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Fosdick  Suspended Member (Idle past 5527 days) Posts: 1793 From: Upper Slobovia Joined: |
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Author | Topic: Descent with Modification v. Larval Hybridization | ||||||||||||||||||||||||
Fosdick  Suspended Member (Idle past 5527 days) Posts: 1793 From: Upper Slobovia Joined: |
In another thread”Message 93”onfire stated the usual perception of how biological evolution occurs:
onfire writes: There is no difference between micro and macro if you take into account transitional fossils. There is no great leap from micro to macro, the whole arguement is wrong, transitional fossils, like that of the whales, shows descent with modification. There is no need, in respect to the whale(and all other species), to have a micro/macro discusion. We classify them as different species giving them the appearence of a micro/macro change but they don't just change from one species to the next. The micro/macro debate is old, many transitional fossils have been found and the debate should have been put to rest, I see it hasn't. But is "descent with modification" the only way biological evolution proceeds? Or can there be huge leaps of change occurring in biological evolution that do not follow the decent-with-modification rule? Could the former case be called "microevolution" and the latter "macroevolution"? And why does there always need to be a transitional fossil? Thus, we have need to debate: "descent with modification" v. "larval hybridization." The former is well known for its role in the evolution of Darwin's finches, for example. The latter would engage more robustly the role of horizontal gene transfer." As a way to focus this discussion, I'll suggest that the genetically free-wheeling affairs of larvae may account for incredible evolutionary leaps between taxa, leaving no evidence behind of descent with modification by way of transitional fossils. In my opinion, Williamson makes a few good points in his book The Origins of Larvae, concerning the role of larvae hybridization in evolution. Such a theory would help to explain certain commonalities between echinoderms and chordates, for example, and it could also lend credence to the disputed Cambrian Explosion. But, in accordance with peer review, recent criticisms of his hypothesis arose in the Letters to the Editors pages of American Scientist (March-April, 2008). Should onfire change his mind about how evolution works? Or is evolution accomplished only by descent with modification? ”HM
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AdminNosy Administrator Posts: 4754 From: Vancouver, BC, Canada Joined: |
Thread moved here from the Proposed New Topics forum.
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gluadys Member (Idle past 4990 days) Posts: 57 From: Canada Joined: |
hootmon writes: In another thread”Message 93”onfire stated the usual perception of how biological evolution occurs:
onfire writes: There is no difference between micro and macro if you take into account transitional fossils. There is no great leap from micro to macro, the whole arguement is wrong, transitional fossils, like that of the whales, shows descent with modification. There is no need, in respect to the whale(and all other species), to have a micro/macro discusion. We classify them as different species giving them the appearence of a micro/macro change but they don't just change from one species to the next. The micro/macro debate is old, many transitional fossils have been found and the debate should have been put to rest, I see it hasn't. But is "descent with modification" the only way biological evolution proceeds? Or can there be huge leaps of change occurring in biological evolution that do not follow the decent-with-modification rule? Could the former case be called "microevolution" and the latter "macroevolution"? And why does there always need to be a transitional fossil? Of course, there does not always need to be a transitional fossil as so few species are ever represented by fossils at all. But descent with modification does require transitions, whether or not any were fossilized.
Thus, we have need to debate: "descent with modification" v. "larval hybridization." The former is well known for its role in the evolution of Darwin's finches, for example. The latter would engage more robustly the role of horizontal gene transfer." I am not sure I follow this. Horizontal gene transfer is a phenomenon well known among bacteria, but not frequent among eukaryotes. Is the hypothesis of larval hybridization intended to provide a mechanism of horizontal gene transfer in animals?
As a way to focus this discussion, I'll suggest that the genetically free-wheeling affairs of larvae may account for incredible evolutionary leaps between taxa, leaving no evidence behind of descent with modification by way of transitional fossils. I am not sure why this would be. We might approach the issue by asking first why does descent with modification require a period of transition. Answer: because it takes time for a useful mutation to move from the one organism in which it originally occurs to every organism in the whole species. How would horizontal gene transfer avoid this? Larval hybridization would only affect the individuals participating in the hybrid mating and their immediate offspring. What comes next? Can these offspring still mate with their un-hybridized cousins? Is this the way they pass on their unique genetic pattern to other members of the species? If this is the proposal, the only difference between normal descent with modification and larval hybridization is that ordinarily the modification begins as a change in the organisms own DNA, while with larval hybridization the modification is accomplished by the acquisition of DNA from another organism. This sort of thing already happens in the cases of endogenous retroviral insertions. The new DNA is introduced by a retrovirus, but is then transmitted by inheritance in the usual way exactly as a mutation to the organisms own DNA is. So how does this really differ from descent with modification? It seems it offers only an alternate mode of modification. It still relies on the transmission of that modification via descent. So it still falls within the category of descent with modification.
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Fosdick  Suspended Member (Idle past 5527 days) Posts: 1793 From: Upper Slobovia Joined: |
gluadys writes:
Yes, gluadys. According to Donald Williamson and Sonya Vickers (in American Scientist, Nov.-Dec. 2007, pp. 509-517):
Is the hypothesis of larval hybridization intended to provide a mechanism of horizontal gene transfer in animals? quote: I am not sure I follow this. Horizontal gene transfer is a phenomenon well known among bacteria, but not frequent among eukaryotes. Is the hypothesis of larval hybridization intended to provide a mechanism of horizontal gene transfer in animals?
Yes. They go on to say:
quote:As such, convergent evolution looks a little shaky, I think. If this is the proposal, the only difference between normal descent with modification and larval hybridization is that ordinarily the modification begins as a change in the organisms own DNA, while with larval hybridization the modification is accomplished by the acquisition of DNA from another organism.
Yes, or its larvae. Their hypothesis relies on the merging of genomes by way of HGT.
So how does this really differ from descent with modification?
Dramatically. In HGT there would be no common ancestor from which certain traits (alleles) could be inherited. Those traits would be acquired instead "from the side." But this is not to say that decent with modification couldn't proceed from there. ”HM Edited by Hoot Mon, : No reason given.
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gluadys Member (Idle past 4990 days) Posts: 57 From: Canada Joined: |
hootmon writes: gluadys writes:
Dramatically. In HGT there would be no common ancestor from which certain traits (alleles) could be inherited. Those traits would be acquired instead "from the side." But this is not to say that decent with modification couldn't proceed from there. So how does this really differ from descent with modification? It is an interesting hypothesis, but I don't see it as all that dramatic. As you say, it would still involve descent with modification to proceed from the initial hybridization, so there is still time needed for the transformation of the species. Transitionals will still be very much part of the scenario, and possibly fossils too. It strikes me that in opposing this to "descent with modification" you are falling into the common trap of thinking that evolution is what happens to an organism in which the DNA is modified. But evolution is not just the initial modification, no matter what the source. Evolution is the changing of a species to incorporate that modification into the species genome.
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Fosdick  Suspended Member (Idle past 5527 days) Posts: 1793 From: Upper Slobovia Joined: |
gluadys writes:
In reply, I might ask you if you think all of these evolutionary scenarios are plausible:
It strikes me that in opposing this to "descent with modification" you are falling into the common trap of thinking that evolution is what happens to an organism in which the DNA is modified. But evolution is not just the initial modification, no matter what the source. Evolution is the changing of a species to incorporate that modification into the species genome. quote: As such, scenario C is another way of demonstrating the role of HGT in evolution, via larval hybridization. It short-circuits the much longer process of descent with modification. ”HM
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Blue Jay Member (Idle past 2725 days) Posts: 2843 From: You couldn't pronounce it with your mouthparts Joined: |
Hi, Hoot Mon.
I won't outrightly disagree with this hypothesis, because it seems like it could be interesting if any evidence is found for higher eukaryote HGT. However, I do notice a couple of problems. Your quote from Williamson and Vickers:
quote: I think the lack of the trochophore larva in cephalopods is more easily explained by the loss of the stage, while clams and snails retained it. The image from your message is provided below for a reference. Cephalopods are most often considered a derived branch or sister-group of the gastropods. The loss of a larval stage isn't unpredecented, either: lots of frogs skip the tadpole stage, and lots of groups of snails don't have trochophores, either. Also, as far as I can tell, the animals with trochophore larvae are still considered to form a monophyletic group (along with various groups wherein the trochophore was apparently lost). I don't believe it actually is hypothesized that the trochophore was developed convergently in these lineages (though I'll admit to a layman's expertise, at best, on this topic): most evidence seems to suggest that the trochophore was a plausible ancestral condition, and all non-trochophores in the clade are the derived condition.
Edited by Bluejay, : dBCodes problem -Bluejay Darwin loves you.
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Blue Jay Member (Idle past 2725 days) Posts: 2843 From: You couldn't pronounce it with your mouthparts Joined: |
Hi, Gluadys. And, welcome to EvC!
gluadys writes: As you say, it would still involve descent with modification to proceed from the initial hybridization, so there is still time needed for the transformation of the species. I disagree with you on this. Assuming he's right that HGT can and, in fact, did happen in eukaryotes, isn't it entirely possible that a new species is formed immediately following an HGT event? Of course it would be difficult for this to happen in a bisexual species unless that specific HGT event was unusually common. But, snails are hermaphroditic, which always permits the possibility of self-fertilization (though I don't know how much of this is actually allowed by gastropods). But, speaking purely theoretically, Hoot Mon could be right that larval hybridization would not require an extended period of descent with modification. -Bluejay Darwin loves you.
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gluadys Member (Idle past 4990 days) Posts: 57 From: Canada Joined: |
hootmon writes:
In reply, I might ask you if you think all of these evolutionary scenarios are plausible: Yes, though for the moment I would consider C more speculative than the others. I'll wait and see how the scientists hash it out.
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gluadys Member (Idle past 4990 days) Posts: 57 From: Canada Joined: |
Bluejay writes: Hi, Gluadys. And, welcome to EvC! Thanks.
Assuming he's right that HGT can and, in fact, did happen in eukaryotes, isn't it entirely possible that a new species is formed immediately following an HGT event? Of course it would be difficult for this to happen in a bisexual species unless that specific HGT event was unusually common. But, snails are hermaphroditic, which always permits the possibility of self-fertilization (though I don't know how much of this is actually allowed by gastropods). But, speaking purely theoretically, Hoot Mon could be right that larval hybridization would not require an extended period of descent with modification. That was one of the things I wanted to have clarification on. Is there any assumption here that the hybrid offspring can breed with non-hybrids or is the hybrid dependent on having other hybrids to breed with, or, as you suggest on self-fertilization. Not all hermaphrodites can or do self-fertilize. I don't know how common it is in snails either.
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.3 |
HootMan writes: (a) is closest to the observed pattern. (b) does not match the evidence where the fossil record is detailed enough to distinguish it from (a), and is indistinguishable from (a) across most of the evidence. As for (c), well, it commits both the error of (b) and compounds it with suggesting an unobserved mechanism for additional change - it could be possible; but until you can show me some compelling evidence? I remain sceptical. (a) then is the more convincing, although it shows a more consistent rate of change than that supported by the data.
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Fosdick  Suspended Member (Idle past 5527 days) Posts: 1793 From: Upper Slobovia Joined: |
Bluejay writes:
You make a good point. This will have to be resolved for Williamson's hypothesis to advance anywhere.
I think the lack of the trochophore larva in cephalopods is more easily explained by the loss of the stage, while clams and snails retained it...Cephalopods are most often considered a derived branch or sister-group of the gastropods. The loss of a larval stage isn't unpredecented, either: lots of frogs skip the tadpole stage, and lots of groups of snails don't have trochophores, either. Also, as far as I can tell, the animals with trochophore larvae are still considered to form a monophyletic group (along with various groups wherein the trochophore was apparently lost).
But trochophore larvae are not monophyletic (again, per Williamson & Vickers' diagram):
...most evidence seems to suggest that the trochophore was a plausible ancestral condition, and all non-trochophores in the clade are the derived condition.
This, of course, is key to the larval-hybridization hypothesis. But I still like Williamson's hypothesis because it challenges old thinking on how evolution occurs, and because I have often wondered if polyphyletic larvae have ever mated. What is a larva, anyway, if not just a genome-dispersal mechanism? From Williamson & Vickers (2007):
quote:This question seems to be huge in terms of how we view the "evolutionary process." It has all sorts of implications, raising questions such as these: Just how dominant is natural selection in the evolutionary process? Could larval hybridization be a viable alternative to NS in some evolutionary processes? Could larval hybridization, or the process of HGT, be the key mechanism of "punctuated equilibrium"? Could HGT explain more about the so-called Cambrian Explosion than descent with modification? ”HM Edited by Hoot Mon, : No reason given.
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Fosdick  Suspended Member (Idle past 5527 days) Posts: 1793 From: Upper Slobovia Joined: |
Mr Jack writes:
Why would it be impossible for the genomes of different species to intermingle by way of larval cross-fertilization? HGT is already a well known process, albeit mostly in prokaryotes. But it certainly cannot be ruled out for eukaryotes.
(a) is closest to the observed pattern. (b) does not match the evidence where the fossil record is detailed enough to distinguish it from (a), and is indistinguishable from (a) across most of the evidence. As for (c), well, it commits both the error of (b) and compounds it with suggesting an unobserved mechanism for additional change - it could be possible; but until you can show me some compelling evidence? I remain sceptical. (a) then is the more convincing, although it shows a more consistent rate of change than that supported by the data.
Do you think Punc Eq can be explained entirely by descent with modification? ”HM
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Dr Jack Member Posts: 3514 From: Immigrant in the land of Deutsch Joined: Member Rating: 8.3 |
Why would it be impossible for the genomes of different species to intermingle by way of larval cross-fertilization? HGT is already a well known process, albeit mostly in prokaryotes. But it certainly cannot be ruled out for eukaryotes. It might be possible; but I'd need evidence of it occuring before I accept it as an explaination; frankly, the letters link you posted in your first post covers the reasons I don't give credence to the idea.
Do you think Punc Eq can be explained entirely by descent with modification? There is no "punc eq"; it doesn't need explaining.
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Blue Jay Member (Idle past 2725 days) Posts: 2843 From: You couldn't pronounce it with your mouthparts Joined: |
Hi, Hoot Mon
Hoot Mon writes: But trochophore larvae are not monophyletic (again, per Williamson & Vickers' diagram): I have always read that all the phyla in that diagram (Sipuncula, Mollusca, Annelida and Rotifera) are grouped in the clade Lophotrochozoa, whose common ancestor was proposed to have had a trochophore larva. I realize that there is some disagreement as to the placement of the phylum Rotifera, which compounds the matter significantly. But, I don't know enough about this subject to make an informed conclusion. I remain skeptical of this larval-hybridization hypothesis, simply because the two alternatives (monophyly and convergence) are apparently at least as robust as it is. It could be tested, I think: you could sequence genes that are active during larval development, and separately sequence adult genes, and plot separate cladograms based on adult and larval genes. If the larval cladogram resolves a trochophore clade while the adult cladogram does not, this could support Williamson's hypothesis. Granted, the process would be a pain in the butt, but the technology exists (I think). If Williamson's hypothesis turned out to be correct, it would be a very interesting development in evolutionary biology. At this point, it seems to be a violation of parsimony to me, but I'll withhold final judgment until it has been tested. -Bluejay Darwin loves you.
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