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Author Topic:   Descent with Modification v. Larval Hybridization
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 1 of 23 (479026)
08-23-2008 12:59 PM


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

Replies to this message:
 Message 3 by gluadys, posted 08-24-2008 1:59 PM Fosdick has replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 4 of 23 (479137)
08-24-2008 8:07 PM
Reply to: Message 3 by gluadys
08-24-2008 1:59 PM


Trochophore larvae
gluadys writes:
Is the hypothesis of larval hybridization intended to provide a mechanism of horizontal gene transfer in animals?
Yes, gluadys. According to Donald Williamson and Sonya Vickers (in American Scientist, Nov.-Dec. 2007, pp. 509-517):
quote:
Rotifers have simple life histories, but these small marine and freshwater animals may have contributed a larval stage to the life histories of other animals, explaining the scattering of so-called trochophore larvae through unrelated phyla. In the above scenario, reading forward in evolutionary time from the bottom, a polychaete worm hybridized with a rotifer, acquiring a trochophore larva (1); this part of the polychaete's genome was acquired by a sipunculan worm in a second hybridization (2)...
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:
Further hybridization with rotifers gave trochophore larvae to the ancestor's of today's clam-like and snail-like mollusks. Their close relatives, the octopuses and squid, lack larvae. In conventional thinking, larval forms arose over time as young and adult forms within a species became more and more different. The similarities among larvae in distantly related species are conventionally explained by convergent evolution”many organisms developing larval stages to solve problems such as dispersal and feeding.
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.

This message is a reply to:
 Message 3 by gluadys, posted 08-24-2008 1:59 PM gluadys has replied

Replies to this message:
 Message 5 by gluadys, posted 08-24-2008 8:22 PM Fosdick has replied
 Message 7 by Blue Jay, posted 08-24-2008 9:06 PM Fosdick has replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 6 of 23 (479144)
08-24-2008 9:04 PM
Reply to: Message 5 by gluadys
08-24-2008 8:22 PM


Re: Trochophore larvae
gluadys writes:
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.
In reply, I might ask you if you think all of these evolutionary scenarios are plausible:
”from Williamson and Vickers (2007):
quote:
From Darwin forward, classical theory has viewed life's evolutionary history as taking place through continuing adaptation and mutation. Darwin's tree of life may be seen as a smoothly branching tree in which each bifurcation indicates common ancestry, and truncated branches indicates extinctions (a). In 1972, Niles Eldredge and Stephan Jay Gould, noting spurts of activity in the fossil record, introduced the concept of "punctuated evolution," pointing out that species appear fully formed (b). Williamson's larval-transfer theory introduces another wrinkle: the notion that one animal can become a larva of another, even distantly related animal by hybridization, thus establishing a connection between the two branches of the tree of life (c). The diagrams trace the ancestry of a hypothetical starfish (red), one of several species that may have acquired larvae through hybridization (blue arrows).
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

This message is a reply to:
 Message 5 by gluadys, posted 08-24-2008 8:22 PM gluadys has replied

Replies to this message:
 Message 9 by gluadys, posted 08-24-2008 10:08 PM Fosdick has not replied
 Message 11 by Dr Jack, posted 08-25-2008 10:38 AM Fosdick has replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 12 of 23 (479202)
08-25-2008 11:52 AM
Reply to: Message 7 by Blue Jay
08-24-2008 9:06 PM


Re: Trochophore larvae
Bluejay writes:
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.
You make a good point. This will have to be resolved for Williamson's hypothesis to advance anywhere.
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:
[The] larval transfer hypothesis proposes that larvae, and the genes that specify them, have been transferred from one hereditary animal lineage to another by cross-species, cross genera and even cross-phyla fertilization. We feel compelled to ask a question that is obvious to those not trapped in conventional thinking: Could animals with larval forms be hybrids, the products of successful fusions of genomes that are expressed in sequence during the animal's life history?
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.

This message is a reply to:
 Message 7 by Blue Jay, posted 08-24-2008 9:06 PM Blue Jay has replied

Replies to this message:
 Message 15 by Blue Jay, posted 08-26-2008 8:57 AM Fosdick has replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 13 of 23 (479206)
08-25-2008 12:08 PM
Reply to: Message 11 by Dr Jack
08-25-2008 10:38 AM


Re: Punc Eq is wrong; this compounds it.
Mr Jack 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.
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) 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

This message is a reply to:
 Message 11 by Dr Jack, posted 08-25-2008 10:38 AM Dr Jack has replied

Replies to this message:
 Message 14 by Dr Jack, posted 08-25-2008 7:11 PM Fosdick has replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 17 of 23 (479340)
08-26-2008 3:19 PM
Reply to: Message 15 by Blue Jay
08-26-2008 8:57 AM


Re: Trochophore larvae
Bluejay writes:
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.
But, so far as the lophotrochozoans are concerned, they are protostomes, not deuterostomes. Yet the trochophore larvae can be found in both protostomes phyla (e.g. annelids) and deuterostomes phyla (e.g., echinoderms)”see this chart. This suggests that larval hybridization may have occurred across the protostomes-deuterostomes boundary, which is a huge leap on any cladogram.
”HM

This message is a reply to:
 Message 15 by Blue Jay, posted 08-26-2008 8:57 AM Blue Jay has replied

Replies to this message:
 Message 21 by Blue Jay, posted 08-26-2008 6:25 PM Fosdick has replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 18 of 23 (479347)
08-26-2008 3:32 PM
Reply to: Message 14 by Dr Jack
08-25-2008 7:11 PM


Re: Punc Eq is wrong; this compounds it.
Mr Jack writes:
There is no "punc eq"; it doesn't need explaining.
So, do you think all evolutionary prcosses can be explained by descent with modification?
”HM

This message is a reply to:
 Message 14 by Dr Jack, posted 08-25-2008 7:11 PM Dr Jack has replied

Replies to this message:
 Message 19 by Dr Jack, posted 08-26-2008 3:46 PM Fosdick has replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 22 of 23 (479393)
08-26-2008 7:38 PM
Reply to: Message 21 by Blue Jay
08-26-2008 6:25 PM


Re: Trochophore larvae
Bluejay writes:
Bluejay writes:
the trochophore larvae can be found in both protostomes phyla (e.g. annelids) and deuterostomes phyla (e.g., echinoderms)
Really? Are you sure it's in echinoderms? I've never heard that before. Williamson and Vickers say it's in echiurans, but those are lophotrochozoan worms.
I'm wrong! Thanks for the biology lesson. Echinoderms do not have trochophore larvae; they have bipinnaria larvae. They do look a little like trochophores, but they don't have the midrift fringe of cilia. Also, trochophore larvae appear to be much more ontogenetically advanced than the bispinnaria larvae of the starfish, for example, which are modified gastrula.
I'm afraid I can't support the idea that larval hybridization occurred between Protostomes and Deuterostomes.
But as you say to Mr Jack in Message 20:
The majority of people don't give a wet slap about worms and slugs.
Probably true, but I like 'em.
”HM

This message is a reply to:
 Message 21 by Blue Jay, posted 08-26-2008 6:25 PM Blue Jay has not replied

  
Fosdick 
Suspended Member (Idle past 5500 days)
Posts: 1793
From: Upper Slobovia
Joined: 12-11-2006


Message 23 of 23 (479403)
08-26-2008 8:20 PM
Reply to: Message 19 by Dr Jack
08-26-2008 3:46 PM


Re: Punc Eq is wrong; this compounds it.
Mr Jack writes:
It depends what you mean by that; there is horizontal gene transfer between organisms, and certain apparently deeply significant events in evolutionary history appear to have occurred by advanced symbiosis - most obviously the history of mitochondria and other organelles in Eukaryotic cells...But the extent to which these events have influenced evolution is not clear, particularly among the "higher" organisms.
HGT between nuclear genomes of a "higher" organisms, so that it makes a difference to their natural history, is probably a stretch. (I haven't yet gotten over "The Fly.")
But the extent to which these events have influenced evolution is not clear, particularly among the "higher" organisms. Also, it is not clear to me that these are not considered most productively as special forms of individual heritable variation (of which mutation and sexual recombination form the two most important examples) upon which the normal processes of natural selection then operate rather than as distinct processes of evolution.
Finally, various non-adaptive effects certainly do occur (genetic drift and the founder effect to name just two) but, again, these are processes that operate with, rather than instead of, descent with modification.
Well said. I have to agree.
Even if larval hybridization does turn out to be correct; it will represent a new means of generating genetic variety rather than a strictly alternative means of evolving.
I suppose one could say that genetic engineering and its use of HGT is a new way of evolving without the need for descent with modification (as discussed in Digital Life Design”What a concept!).
”HM

This message is a reply to:
 Message 19 by Dr Jack, posted 08-26-2008 3:46 PM Dr Jack has not replied

  
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