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Author Topic:   Descent with Modification v. Larval Hybridization
Fosdick 
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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


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AdminNosy
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Message 2 of 23 (479030)
08-23-2008 1:42 PM


Thread moved here from the Proposed New Topics forum.

  
gluadys
Member (Idle past 3301 days)
Posts: 57
From: Canada
Joined: 08-22-2008


Message 3 of 23 (479083)
08-24-2008 1:59 PM
Reply to: Message 1 by Fosdick
08-23-2008 12:59 PM


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.


This message is a reply to:
 Message 1 by Fosdick, posted 08-23-2008 12:59 PM Fosdick has responded

Replies to this message:
 Message 4 by Fosdick, posted 08-24-2008 8:07 PM gluadys has responded

  
Fosdick 
Suspended Member (Idle past 3838 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 responded

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

  
gluadys
Member (Idle past 3301 days)
Posts: 57
From: Canada
Joined: 08-22-2008


Message 5 of 23 (479139)
08-24-2008 8:22 PM
Reply to: Message 4 by Fosdick
08-24-2008 8:07 PM


Re: Trochophore larvae
hootmon writes:

gluadys writes:

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.

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.


This message is a reply to:
 Message 4 by Fosdick, posted 08-24-2008 8:07 PM Fosdick has responded

Replies to this message:
 Message 6 by Fosdick, posted 08-24-2008 9:04 PM gluadys has responded
 Message 8 by Blue Jay, posted 08-24-2008 9:14 PM gluadys has responded

  
Fosdick 
Suspended Member (Idle past 3838 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 responded

Replies to this message:
 Message 9 by gluadys, posted 08-24-2008 10:08 PM Fosdick has not yet responded
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Blue Jay
Member (Idle past 1036 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 7 of 23 (479146)
08-24-2008 9:06 PM
Reply to: Message 4 by Fosdick
08-24-2008 8:07 PM


Re: Trochophore larvae
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:
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.

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.


This message is a reply to:
 Message 4 by Fosdick, posted 08-24-2008 8:07 PM Fosdick has responded

Replies to this message:
 Message 12 by Fosdick, posted 08-25-2008 11:52 AM Blue Jay has responded

  
Blue Jay
Member (Idle past 1036 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 8 of 23 (479147)
08-24-2008 9:14 PM
Reply to: Message 5 by gluadys
08-24-2008 8:22 PM


Re: Trochophore larvae
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.


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

Replies to this message:
 Message 10 by gluadys, posted 08-24-2008 10:15 PM Blue Jay has not yet responded

  
gluadys
Member (Idle past 3301 days)
Posts: 57
From: Canada
Joined: 08-22-2008


Message 9 of 23 (479154)
08-24-2008 10:08 PM
Reply to: Message 6 by Fosdick
08-24-2008 9:04 PM


Re: Trochophore larvae
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.


This message is a reply to:
 Message 6 by Fosdick, posted 08-24-2008 9:04 PM Fosdick has not yet responded

  
gluadys
Member (Idle past 3301 days)
Posts: 57
From: Canada
Joined: 08-22-2008


Message 10 of 23 (479155)
08-24-2008 10:15 PM
Reply to: Message 8 by Blue Jay
08-24-2008 9:14 PM


Re: Trochophore larvae
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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 Message 8 by Blue Jay, posted 08-24-2008 9:14 PM Blue Jay has not yet responded

  
Dr Jack
Member (Idle past 443 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 11 of 23 (479183)
08-25-2008 10:38 AM
Reply to: Message 6 by Fosdick
08-24-2008 9:04 PM


Punc Eq is wrong; this compounds it.
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.


This message is a reply to:
 Message 6 by Fosdick, posted 08-24-2008 9:04 PM Fosdick has responded

Replies to this message:
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Fosdick 
Suspended Member (Idle past 3838 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 responded

Replies to this message:
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Fosdick 
Suspended Member (Idle past 3838 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 responded

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

  
Dr Jack
Member (Idle past 443 days)
Posts: 3507
From: Leicester, England
Joined: 07-14-2003


Message 14 of 23 (479262)
08-25-2008 7:11 PM
Reply to: Message 13 by Fosdick
08-25-2008 12:08 PM


Re: Punc Eq is wrong; this compounds it.
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.


This message is a reply to:
 Message 13 by Fosdick, posted 08-25-2008 12:08 PM Fosdick has responded

Replies to this message:
 Message 18 by Fosdick, posted 08-26-2008 3:32 PM Dr Jack has responded

  
Blue Jay
Member (Idle past 1036 days)
Posts: 2843
From: You couldn't pronounce it with your mouthparts
Joined: 02-04-2008


Message 15 of 23 (479293)
08-26-2008 8:57 AM
Reply to: Message 12 by Fosdick
08-25-2008 11:52 AM


Re: Trochophore larvae
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.


This message is a reply to:
 Message 12 by Fosdick, posted 08-25-2008 11:52 AM Fosdick has responded

Replies to this message:
 Message 16 by Dr Jack, posted 08-26-2008 12:32 PM Blue Jay has responded
 Message 17 by Fosdick, posted 08-26-2008 3:19 PM Blue Jay has responded

  
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