argument is that natural selection and genetic drift, all the processes that select or isolate a portion of a population, do bring about the change called evolution but also always reduce genetic variability, which is the opposite of what evolution needs. Paul K answered my opening post on that thread this way: If you accept for the sake of argument that new variation is entering the population through mutation then what happens depends on the rate at which new variations appear against the rate at which variation is lost through selection. Only if the rate at which variation is lost is greater than the rate at which new variation enters the population will there be a net decrease in variation. This is the answer that is always given to this argument, and Pink Sasquatch gives another version of the same answer in her post that follows PaulK's. I answered in turn many times that increase in variation (sometimes called "information") doesn't prevent this ultimate reduction -- you can have as much variability as mutation or any other source of genetic variability can provide, but the processes that select and isolate, which are considered essential to evolution, inevitably work to reduce the needed genetic variability, and this spells the end of evolutionary processes. It seems to be generally overlooked that for evolution to occur, alleles must be eliminated, thus reducing genetic diversity. But I realize this has to be demonstrated. Genetic drift is demonstrated at Wikipedia with a series of jars representing subsequent generations of a population through which drift occurs, demonstrating that while the population starts out with equal proportions of blue and pink alleles, over time drift completely eliminates the pink ones leaving only the blue, a complete elimination of one allele. The same thing happens but more systematically if natural selection is doing the eliminating. Then if you think mutation can save the day, all that happens is that the mutated allele gradually eliminates all the other alleles, once again eliminating genetic diversity. This is only what happens to one gene of course, but the trend is inexorable. There is no way to get a trait established in a population if alleles in competition with the allele for that trait are not eliminated. However, there's usually more going on than just the establishment of a single allele. Even in natural selection for a single trait you have to remember that individuals carry these traits and their alleles, and its the individual that is selected for its trait. This individual's genome is full of other genes for other traits, so when it passes on its selected allele it also passes on all the rest and over time if this trend continues the population will simply become more like that individual all around and alleles for that individual's traits will all have been reduced or eliminated, substantially reducing the genetic variability in the gene pool as a whole. However, it is possible of course for the single allele to work its way through the population without remaking the whole population in the image of this individual, if it is that particular trait that has special reproductive value from generation to generation. This seems to be the point Pink Sasquatch was making. Then you will only have eliminated alleles for that trait. You'll have a population of frogs with better ability to catch flies as she was saying, but you'll be pretty far short of a new species. Small changes occur in populations all the time for various reasons without getting anywhere near speciation. With a bottleneck, or domestic selection, on the other hand, where a phenotype is selected (or randomly isolated in the case of bottleneck) and is completely reproductively isolated, then you get the dramatic changes in many traits at once, and when you have that you also have dramatic genetic reduction for all those traits. Often this situation leads to inability to interbreed with former populations and is known as a new species. I've always liked the cheetah example because it is a case of a wonderfully selected animal that demonstrates extreme genetic reduction, to the point of fixed loci for many traits. But whether we are talking only about a change in a single trait or in many traits at once, the trend is ALWAYS toward genetic depletion. You can add as many new alleles as you think mutation can come up with at any point in this progression, but when these selection and isolating processes go to work on them the very same thing happens. You may get a new trait but you'll always get it at the expense of all the other genetic possibilities, and when this occurs with many traits you eventually get speciation, fixed loci, and such limited ability for further variation evolution is for all intents and purposes at an end. To my mind this absolutely spells the end of evolution. Evolution itself defeats evolution. Edited by Faith, : to fix url Edited by Faith, : No reason given. Edited by Faith, : No reason given. Edited by Faith, : No reason given. Edited by Admin, : Fix quote: [q] => [qs]
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
Hello Paul K.
This isn't a simple addition and subtraction problem. If you start with twenty alleles in a population for one gene and one of them becomes crucial for a particular environment and therefore gets selected, either rapidly or slowly depending on the selection pressure, you will lose the other nineteen alleles as the one selected comes to determine this particular trait. One allele causes the subtraction of nineteen, reducing genetic diversity. If it happens to have been a mutation, an addition, it will still have the same effect, subtracting twenty in that case. It would make no sense to add new alleles for that trait either, as it has been selected because of its value for the species. Selection REQUIRES the elimination of alleles. Addition would only kill selection and kill evolution. Addition means stasis, not evolution. Selection (or isolation of a portion of the population) is essential for evolution to occur. Variability may remain in the rest of the population, as Pink Sasquatch emphasized, but that isn't evolution. Evolution requires this subtraction process. And again, NS keeps subtracting everything except what it selects for its survival/reproductive value. If you start with twenty alleles in a population for one gene and 10% of the population migrates to new territory taking five of those alleles with them, you've already lost fifteen and the new population will develop a characteristic phenotype from the five it had, along with whatever proportion of other alleles for every other trait in the population that also went with it. This may be the most common way speciation occurs. Ring species apparently develop this way. New populations are formed from a few members of the previous population, and at each migration new traits come out as many alleles from the old population are left behind. Again, you only get new phenotypes by favoring particular alleles at the expense of others. That can occur by losing them altogether in a migration as well as by selection and drift within a population. I believe my little drawings at the links show this. Simply denying it isn't evidence against it.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
Hi Faith. Welcome back. The problem behind your theory is that it's contradicted by real-world observations. Your predictions are invalidated, because observed evolution has not been reducing genetic capacity for variance. Hi Rahvin. Actually, observed evolution does demonstrate this when you focus on endangered species. It is probably not Natural Selection that has brought about their endangered condition since supposedly that would be adaptive and not endangering, but it is brought about by processes that isolate a small portion of the genetic variation formerly available to the whole population, which is just a more drastic version of what NS does. As I imply above in my response to Paul K I believe perfectly viable healthy populations can develop from such random reductions in genetic diversity, as most likely is the case in Ring Species. A decimated population such as the seals which were hunted to near extinction, may actually come back in large numbers, but they will come back with much reduced genetic variability compared to their original population. Surely this is obvious? Unfortunately in many cases such a situation does threaten the survival of a species and conservationists are always having to deal with these situations.
Year after year, undergraduate students directly observe evolution in action as they show changes in allele frequency in fruit flies. Other students observe the case of drug resistance spontaneously forming in a population of bacteria. It's certainly true that we don't have to worry about fruit flies and bacteria becoming endangered species. You are going to have to prove to me that you can get significant changes in fruit fly phenotypes without a reduction in genetic diversity, that is, you can bring about a new population characterized by this change without losing genetic diversity. As for bacteria I'm not enough up on the genetics involved, but what you are describing is some sort of mechanism for increasing their variation, not the selection that reduces it, which is what I'm focusing on. Their ability to evolve a drug-resistant strain even when reduced to a single allele is very interesting but it MUST be accompanied by a severe genetic reduction leaving ONLY the allele for that particular strain no matter how the original came about. Or are you claiming that you see a multiplication of new alleles from a condition of total genetic depletion? Again, even if you are, again this is increased variation, not selection and when you have it you no longer have evolution, you no longer have a drug-resistant strain or whatever else you were aiming to get. It's selection and isolation that bring the desired trait to expression in the phenotype, drug resistance in the case of bacteria, and this always involves decreased genetic variability. At least you haven't shown me how it doesn't.
The process doesn't stop. Variation continues, unimpeded. There is no reduction in the possibilities derived from mutation guided by natural selection. At no point to we reach an evolutionary "endpoint" where no more change is possible. There may, hypothetically anyway, be no "reduction in the possibilities derived from mutation," but when you add "guided by natural selection" you are implying something that can't in fact happen. Natural selection "guides" by doing what I've been describing, by eliminating all that variation mutation has brought about so that the selected variant can come to characterize the population.
And that's just the examples that we directly observe. The fossil record and the other extant life we see today has variety beyond comprehension. {Edit: I have to add in here somewhere a warning to myself as well as you that it's easy to get the terms "variation" and "variability" and related concepts confused with each other and even lose the whole point. This has happened to me and I'm trying very hard to avoid it but it will probably still happen. The fact that there is lots of VARIETY in nature, in phenotypes even within the same species, is a different matter from the amount of genetic VARIABILITY that is present in a population} The fact that great variety exists in nature is a very good thing, but this doesn't change the fact that when evolution occurs in a population it HAS to reduce genetic diversity. That's the only way you get a new variety. I have to postulate that there used to be much much more variability in all species than there is today in most species, BECAUSE of the evolution that has been going on creating new varieties (or "species" if they can't interbreed with their parent population) over millennia. Some species retain enormous variability nevertheless -- dogs for instance. The most amazing varieties of dogs have been brought about and yet they can still interbreed -- THAT's enormous built-in variability there. But other species don't have that much variability, or have lost it in their successive branchings down the centuries. Consider the dog example while we're at it. Every breed of dog MUST show reduced genetic variability compared to its population of origin because if you want it big you're going to have to eliminate everything that tends to smallness, if you want it good natured you have to eliminate everything that breeds for ferocity, and so on. Dogs as a species have enormous genetic variability but a particular dog variety or species has to have very little. You are ELIMINATING alleles in order to bring about your favored breed. Assuming that natural selection operates in a similar fashion in nature, that's what has to happen there too. You are not going to get a new variety, breed or species without a loss of genetic variability. This is really a law of genetics.
The genetic and morphological evidence for common ancestry of virtually every living thing on the planet is overwhelming, to the point that it's better established than the Theory of Gravity. Given that this is the case, and populations continue to diversify into distinct sub-groups before our very eyes, it would seem that your premise, that evolution should grind itself to a halt through some sort of genetic entropy, is falsified. The evidence for common ancestry is going to have to be rethought if it turns out that evolution beyond a series of built-in variations is impossible because of the laws of genetics. Edited by Faith, : No reason given.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
Hi Subbie:
You are ignoring a key aspect of evolutionary diversification; reproductive isolation. I usually have reproductive isolation uppermost in my mind as THE factor that brings about the new phenotype BY ensuring the reduction in genetic diversity or variability. Isolation is THE engine here, not just Natural Selection, which is really a very weak form of isolating factor, or genetic drift or bottleneck but all these processes. They all isolate a portion of a population with respect to a selected trait in the case of NS, or randomly with respect to some unknown factors in the case of genetic drift, or randomly with respect to whatever complement of alleles are contained in the new population in the case of bottleneck and founder effect. It's the isolation from other alleles that does all this. It effectively eliminates all those other alleles from the population under consideration. Alleles can be eliminated by killing them off as in a catastrophe that leaves only a few living members of a species, or by keeping them from affecting the evolving population one way or another. Absolutely reproductive isolation is not being ignored, it's of the essence of what I'm describing. But I'm not smart enough to be able take up more than one piece of this puzzle at a time.
You are correct that if one trait becomes so dominant in a population that it drives competing traits out of existence, that would result in a decrease in variability in that population. Well, take that a bit further and agree with me that this IS what natural selection must do if a new adaptive trait is to be established. That is, this IS evolution by natural selection. (There are certainly less drastic versions of this, such as the "peppered moths" example that adapt to the color of their resting place, which as far as I know have always retained the ability to revert to the previous type and so, while they are an example of natural selection, they aren't an example of evolution to the point of speciation where you actually get a new "species.")
What you don't address is the fact that there are still other populations that possess the "eliminated" traits. To illustrate this, let me expand on the Wiki example you use. Let me just comment before I get to the Wiki example that I haven't addressed this YET. I've certainly had it in mind. In the case of Ring Species, alleles no longer possessed by one population in the series can be found in the former populations. Same in the case of a bottleneck where a small number of individuals are merely separated from a much greater number -- the greater population will retain what the new population has lost. The new population will develop a "new look" like Darwin's Galapagos turtles, and soon not be able to interbreed any more with the old population because of genetic differences, and they will have much reduced genetic diversity compared to the mother population. The point I'm making is that WHEREVER EVOLUTION IS GOING ON, THAT'S WHERE you'll have a reduction in genetic diversity, and that is completely at odds with the theory of evolution. It should be exactly at the point a species is evolving that you are getting increasing variability, but in reality you are getting the opposite. You may have tons of variability/diversity left in the mother population but that's not the evolving population.
Let's say we take a relatively homogeneous population and separate it into two jars, and keep track of the pink and blue alleles. It's entirely conceivable that either the pink or the blue might disappear from one population over time through nonselective processes. It's even possible that the pink might disappear from one and the blue from the other. However, since these populations were reproductively isolated from one another, overall there has been no total loss This is correct, as I anticipated in my paragraph above. Again, it is the separated populations that are evolving, through genetic drift in this case apparently, and it's where we see evolution that we see the reduction in genetic diversity. If you recombine these two populations before they've reached such genetic incompatibility that they can't interbreed the evolution stops. Gene flow interferes with evolution. You've got to have isolation and reduced genetic possibilities for evolution, for change in the phenotype, to occur.
While separating the populations into two jars is an artificial isolation, this type of isolation happens all the time in the real world. It is this isolation that creates different species. (Well, not entirely true, as speciation can occur over time as well, but that's not really relevant to this discussion. I mention it only so someone else doesn't come in and point out the error.) I agree. From what I've read, speciation most often occurs from this sort of random isolation of populations, as in ring species. Yes, it can also occur within a population by factors that bring about reproductive isolation even there, as you point out, but the example of simple geographic isolation without the possibility of gene flow (=reproductive isolation) appears to me from my reading to me THE main way speciation comes about. Again, the point I am making is that wherever you can point to an evolving population due to such isolating or selecting factors, you are inevitably going to have reduced genetic diversity. It doesn't matter how much diversity is left in OTHER populations. The point is that WHERE THE POPULATION IS EVOLVING THAT IS WHAT IS INEVITABLY HAPPENING.
[qs]I doubt that few here would disagree that from, time to time, some traits will disappear from some populations, and as a result, the total variability in that population is reduced. However that is not nearly the same thing as saying that evolution "inevitably work[s] to reduce the needed genetic variability." [/qs] If what you mean by some traits disappearing from some populations is that those populations are evolving with respect to those traits, the phenotype is changing, the gene frequencies are changing etc., then the variability that is lost as a result IS the same thing I'm talking about. No matter how slight the tendency, the tendency is ALWAYS in the direction of reducing genetic variability. And if genetic variability is needed for evolution, as it is, then there you have it.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
I would like to try to deal with every argument in this thread so I may get bogged down with a particular poster from time to time, as with Phage here, if I can't quite grasp what is being said. (I'm not going to answer Coyote of course. Nobody's conceded evolution is lost yet so wait until that happens ).
My argument is that natural selection and genetic drift, all the processes that select or isolate a portion of a population, do bring about the change called evolution but also always reduce genetic variability, which is the opposite of what evolution needs. That is true; if only a subset of the population reproduces then the variability in the set that did not reproduce is lost. Well stated, thanks.
Your argument appears to be based on the concept that because a subset of all possible mutations is being selected, the population must necessarily trend toward a single genetic standard. I'm not sure I'm following you here. I've been keeping my focus as much as possible on a SINGLE mutation (or allele) for the sake of simplicity and clarity (I hope), knowing that of course reality is much more complicated -- rather than a "subset" of mutations, but perhaps that isn't the problem for me so much as "a single genetic standard." That's what loses me I think. Being focused on a single mutant or allele means I'm focused on a single trait and tracing it through a number of generations in which it is reproductively "successful" as they say, during which series of reproductions it gradually replaces (eliminates) other alleles for the same trait that are continuing to be passed down by other members of the population as well, only not with quite as great success. Eventually this allele will replace them all if the selective pressures or other factors continue to favor it reproductively. This isn't a concept I start with, it's where these processes end up. So I'm not assuming anything about a "single genetic standard" which I don't think I understand in any case, but constructing an argument to show that selection and isolation single out a particular trait by eliminating all its competition and ultimately make that trait characteristic of a new population that emerges from these processes. Having said that let me try to grapple with the rest of your post:
To illustrate where this goes wrong, consider this hypothetical bloodline: An organism has 5 genetically distinct offspring (they each have their own unique variations). Two of these do not reproduce, the remaining three having been "selected". OK, a source of confusion for me is that you have so many different variations going on whereas I've been trying to keep the focus on one at a time. But I'll try to follow you out. So we now have three offspring that will go on to reproduce. The focus shifts from a single allele for a single gene to a whole genome full of variations from which what is being selected is beyond our ability to know. All I can say is that EACH of these selected individuals, as long as its offspring remain "pure" and its line continues to stay selected, not sharing genes with the other selected individuals, will progressively lose genetic variability down the generations as its traits become established in those generations to the point of forming a new population. The likelihood that there will be no interbreeding with the other lines is very low, however, and therefore the likelihood of preserving the selected type is low, so it's doubtful whether we're even talking about the processes of evolution I have in mind. (Each "selected" line would have to move to its own private isolated island to show what I'm trying to show).
These three in turn have 5 genetically distinct offspring each, 2 each of which do not survive (6 lost, 9 reproducing). You seem to be claiming that because the theoretical possibility of having 25 genetically distinct organisms descended from 5 bloodlines is being reduced through selection, that the genetic diversity must necessarily trend toward zero. The genetic diversity in EACH of those "bloodlines" DOES necessarily trend toward zero AS LONG AS THEY MAINTAIN ISOLATION FROM OTHER LINES. You have to keep the focus on the SINGLE line that is evolving, even if there are three or five separate such lines. In EACH line the traits will be changing according to the "founder" at the SAME time alleles for competing variations on those traits will be replaced or lost, and finally ultimately completely lost if the line ever reaches the point of speciation or inability to interbreed with its cousins.
The problem is that in this example we are left with 9 genetically distinct organisms descended from 3 distinct bloodlines, when we started out with one. The series [1, 3, 9] does not appear to trend toward zero in my view. I did have to take this step by step because it was hard for me to follow, but now I think I can say that the problem here is that you are treating the number and variety of "grandchildren" as a collection rather than as a line of evolution. Or something like that. As long as they remain part of the same population with the constant possibility of interbreeding you don't have evolution as I am describing it. Yes, any individual with its genetic peculiarities COULD be the starting point of a completely new evolved population, and yes, any reproductive success is a step on the way to evolution, but you've got to eliminate gene flow and favor/select a particular line to the COMPLETE EXCLUSION of other lines to get what I'm trying to point out here. That's where you see the trend to reduced genetic diversity. In your scenario, assuming the trend continues, you will continue to get new generations of genetically distinct individuals, some passing on their distinctions, some not, but in the end you're only going to have a bigger population of the same species with all its usual variation and variability scattered throughout. Genetic drift (or if you prefer the "selection" that confers reproductive success) will probably have altered its overall "look" somewhat by that time, but this isn't evolution in the strict sense. It's a very healthy state for a population to be in, but it's not evolution except in the most trivial sense. I hope I grasped your point. Edited by Faith, : added a line
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
Greetings, nwr:
My argument is that natural selection and genetic drift, all the processes that select or isolate a portion of a population, do bring about the change called evolution but also always reduce genetic variability, which is the opposite of what evolution needs. That's not quite right. Yes, selection reduces variation. More to the point I'm trying to get at, it reduces genetic variability or the number of alleles for the trait selected.
As far as I know, genetic drift does not affect variation. And mutation increases variation. Yes, mutation increases variation (although I'd point out that it does so by replacing another probably quite viable allele, and since many mutations are not desirable, the net effect may be a loss but I digress) and according to the Wikipedia demonstration I linked, which I will link again here, genetic drift DOES reduce variation and variability. And it makes sense that it would since the idea is that it brings about a change in gene frequencies just as all the other evolutionary processes do, only through a sort of random selection process within a population.
You are correct, that if there were only processes that reduce variation, then eventually evolution would run out of variation and would stop. But as long as there are also processes that increase variation, there is no reason to expect evolution to stop. I have to refer you to my answer to Paul K here. This isn't a simple addition/subtraction. The subtraction that is done by the selective and isolating processes (NS, drift, bottleneck etc.) actively GETS RID OF competing alleles for that trait. So if by some highly improbable chance mutations simultaneously produced thirty new alleles for the trait (again by replacing thirty of a type that was probably perfectly viable in thirty individuals) IF ONE OF THEM IS SELECTED AGAINST ALL THE REST, then all the rest might as well not have existed at all as the selected one will gradually (or possibly rapidly) eliminate them all. Of course under some circumstances in which the selected trait gets completely isolated from the mother population those many new traits may continue on in the greater population, but as far as the selected population goes, it will gain its new adapted phenotype THROUGH the loss of all the other alleles. And then that mother population with all the new traits will have an interesting array of variations at that locus (29 different hair types, 29 different coat patterns, 29 different ear shapes or whatever) but this isn't evolution, unless you want to completely rethink and redefine what evolution is. According to the usual definition, announced quite persistently by Dawkins for instance, it's always evolution BY natural selection, that's what evolution IS.
As far as I know, what is mostly noticed is that variation stays fairly constant. In populations where evolution is not going on this is true. I believe that was the observation embodied in the Hardy-Weinberg principle.
A bottle neck, such as caused by isolation of a small population, can result in reduced variation. No doubt, it DOES, and severely reduced GENETIC variability too. Along with some very interesting phenotypes that may even qualify as new species.
But the variation is rebuilt during succeeding generations. I bolded that statement because I believe that is wishful thinking that is not borne out in reality. If this were true conservationists wouldn't be wringing their hands over the small numbers of salmon that get themselves up a tributary where undesirable reproductions bring undesirable traits to the fore along with extremely reduced genetic possibilities for further change; and there wouldn't be a club for the preservation of the cheetah. While all along in this discussion I am assuming for the sake of argument that useful mutations do occur at a rate that allows them to become the basis for selected traits, in reality this simply doesn't happen, as all too many conservationist scientists in the field ought to be able to testify. HOWEVER, AGAIN, EVEN IF the variability COULD be rebuilt, you'd STILL be facing the exact same situation. You'll either once again have a nonevolving population, OR with selection a portion of it will vary/evolve possibly on out to speciation as it loses its genetic variability. When there is active evolution, the production of new phenotypes, there is also genetic loss. I know the objections to this are going to die hard, and may not die at all, but if they don't it will be through denial.
The type of argument you are making could perhaps be used to suggest that the theory overemphasizes selection and underemphasizes the production of new variation. But you won't be able to refute evolution this way, because the empirical evidence shows that variation does build up again if it has been reduced - unless, of course, that particular line goes extinct. Here I shall request that you produce this empirical evidence. The cheetah has lasted quite a long time without going extinct AND without acquiring one iota of variability. Perhaps someone should look into that population of seals that grew so large after being nearly exterminated -- Bet you a good collection of DNA samplings would confirm seriously reduced genetic diversity among them. Somebody get a grant to read thousands of genomes from the different populations in a ring species -- salamanders, seagulls, chipmunks, all of the above, or find a small highly variable creature and create a ring series in a laboratory for the same empirical test. I predict that the further you go down the series the less genetic variability you will find. If I had the wherewithall to fund such a study, believe me I'd do it. Edited by Faith, : No reason given.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
I have to take a break here. I only got as far as nwr. There are some coming up I look forward to grappling with but overall I can see that the same arguments are going to continue to come up (and the general tone is losing its equanimity already. Sigh).
It makes perfect sense that this would happen of course but it's hard on my poor head, so I have to ask that further contributors to this thread PLEASE read ALL the posts before responding. I know, you probably won't see this until it's too late, but Oh well.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
Hello Taz:
If you start with twenty alleles in a population for one gene and one of them becomes crucial for a particular environment and therefore gets selected, either rapidly or slowly depending on the selection pressure, you will lose the other nineteen alleles as the one selected comes to determine this particular trait.
But this isn't how evolution works. The other 19 alleles don't just disappear unless there are selective pressures against them.You're still thinking in black and white. Well, I'm trying to keep things simple at least for starters to try to prevent just this sort of misunderstanding, but I guess nothing will work. The 19 other alleles are FOR THE SAME TRAIT, which is clear enough in the quote if you read very carefully, but since it is a very high number of alleles for one trait I can understand that I contributed to the problem by trying to be all-inclusive of all possibilities in this simple layout. Of course the one selected allele WILL over time eliminate all the other nineteen in the population if it contributes that much adaptive benefit that it is passed on to that much greater an extent than the others FOR THAT SAME TRAIT. In other words there IS selective pressure against these nineteen in the form of this highly desirable and successful single allele. I am NOT talking about a zillion other alleles for other genes in the population.
What happens is by some selective pressure, say environmental or predatory, begins to favor one trait out of the 20, we will begin to see a steady increase of that one trait in the population. But the other 19 still remain, perhaps in lower number than before. The other 19 in this case remember are COMPETITORS at this same gene locus. They will remain until they are replaced by "the steady increase of that one trait in the population."
In other words, despite selective pressure favoring one or two or a few traits doesn't mean the overall variation of the gene pool will necessarily decrease. Nor have I said that it would. Again, I'm focusing on ONE selected allele for the purpose of demonstrating that its selection will either rapidly or eventually replace all competing alleles for the same trait. This isn't meant to be a complete picture of how evolution works but an example pared down to its essentials to make the point I want to make. The graphic for genetic drift that I linked from Wikipedia in my OP is the model for everything I've been saying here. They isolated a single gene, gave it two alleles equally divided among twenty individuals and showed how drift eventually replaces one with the other. Edited by Faith, : No reason given.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
Consider the dog example while we're at it. Every breed of dog MUST show reduced genetic variability compared to its population of origin because if you want it big you're going to have to eliminate everything that tends to smallness, if you want it good natured you have to eliminate everything that breeds for ferocity, and so on This is only true if the "first dog" had all possible dog genetic information, and subsequent dogs were created by taking out all the stuff that wasn't necessary for that breed of dog. This idea is ridiculous. While you might be right about the original genetic situation (it's a possibility but there may be other ways it happened) there is nothing in my quote that doesn't describe what is actually observable in domestic breeding. As I recall, either Dawkins or Coyne described this in one of their books (which I've been reading recently) -- there's nothing odd about what I'm saying, it's strictly factual. That is, if you want to maximize certain characteristics of your dog in its offspring, what you have to do is make sure it can't breed with dogs that have different characteristics. It's a process of eliminating what you don't want. Elementary my dear Subbie. Edited by Faith, : No reason given.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
My my my, Dr. A with his flat assertive pronouncements as if from on high, how very scientific of you.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
A decimated population such as the seals which were hunted to near extinction, may actually come back in large numbers, but they will come back with much reduced genetic variability compared to their original population. Surely this is obvious? No, it's not obvious, because you are using it in the wrong way. It's like saying each individual atom of my computer is colorless therefore my computer is colorless. There's a fallacy for that. Try to guess what it is. Offending Taz's sense of propriety probably. I can see that I'm going to be up against denials of the obvious here for some time. It SHOULD be obvious, Taz. It's a BOTTLENECK!!! There were very few individuals of the original population left, from which a new very large population grew, OF COURSE based entirely on the blending of the limited genetic diversity in those few. Sheesh!
While it is true that the seal population came back with less genetic variation than before, we're talking only a couple generations. You are trying to apply a couple generations of seal as an example of evolution. If I didn't get drawn in by your honest tone, I would have said strawman. Please bear in mind that I did not discuss ANYTHING OTHER than the situation as it NOW EXISTS. I did not extrapolate to the future as you are insisting either that I did or I should or who knows what. What NOW EXISTS is a very large population of seals with reduced genetic diversity as a result of the bottleneck they experienced because of the hunters. If you want to take this into the future somewhat compare them to the cheetah. How many generations has it struggled along on its reduced genetic diversity because of the bottleneck it experienced? At what point, how many years or how many generations or whatnot, do you expect to see variability increase in the population? And it would be nice if you could include some actual FACTS in your discussion. (I see that by the end of your post you do at least include some speculations if not facts -- 50 generations you say).
What happened with the seal population you described is called a bottleneck, where an event triggered a loss of many traits and the resulting allele frequency is completely different than the one before. In this particular case with the seal, the event is called over-hunting. Too bad you missed that I already made that perfectly clear.
Because we know for a fact that each individual in that population carries at least several mutations compared to its parents, DO YOU KNOW THAT "FOR A FACT???" Have you sequenced the DNA of a good sample of the seals? Or are you aware of a scientific report on that very experiment? And of what quality are these mutations and are they getting passed on or eliminated in further reproductions? Etc. etc. etc. Do the cheetahs carry several such mutations too? This is NOT something you "know for a fact," this is something you ASSUME because of your preconceptions. But after all that, EVEN IF THERE WERE SUCH AN INCREASE IN VARIATION IN THE POPULATION THROUGH MUTATION, and if you'd read more of this thread than you obviously did you'd know I've discussed this very situation many times already, EVEN IN THAT CASE, if from among these new traits one is selected and is reproduced at a higher rate than its competitors for a number of generations, it will eliminate any competing alleles for that trait/gene that may already be present anyway. The principle I'm talking about operates whenever selection or isolation of a trait occurs. It doesn't matter how much variability you get in between such events. If speciation is going to occur, alleles are going to have to be eliminated.
if left undisturbed it is inevitable that genetic variation in that population will increase given enough time. By enough time, I'm talking about at 50 generations or so, not a couple. You don't know if it WILL, even over 50 generations, you just assume that it must. But again, if it does, this is not the subject I'm focused on here. I'm talking about how the selection and isolation processes reduce this very variability no matter how it is originally acquired.
As a side note, the rattlesnake population in the southwest are going through a bottleneck event as we speak. People there are hunting down every rattlesnake they could find, which are usually the ones that make a lot of noise. The very trait that helped keep their ancestors from being trampled on are now working against them with humans. There are reports of increasing number of silent rattlesnakes crawling around. Goddamn rednecks... Yes? This is typical natural selection/evolution. And what's happening is that the allele(s) for noisy rattlers are BEING ELIMINATED FROM THE POPULATION allowing a new phenotype, a deadly silent nonrattler, to emerge.
I'm sure that one day in the distant future, our children's children will label this period as the great bottleneck era for most species on Earth. Man has been changing and molding population genetics to our liking. I'm sure we'll look back one day and realize the vast changes we've made to wild populations everywhere. Could be, I haven't been following that aspect of the situation, But whether human beings or something else in nature is bringing about the changes, such change always happens through the elimination of some traits which is the reduction of genetic diversity I'm talking about. (I must apologize if I sound exasperated. I'm not up to rewriting this. It's just frustrating to be so indignantly told I'm wrong when you've completely misunderstood what I've said.)
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
I haven't got down to your posts, Dr. A, but I will, I will. Patience.
Edited by Faith, : No reason given.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
This isn't a simple addition and subtraction problem. At heart it is. Granted there are complications in the details, but there is no doubt that there is addition as well as subtraction. No doubt at all, PaulK, nor have I implied that there is no addition, I'm simply talking about what happens when the existing variation is acted upon by selection or isolation.
If you start with twenty alleles in a population for one gene and one of them becomes crucial for a particular environment and therefore gets selected, either rapidly or slowly depending on the selection pressure, you will lose the other nineteen alleles as the one selected comes to determine this particular trait. And that allele will still be subject to mutations and new alleles will be derived from the one survivor. How unfortunate! This poor allele was selected only to be morphed into something else after doing its duty for the species? Wouldn't the adaptive benefit its selection conferred on the population then be lost again? What kind of "evolution" is going on here? Sorry, it's past my bedtime. I may have to answer your post again tomorrow to do it justice. But I AM trying to make a point. If you have THAT much mutation going on, even to the point that the allele that is selected for its adaptability is mutated away, it's hard to see that anything deserving of the name evolution is really going on here. You've got changes galore but a scattered mix of them rather than evolution in a particular direction. That's why I called this "stasis" before. The population in this condition is not evolving. It may even be reverting to an earlier condition from the sound of it.
Meanwhile other genes will also be mutating, producing new alleles. There is addition as well as subtraction. Indeed there is, but what is going on in the rest of the population is not my subject here, as I'm trying to keep the focus on what happens when there is SELECTION and ISOLATION, not increased variation. What you get from mutation is the raw material for evolution, you do not get evolution. Any number of writers on this subject will tell you that, especially Dawkins I believe. I suppose I should look it up at some point.
It would make no sense to add new alleles for that trait either, as it has been selected because of its value for the species. Selection REQUIRES the elimination of alleles. Addition would only kill selection and kill evolution. Addition means stasis, not evolution. Selection (or isolation of a portion of the population) is essential for evolution to occur. Addition certainly does not mean stasis. It cannot, because addition is the arrival of new alleles, not previously existing in the population. That is an example of change, not stasis. Selection gives direction to change, making it more than a random walk but mutation and drift ensure that change would happen, even in the absence of selection. You are right, the word was not apt. Of course mutation is change. But it's not evolution. Evolution requires selection from among those variations.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
The point I'm making is that WHEREVER EVOLUTION IS GOING ON, THAT'S WHERE you'll have a reduction in genetic diversity, and that is completely at odds with the theory of evolution. This is simply not true. As an example, let's discuss Darwin's Finches, 15 different species of tanagers that Darwin found on the Galapagos islands. The most important difference among the various species is the size and shape of their beaks. These differences did not come about by the elimination of alleles, but by changes in alleles in different populations, resulting in different species. Thus, in this example, evolution resulted in an increase in genetic diversity. In fact, this sort of increase in genetic diversity is what is most commonly seen in speciation events. I want to address this:
These differences did not come about by the elimination of alleles, but by changes in alleles in different populations, resulting in different species. My understanding of speciation is that, just as with domestic selection, the selected trait is isolated from other alleles for that trait so that it can disperse through the new population down the generations and thus come to characterize the new species. If the alleles for different kinds of beaks were not eliminated you would not have this new species. This is what must have happened with each of the finch types. A beak type got selected for its usefulness with a particular kind of function, and that got passed on and came to characterize a whole population because the alleles for the other beak types were eliminated from the reproductive pool. The same thing happened with other beak types as each found its peculiar adaptation and became isolated from the other types. It's not exactly that there were "changes in alleles" but that those alleles that were already available in the original population and probably haphazardly expressed phenotypically at that stage -- quite a few types obviously -- were selected down to a single type for each new population, and in the process all the other beak types were eliminated. The principle I'm hammering away at does hold up you see. Edited by Faith, : No reason given. Edited by Faith, : No reason given. Edited by Faith, : No reason given.
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Faith  Suspended Member (Idle past 1704 days) Posts: 35298 From: Nevada, USA Joined: |
Good morning, Percy.
It's taking forever to get through the posts, but I do want to get to all of them if I can. Genetic diversity can go in any direction after reproductive isolation. Got to answer this right off the bat. The point I keep trying to get in focus here is the genetic diversity CAUSED BY reproductive isolation. What happens AFTERWARD is another subject. Reproductive isolation is THE way new phenotypes/variations get established, whether the mechanism is natural selection, genetic drift or bottleneck etc. Reproductive isolation keeps the selected -- whether randomly or adaptationally -- alleles from being mixed with others that would perpetuate the characteristics of the old population, so that a new population with new characteristics can emerge. This is how a ring species forms for instance -- a few migrate away from the original population and start their own population and if there is no gene flow with the original population due to geographic or other kinds of barrier, after a few generations the new population will come to have its own peculiar characteristics that set it apart from the original population. This is just the effect of the alleles already present in the new population coming to expression, in most cases a relatively small portion of the original gene pool, whereas in the original population other alleles dominated the phenotype.
For example, consider a relatively homogeneous population that becomes divided in two when a river changes course. There are now two populations, both with pretty much the same alleles and allele frequency. That can happen I'm sure, if the original population was large enough that dividing it fairly equally wouldn't much change the frequencies, but then it's not an example of what I'm talking about here. But perhaps you are making a different point.
Mutations experienced in one population will no longer be shared with the other and the populations will evolve along different paths. If this continues for a sufficient period then they could lose their mutually interfertile quality and become two species. OK, I can see that mutation appears to be the explanation for all change in the minds of evolutionists here so that it's hard to get across what seems to me to be the much more common scenario I've been spelling out -- that an already-present complement of alleles is split between the populations and it is this that brings about the differences between the two as they go their separate ways mixing their genes in isolation from each other. It's the already-present alleles that "are no longer shared" between the populations, not mutations. The already-present alleles are sufficient to establish two different varieties without any help from mutations, and even to establish separate species according to the definition that they can no longer interbreed. This wouldn't happen in the case where the alleles are equally distributed between the two as you define this situation, however. I believe my scenario is recognized by all the major writers on this kind of thing, though, it's only here that I'm running into this dependence on mutation to power all variations. It's kind of curious, really. Many sources on the subject give the usual definition that it's relatively rare, usually deleterious, responsible for thousands of genetic diseases and known to have been beneficial in only a few odd circumstances that involve playing off a detriment against a benefit and so on, and yet people here talk as if mutation is the engine of evolution. And I get the impression you don't all agree with each other about exactly how it works, either.
If both populations thrive then diversity could increase in both. But if one or both populations suffer some disaster such as flood or famine or an invasive predator or disease that greatly reduces population size, then diversity would be reduced. It all depends upon what happens to the populations. IF mutation is the source of all variation and IF it occurs at a sufficient rate that enough useful mutations become established in a population to affect the phenotype while the larger proportion of deleterious mutations are eliminated by selection, then you'd be right that diversity could increase (over a few thousand years or what though?) but this is so hypothetical I don't really see the point. And you've also got one or both of these populations suffering from a disaster which could cause a bottleneck, or a predator or disease, all reducing the population size and of course these reduce genetic diversity just as natural selection does or migration of a small portion of the population as in ring species and so on and so forth, which is where I've been trying to focus from the beginning. When you get a smaller number from a population isolated that's when you get reduced genetic diversity. It's what has to happen in natural selection and it's what happens in speciation. {edit: not ALL natural selection, I hasten to add, as some doesn't reduce the population but simply works its way through a population -- but sometimes it does isolate a small portion of a population:} You get a new phenotype, even such a dramatically new phenotype with such changed genotype that it can no longer interbreed with the former population. Even if there are some mutations that contribute to the variation from which the new phenotype emerges, the bulk of the contribution is from the already-present alleles, and in any case it's when natural selection or any of the other isolating processes are at work that the phenomena I'm talking about occur, and since these are intrinsic to evolution then evolution depends on reducing genetic diversity in order to get a new variation or species and you can see that this chain of events MUST be self-limiting. Edited by Faith, : No reason given. Edited by Faith, : No reason given. Edited by Faith, : typo
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