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Thread: Can Natural Selection explain cell differentiation?

  1. #1 Can Natural Selection explain cell differentiation? 
    Forum Freshman starlarvae's Avatar
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    Darwin used the phrase "Descent with modification" to summarize his theory of evolution. Despite many particulars that more recent science has amended to Darwin's theory, the basic idea of descent with modification remains. The descendant modifications are taken to be driven by varying degrees of "fitness" to environments. Even according to today's NeoDarwinian (or Modern Synthesis, or Extended Synthesis) model, the modifications observed in descendants are taken to be the result primarily of adaptations to environmental contingencies, operating under the mechanism of variation + selection, which runs without the benefit of any plan or program that might provide direction. And this "blind" process has produced all the phenotypes that were or are or will be.

    However, another case of biological descent with modification apparently does benefit, or is assumed to benefit, from a guiding plan, or program. That is the descent of various tissue types from an undifferentiated zygote during ontogeny. This poses a paradox.

    If natural selection is so powerful a causal agent that it can generate all the phenotypes that make up an ecosystem, then why is it necessary to suppose that there occurs in a zygote some sort of genetic plan or program that guides development of the organism? Why not just chalk it up to natural selection -- competition and cooperation among the cells in the organism? What evidence is there of a developmental program?

    All the cell types that make up the body of a complex organism share the same genotype but differ as to which genes are active and which not. And that info must be heritable, hence a source of variation ("copying errors"). But any variation among cells in an embryo might provide an advantage to some cells and/or disadvantage to others. So, the stage is set for natural selection.

    The tissues that make up a complex body and their symbiotic interdependencies are just the happenstance of competition among the cells -- is that a defensible proposition? The fit survive and go on to take their place in the somatic ecosystem of the body. The unfit go extinct. A clear case of unguided evolution. No need for a developmental program.

    I am NOT proposing that this is what happens. I am only asking the question: What OBSERVATION could disprove this argument -- that the cells descend with modification from their common ancestor, a zygote, through a process of variation + selection?


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  3. #2  
    Universal Mind John Galt's Avatar
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    Do you think each cell in the developing embryo is in the same environment?

    If you do not, then your question is already answered.

    If you do, then you need to study some more embryology.


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  4. #3  
    Forum Freshman starlarvae's Avatar
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    Quote Originally Posted by Ophiolite
    Do you think each cell in the developing embryo is in the same environment?

    If you do not, then your question is already answered.

    If you do, then you need to study some more embryology.
    Each cell is in the same environment insofar as all are in the womb. But even in the womb there will be variation in chemistry from place to place. As soon as the embryonic cells start to differentiate, they enter different environments. Some will be internal to the organism and some will be on the surface, exposed directly to the womb. The inside of a developing organism is itself a collection of environments.

    So the variation in environments explains the variation in cellular phenotypes, because the cell types differentially adapt ?

    Even if we say that all the cells encounter the same environment, they can differentiate, due to chance mutations that increase or decrease fitness of the mutants.

    So, unanswered is the question as to what causes the differentiation? An ontogenetic developmental program? or natural selection in the womb and inside the developing organism?
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  5. #4  
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    Thank you for your reply. You have clarified the situation. You are speculating about matters which you have not studied, or have studied poorly and for which you have little or no even basic understanding of the relevant material.

    I could attempt to explain your errors and oversights, but I strongly suspect that you would glibly ignore these corrections and sail on with your meaningless speculations. If I am mistaken in this - and I hope I am - your honest response will reveal that. We can then look towards for some education for both of us.
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  6. #5 Re: Can Natural Selection explain cell differentiation? 
    Moderator Moderator TheBiologista's Avatar
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    Quote Originally Posted by starlarvae
    However, another case of biological descent with modification apparently does benefit, or is assumed to benefit, from a guiding plan, or program. That is the descent of various tissue types from an undifferentiated zygote during ontogeny. This poses a paradox.
    Hang on, what evidence is there that cells undergo descent with modification during development? The differences between cell types are in the phenotypes, not the genotypes of those cells. With a few exceptions, all of the cells of your body have the same genome. That's very well-established in evidence.

    The answer to your question of why the body does not utilise natural selection instead of a programmed ontogeny is because natural selection is random and wasteful. It generates huge diversity, but at the cost of massive losses of unfit individuals. Any organism widely utiling such a method for development would have a hugely negative cost:benefit ratio and would not persist in the population.

    The answer to your question on whether there's evidence of a programmed ontogeny is yes of course there is. And it's readily accessible on the internet which makes me wonder whether you arrived at a conclusion before actually asking the question and doing the basic reading first...
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  7. #6  
    Forum Freshman starlarvae's Avatar
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    Quote Originally Posted by Ophiolite
    Thank you for your reply. You have clarified the situation. You are speculating about matters which you have not studied, or have studied poorly and for which you have little or no even basic understanding of the relevant material.

    I could attempt to explain your errors and oversights, but I strongly suspect that you would glibly ignore these corrections and sail on with your meaningless speculations. If I am mistaken in this - and I hope I am - your honest response will reveal that. We can then look towards for some education for both of us.
    I'll reduce my rhetoric to two simple, scientific questions:

    1) What observation -- under the microscope or in the field -- could distinguish a process of descent with modification that is the result of natural selection from one that is directed by a developmental program?

    2) If there is in theory no possible observation that could distinguish the two processes, then what justifies making the distinction when it comes to ontogeny and phylogeny?

    I know what the textbooks say. Don't lecture me on that. But I don't find in the literature any empirical justification for the distinction. The distinction tends instead to be justified on doctrinal grounds.
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  8. #7  
    Moderator Moderator TheBiologista's Avatar
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    Quote Originally Posted by starlarvae
    Quote Originally Posted by Ophiolite
    Thank you for your reply. You have clarified the situation. You are speculating about matters which you have not studied, or have studied poorly and for which you have little or no even basic understanding of the relevant material.

    I could attempt to explain your errors and oversights, but I strongly suspect that you would glibly ignore these corrections and sail on with your meaningless speculations. If I am mistaken in this - and I hope I am - your honest response will reveal that. We can then look towards for some education for both of us.
    I'll reduce my rhetoric to two simple, scientific questions:

    1) What observation -- under the microscope or in the field -- could distinguish a process of descent with modification that is the result of natural selection from one that is directed by a developmental program?

    2) If there is in theory no possible observation that could distinguish the two processes, then what justifies making the distinction when it comes to ontogeny and phylogeny?

    I know what the textbooks say. Don't lecture me on that. But I don't find in the literature any empirical justification for the distinction. The distinction tends instead to be justified on doctrinal grounds.
    I answered your first question in my previous comment. The second one is rendered moot by the answer to the first. Cells within a multicellular organism do not undergo descent with modification.
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  9. #8  
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    starlarvae,

    The key observations are the processes of development and evolution themselves - they are fundamentally different and understanding them better - even a little - is all that is required.

    Here are a few observations/experiments:

    What do you think would happen if you knocked out the SRY gene in a XY zygote? Does this transcription factor regulate the development of certain tissues or not? How would you describe testis development as occurring by a process of "variation + selection"? Where are the myriad cells that failed to hit upon the right path to testis development? How does your model explain any of the dozens of well-characterised developmental errors? Have you never studied the role of Hox genes in Drosophila development?

    I suggest you read a paper called "Evolution of mitotic cell-lineages in multicellular organisms" by Torbjörn Fagerström,
    David A. Briscoe and Paul Sunnucks. It's online and is about the closest non-wacko source I can find that comes even remotely close to what you describe.


    Incidentally, since so-called "Junk DNA" evolves in a completely non-Darwinian fashion, how can it play a role in your suggestion, where selection is key?
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  10. #9  
    Forum Freshman starlarvae's Avatar
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    Quote Originally Posted by TheBiologista
    Hang on, what evidence is there that cells undergo descent with modification during development? The differences between cell types are in the phenotypes, not the genotypes of those cells. With a few exceptions, all of the cells of your body have the same genome. That's very well-established in evidence.
    Right. DNA is conserved during cell differentiation in a body, radically. And DNA is conserved during species differentiation in evolution, less radically, but enough so that the literature on evo-devo is full of references to surprise findings when it comes to the similarity of genotypes across diverse phenotypes.

    As we're learning from evo-devo, a diversity of phenotypes need not be supported by any correspondingly proportionate diversity of genotype. A conserved genotype can express phenotypic diversity through the use of regulatory "switches" that turn genes on and off, here and there at various times. If the regulatory "switch settings" are heritable, as they must be during both cell differentiation and species differentiation, then descent with modification can occur despite a conserved genotype.

    Quote Originally Posted by TheBiologista
    The answer to your question of why the body does not utilise natural selection instead of a programmed ontogeny is because natural selection is random and wasteful. It generates huge diversity, but at the cost of massive losses of unfit individuals. Any organism widely utiling such a method for development would have a hugely negative cost:benefit ratio and would not persist in the population.
    Some examples of natural selection no doubt are messier than others. But the messiness is not necessary for natural selection. It is not a requirement. All that is required is variation in a population, with some variants enjoying greater fitness than others. I suppose the messiness will be a product of the variability. As for the "massive losses of unfit individuals," cells in your body are dying all the time. During a lifetime there are massive losses of fit and unfit (cellular) individuals, just as there are massive losses of fit and unfit individuals in evolution.

    Quote Originally Posted by TheBiologista
    The answer to your question on whether there's evidence of a programmed ontogeny is yes of course there is. And it's readily accessible on the internet which makes me wonder whether you arrived at a conclusion before actually asking the question and doing the basic reading first...
    How did I know that was coming? I'm interested in what you can tell me, not in what Google can tell me. I want dialog.

    OK, let me backtrack. I just Googled - evidence of programmed ontogeny - and the most consistent linkage of programming and ontogeny has to do with programmed cell death, so I'm glad I took the time, because that's one more parallel between ontogeny and phylogeny: programmed death.
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  11. #10  
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    Quote Originally Posted by Zwirko
    What do you think would happen if you knocked out the SRY gene in a XY zygote? Does this transcription factor regulate the development of certain tissues or not?
    If you alter an expressed gene in a zygote, the phenotype of the organism will be modified in some manner, which might be more or less predictable. The question is whether the observable modification would be due to your causing a change in a developmental program or just a change in the variability of descendant cells. In the latter case, you would be limiting or expanding the variants from which natural selection could select. And whatever outcome resulted you could chalk up to the persistent cellular phenotypes being better adapted or more fit than whatever alternative variants they competed against.

    Quote Originally Posted by Zwirko
    How would you describe testis development as occurring by a process of "variation + selection"?
    The same way evolution theory explains all kinds of phenotypes: Whatever is observed is taken to be the better adapted of whatever variants got expressed in the local population. The cells of the testis got to be how they got to be the same way the other cells in the body got to be how they got to be. They out competed (or out cooperated) their rivals.

    Quote Originally Posted by Zwirko
    Where are the myriad cells that failed to hit upon the right path to testis development?
    They're gone. After all, they failed to hit upon the right path, namely that of "adaptive advantage" which would have conferred "fitness." As in evolution, there are, admittedly, missing transitional forms. We might surmise various reasons for this. Maybe they just were not of types conducive to becoming fossilized. Stem cells do survive in our bodies -- bizarre "living fossils."

    I hope you can tell that I'm having fun with this. But the sarcasm is meant to make a point.

    Quote Originally Posted by Zwirko
    How does your model explain any of the dozens of well-characterised developmental errors?
    Those aren't developmental errors. They are examples of less-well-adapted phenotypes. The fossil record is littered with maladapted phenotypes (the extinct species). However, natural selection produced them (I'm assuming it did. Or, do you think neanderthal was a developmental error?)

    Again, if the "error" involves modification of an expressed gene (or genes) then whatever phenotype results can be explained by the "error" (or mutation) affecting the variability of descendant cells upon which natural selection then can act.

    Quote Originally Posted by Zwirko
    Have you never studied the role of Hox genes in Drosophila development?
    No such examples can torpedo the proposition that cells in a body differentiate by way of natural selection. Or, tell me how the action of Hox genes demonstrates programmed development instead of natural selection among differentiating cells. If the genes affect cellular phenotypes, then they simply give selection more or fewer variant types from which to select.

    Quote Originally Posted by Zwirko
    I suggest you read a paper called "Evolution of mitotic cell-lineages in multicellular organisms" by Torbjörn Fagerström,
    David A. Briscoe and Paul Sunnucks.
    Got it. Thanks.

    Quote Originally Posted by Zwirko
    Incidentally, since so-called "Junk DNA" evolves in a completely non-Darwinian fashion, how can it play a role in your suggestion, where selection is key?
    Tell me more about non-Darwinian evolution.
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  12. #11  
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    Quote Originally Posted by starlarvae
    Tell me more about non-Darwinian evolution.
    A good example would be your posts. Despite being increasingly unfit for their environment (a science forum) they continue to propagate and flourish and change character from bizarre to downright dumb.
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  13. #12  
    Forum Professor Zwirko's Avatar
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    starlarvae,

    Your responses don't address the points I've raised - you've simply waved them away, sweeping reality under the carpet, because you don't understand. Or, it would appear, don't even want to understand. There is a truly mammoth (and, so far incomplete) literature available on many aspects of developmental biology, describing in great detail the molecular and genetic events that underpin the process. There is a wealth of empirical evidence that has tracked the fate of cells in the early embryo, describes the morphology and molecular characteristics of embryonic and fetal tissues at each and every stage and describes what happens when the process goes awry. In some organisms the development of every single last cell in the adult can be accounted for and followed from the zygote stage - a repeatable process that is invariant from individual to individual. That evolutionary processes, natural selection in particular, can explain development is so badly muddled and nonsensical that it's not even wrong, as they say.


    If the process was operating the way you suggest then a zygote would simply develop into a mass of tumors incapable of any organismal-level function at all. Two zygotes from the same species would never develop in the same way. It'd be impossible. What you are suggesting makes no sense. It's factually incorrect and has no supporting evidence. I can't for the life of me understand why you are persisting with the idea, unless you're simply playing a game for amusement.
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  14. #13  
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    Quote Originally Posted by starlarvae
    As for the "massive losses of unfit individuals," cells in your body are dying all the time.
    Yes, but because they're terminally differentiated. There's a pattern to the cell deaths. You don't see them dying at random in massive numbers unless the organism is sick or dying. No so with natural selection, where there is no consistent pattern to organism deaths.

    Quote Originally Posted by Zwirko
    Two zygotes from the same species would never develop in the same way.
    This is, I think, the deal breaker. If natural selection were the driving force behind development, you would expect zygotes to start the same and then develop along a wide variety of paths following some random distribution.
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  15. #14  
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    Quote Originally Posted by Ophiolite
    Quote Originally Posted by starlarvae
    Tell me more about non-Darwinian evolution.
    A good example would be your posts. Despite being increasingly unfit for their environment (a science forum) they continue to propagate and flourish and change character from bizarre to downright dumb.
    I'll leverage your dismissal to make a point.

    The fact that the posts propagate is the only criterion that you have by which to assess their fitness. Hence, they are fit.

    This demonstrates one of the problems with the orthodox view. Whether a phenotypic variant bestows an adaptive advantage or not can be assessed only in arrears. A given phenotype might or might not enjoy reproductive success. This much we can observe.

    But then the Darwinians move in and muddy the waters with mystical abstractions, such as "adaptive advantage" and "fitness." These things we cannot observe.

    Sorry for being such an empiricist, but I try to take scientific method seriously.
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  16. #15  
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    Quote Originally Posted by starlarvae
    Sorry for being such an empiricist, but I try to take scientific method seriously.
    You are not trying hard enough.
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  17. #16  
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    Quote Originally Posted by TheBiologista
    Quote Originally Posted by starlarvae
    As for the "massive losses of unfit individuals," cells in your body are dying all the time.
    Quote Originally Posted by TheBiologista
    Yes, but because they're terminally differentiated.
    Some are and some aren't. There must be some embryonic cells, stem cells, primordia, various pluripotent cellular varieties that die. They're not terminally differentiated.

    "Terminally differentiated" is an interesting idea, though. Does it mean that no phenotypic variation is possible beyond a certain point? Can a species be terminally differentiated? I think Zwirko is referring to C. elegans as the creature whose ontogeny has been mapped down to each cell. I don't know what that proves, unless you and he are making the case that there is a state called "terminal differentiation" that precludes any possibility of further evolution / development. But that doesn't work, because there's always the possibility of new evolution / development via mutation.

    Quote Originally Posted by TheBiologista
    There's a pattern to the cell deaths. You don't see them dying at random in massive numbers unless the organism is sick or dying.
    Really? What's the pattern? Some die of old age. Some of infection. Some of starvation. Others of unfitness of various kinds. Some die in the skin. Some in the kidney. Some in the spleen. What's the pattern?

    Quote Originally Posted by TheBiologista
    No so with natural selection, where there is no consistent pattern to organism deaths.
    This is incoherent. Where is the consistent pattern?

    Quote Originally Posted by Zwirko
    Two zygotes from the same species would never develop in the same way.
    Quote Originally Posted by TheBiologista
    This is, I think, the deal breaker. If natural selection were the driving force behind development, you would expect zygotes to start the same and then develop along a wide variety of paths following some random distribution.
    To make sense of this objection would require operationalizing the terms.

    What would constitute developing in "the same way"? An eskimo and a tutsi are the same species. A poodle and a great dane are the same species. But in these cases phenotypic differences are conspicuously apparent. What degree of distinction constitutes "a wide variety of paths"?

    Zygotes of the same species will tend to develop in the same environment, so nat sel will tend drive them toward similar phenotypes. The selection pressures will be the same, so given similar genetic starting points, it's no surprise that zygotes of the same species will develop similarly. The similarity in selection pressures should do it. There's no need for a developmental program.
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  18. #17  
    Forum Professor Zwirko's Avatar
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    Evolutionary outcomes are statistical in nature - natural selection is a probabilistic sampling process. Development is not a probabilistic sampling process. It's really that simple. If you think otherwise, provide evidence to show it.

    What studies on C. elegans show is that there is no probabilistic sampling process operating during the development of this organism, therefore no natural selection. Your hypothesis makes the prediction that the cells in the embryo of C. elegans are exploring sample space randomly (presumably by altering patterns of gene expression) and that the cells that have hit upon a certain phenotype (presumably some optimal solution) survive to make up the adult organism. This is not observed - the history of every last somatic cell can be traced. Therefore, it's not natural selection. There should be thousands to millions, possibly billions, of cells off exploring other possibilities or even finding other, equally optimal, solutions (even though there are only ~1,000 cells in the adult). This is not observed. Therefore, it's not natural selection. Somatic stem cells in C. elegans differentiate directly along predictable and invariant pathways. Therefore, it's not natural selection. Tight regulation of cell replication and differentiation is necessary to form the adult. Therefore, it can't be natural selection. What it is, is a developmental program interacting with the environment. You can pick any cell at any point during development of C. elegans and know the fate of that cell beforehand. Therefore, it's not natural selection (which is utterly blind to future outcomes).
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  19. #18 Re: Can Natural Selection explain cell differentiation? 
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    Quote Originally Posted by starlarvae

    If natural selection is so powerful a causal agent that it can generate all the phenotypes that make up an ecosystem, then why is it necessary to suppose that there occurs in a zygote some sort of genetic plan or program that guides development of the organism? Why not just chalk it up to natural selection -- competition and cooperation among the cells in the organism? What evidence is there of a developmental program?
    Think of a hive of bees. The same queen produces workers, drones, and other queens. (For some types of insect I think there is also a warrior class, but I'm not sure which ones have them and which ones don't.)

    Because of their relationship with each other, each worker is facing a different aspect of the environment depending on their assigned tasks, but the only ones that determine the DNA are the drones and the queen. And, just as in the human body, the drones and queen do the least interacting with that environment, being sheltered as best as possible by the workers (and warriors). (Similarly, we humans try to protect our sexual organs as best as possible.)

    But, here's the thing: if the queen/drones' worker DNA is flawed, those workers will fail to protect them and feed them well. So, the queen/drones evolve better worker DNA over time, not based on the survival of the workers, but based on their own survival.

    Quote Originally Posted by TheBiologista
    Quote Originally Posted by starlarvae
    However, another case of biological descent with modification apparently does benefit, or is assumed to benefit, from a guiding plan, or program. That is the descent of various tissue types from an undifferentiated zygote during ontogeny. This poses a paradox.
    Hang on, what evidence is there that cells undergo descent with modification during development? The differences between cell types are in the phenotypes, not the genotypes of those cells. With a few exceptions, all of the cells of your body have the same genome. That's very well-established in evidence.
    I hope you'll forgive my ignorance, but isn't descent with modification the main reason why we grow old and die? If I understand the process right, descent with modification does indeed occur, but it's a wholly destructive process. The descendant cells deviate further and further from the originals until they just plain aren't functional any more.

    So, the reason we survive as long as we do is because the "programmed ontogeny" you mentioned is the dominant force, but both effects are still present, right?
    Some clocks are only right twice a day, but they are still right when they are right.
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  20. #19  
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    Quote Originally Posted by Zwirko
    starlarvae, Your responses don't address the points I've raised - you've simply waved them away, sweeping reality under the carpet, because you don't understand. Or, it would appear, don't even want to understand. There is a truly mammoth (and, so far incomplete) literature available on many aspects of developmental biology, describing in great detail the molecular and genetic events that underpin the process. There is a wealth of empirical evidence that has tracked the fate of cells in the early embryo, describes the morphology and molecular characteristics of embryonic and fetal tissues at each and every stage and describes what happens when the process goes awry. In some organisms the development of every single last cell in the adult can be accounted for and followed from the zygote stage - a repeatable process that is invariant from individual to individual. That evolutionary processes, natural selection in particular, can explain development is so badly muddled and nonsensical that it's not even wrong, as they say.
    I know that intra- and intercellular events can be tracked precisely and that, for the most part, endogenous factors have been assigned a directive power as cells differentiate and developing creatures take form. That's not terribly interesting.

    What is more interesting is that the so-called Extended Synthesis assigns more directive power to endogenous factors in shaping phenotypes that did the Modern Synthesis, and so acknowledges this "ontogenetic" aspect of phylogeny. But, the scientific establishment, for the most part, resists admitting that the theory of natural selection is significantly compromised by the encroachment of endogenous factors. So, as research continues, the scientists will have to quantify this stuff. How much endogenous direction can be allowed to infiltrate before we have to demote natural selection to a secondary, or tertiary, shaper of phenotypes?

    Quote Originally Posted by Zwirko
    If the process was operating the way you suggest then a zygote would simply develop into a mass of tumors incapable of any organismal-level function at all. Two zygotes from the same species would never develop in the same way. It'd be impossible.
    Which is it? Would they develop into a mass of tumors incapable of organismal-level function (i.e., they would remain phenotypically the same)? Or, would they never develop in the same way (i.e., they would become phenotypically distinct)? Which is it? Would they remain the same or become different? You can't say that my model predicts both outcomes. OR?

    Quote Originally Posted by Zwirko
    Evolutionary outcomes are statistical in nature - natural selection is a probabilistic sampling process. Development is not a probabilistic sampling process. It's really that simple. If you think otherwise, provide evidence to show it.
    These are just assertions.

    Part of my point is that there's no evidence to be had when it comes to distinguishing a programmed process from one of natural selection. WHATEVER is observed can be assigned the status of "better adapted" and it earns that status simply by existing. That's part of the hollowness of selectionism to which Fodor and others are calling attention.

    Quote Originally Posted by Zwirko
    What studies on C. elegans show is that there is no probabilistic sampling process operating during the development of this organism, therefore no natural selection.
    "probabilistic sampling process"? Are you just making up terminology?

    If there's variability among the phenotypes (caused by whatever - regulatory networks, mutations, chance - it doesn't matter) and some phenotypes persist for more generations (enjoy greater "fitness") than others, then you've observed all there is to observe -- according to selectionism, and the "fit" types are the effect caused by natural selection.

    Quote Originally Posted by Zwirko
    Yourhypothesis makes the prediction that the cells in the embryo of C. elegans are exploring sample space randomly (presumably by altering patterns of gene expression) and that the cells that have hit upon a certain phenotype (presumably some optimal solution) survive to make up the adult organism. This is not observed - the history of every last somatic cell can be traced. Therefore, it's not natural selection.
    Not so fast. Pick any individual creature (or plant) you want. It's obviously not feasible in practice to identify every individual ancestor of that creature, but in theory the history of every extant creature could be traced -- back to its progenitor, and back another generation, and so on -- as far as you care to go. An unbroken lineage. Therefore it's not natural selection? All you're saying about the C. elegans cells is that they descend from an unbroken lineage.

    Also, be careful. If you're saying that tight endogenous constraints on phenotypic expression preclude the operation of natural selection, then you have to face the prospect that the same thing applies at the level of phylogeny. Again, that's one of the points underscored by Fodor, et al. The Extended Synthesis throws more weight behind endogenous constraints as shapers of phenotypes and so deprives selectionism of some of its traditional turf.

    Quote Originally Posted by Zwirko
    There should be thousands to millions, possibly billions, of cells off exploring other possibilities or even fi[nding other, equally optimal, solutions (even though there are only ~1,000 cells in the adult). This is not observed. Therefore, it's not natural selection.
    So endogenous constraints can not only limit, but even preclude the possibility of, natural selection? Interesting.

    Quote Originally Posted by Zwirko
    Somatic stem cells in C. elegans differentiate directly along predictable and invariant pathways. Therefore, it's not natural selection. Tight regulation of cell replication and differentiation is necessary to form the adult. Therefore, it can't be natural selection. What it is, is a developmental program interacting with the environment. You can pick any cell at any point during development of C. elegans and know the fate of that cell beforehand. Therefore, it's not natural selection (which is utterly blind to future outcomes).
    You're assuming that C. elegans is a model organism. You think every cell in your body descended via an invariant lineage from the first division of a zygote? If you say Yes, then it's an amazing claim (but, who knows?). If No, then I propose selection as the phenotypic driver.
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    starlarvae,

    My statements that "Evolutionary outcomes are statistical in nature" and that "natural selection is a probabilistic sampling process" are not assertions nor are they made up terminology. That you fail to understand these phrases is a perfect illustration of the fact that you haven't the faintest clue what you are talking about. Your whole problem boils down to the simple fact that you are exceedingly ignorant about the basic underlying concepts of natural selection. Understand these phrases and you will see your error in claiming that development proceeds via a process of natural selection.

    I've tried to help you, but you're clearly not interested in learning anything. So, with respect, I'll leave you to it.
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    Quote Originally Posted by starlarvae
    What is more interesting is that the so-called Extended Synthesis assigns more directive power to endogenous factors in shaping phenotypes that did the Modern Synthesis, and so acknowledges this "ontogenetic" aspect of phylogeny. But, the scientific establishment, for the most part, resists admitting that the theory of natural selection is significantly compromised by the encroachment of endogenous factors. So, as research continues, the scientists will have to quantify this stuff. How much endogenous direction can be allowed to infiltrate before we have to demote natural selection to a secondary, or tertiary, shaper of phenotypes?
    It's not threatened by them at all. Because the mutations of cells within your own body have no impact on your offspring (unless those mutations are occurring in your reproductive system), they also have a perfect zero impact on the evolution process. They're part of a colony of cells (your body) wherein the entire colony is destined to go extinct. If the whole colony dies, then what does it matter how any individual lineage of cells had mutated prior to their extinction?
    Some clocks are only right twice a day, but they are still right when they are right.
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    Quote Originally Posted by Zwirko
    starlarvae,

    My statements that "Evolutionary outcomes are statistical in nature" and that "natural selection is a probabilistic sampling process" are not assertions nor are they made up terminology. That you fail to understand these phrases is a perfect illustration of the fact that you haven't the faintest clue what you are talking about. Your whole problem boils down to the simple fact that you are exceedingly ignorant about the basic underlying concepts of natural selection. Understand these phrases and you will see your error in claiming that development proceeds via a process of natural selection.

    I've tried to help you, but you're clearly not interested in learning anything. So, with respect, I'll leave you to it.
    Ok, thanks for the engagement. But my failure is not one of understanding. I understand the orthodox view. I fail to concur.
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    Quote Originally Posted by starlarvae
    I understand the orthodox view.
    We have here a thread full of posts that demonstrate clearly that you do not.

    I sit patiently waiting for the next revolution in science, no matter which field. I long for the conventional and the orthodox to be overturned. You haven't got it.
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    [quote="kojax"]
    Quote Originally Posted by starlarvae
    But, the scientific establishment, for the most part, resists admitting that the theory of natural selection is significantly compromised by the encroachment of endogenous factors. So, as research continues, the scientists will have to quantify this stuff. How much endogenous direction can be allowed to infiltrate before we have to demote natural selection to a secondary, or tertiary, shaper of phenotypes?
    Quote Originally Posted by kojax
    It's not threatened by them at all. Because the mutations of cells within your own body have no impact on your offspring (unless those mutations are occurring in your reproductive system),
    A mutation of cells in a body will alter the phenotype of the body. An alteration in phenotype affects the variability from which natural selection can select. Hence, it affects evolution. It doesn't have to involve the gametes directly. The point is that the tight endogenous constraints on phenotypes deprive selection of the variability that it needs to do what it is supposed to do.

    Quote Originally Posted by kojax
    They're part of a colony of cells (your body) wherein the entire colony is destined to go extinct. If the whole colony dies, then what does it matter how any individual lineage of cells had mutated prior to their extinction?
    You could say the same thing about species. Any generation of a particular species will die, too, and the species itself might go extinct. What's your point? It's like gene selection vs phenotype selection vs group selection. What's good for the individual, like selfishness, might be bad for the group. So what gets passed to the next generation? What's good for the individual or what's good for the group?
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    Quote Originally Posted by starlarvae
    Quote Originally Posted by kojax
    Quote Originally Posted by starlarvae
    But, the scientific establishment, for the most part, resists admitting that the theory of natural selection is significantly compromised by the encroachment of endogenous factors. So, as research continues, the scientists will have to quantify this stuff. How much endogenous direction can be allowed to infiltrate before we have to demote natural selection to a secondary, or tertiary, shaper of phenotypes?
    Quote Originally Posted by kojax
    It's not threatened by them at all. Because the mutations of cells within your own body have no impact on your offspring (unless those mutations are occurring in your reproductive system),
    A mutation of cells in a body will alter the phenotype of the body. An alteration in phenotype affects the variability from which natural selection can select. Hence, it affects evolution. It doesn't have to involve the gametes directly. The point is that the tight endogenous constraints on phenotypes deprive selection of the variability that it needs to do what it is supposed to do.
    You understand that your body does mutate as you age, right? It mutates into an old person, and eventually into a corpse.

    The problem is that the environment isn't strongly selective. Most cells live out their individual life spans regardless of their fitness or un-fitness because the environment inside your body is just super-comfortable for them. No selective pressure means no direction. No direction means no change in phenotype. Or rather, a bazillion changes in phenotype all in different directions that ultimately just lead to a mass of cells that aren't cooperating very well.


    Also there's no sexual reproduction, and not a lot of migration for certain types of tissue. The skin cells growing in the tip of your finger are unlikely to migrate from there to the tip of your toe, and even if they did they wouldn't be having sex with any of the cells in your toe, so whatever change is happening in their DNA isn't going to spread.

    Quote Originally Posted by kojax
    They're part of a colony of cells (your body) wherein the entire colony is destined to go extinct. If the whole colony dies, then what does it matter how any individual lineage of cells had mutated prior to their extinction?
    You could say the same thing about species. Any generation of a particular species will die, too, and the species itself might go extinct. What's your point? It's like gene selection vs phenotype selection vs group selection. What's good for the individual, like selfishness, might be bad for the group. So what gets passed to the next generation? What's good for the individual or what's good for the group?
    [/quote]

    Dividing it into "generations" is purely arbitrary. You could just as easily divide it into individual members and argue that since every individual member eventually dies, the whole colony eventually dies. The thing with a human body is that the whole colony dies without offspring.

    The only cells in your body that are capable of having offspring that shall survive this cataclysm are the ones that produce eggs and sperm. All the other cells have only about 80 years to "evolve" into something new.
    Some clocks are only right twice a day, but they are still right when they are right.
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