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Author
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Topic: Ontogenetic Depth and the Origin of Animals
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yersinia
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Member # 324
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posted 08. February 2003 02:08
quote:
I think your quote from Raff must be from an earlier paper. Hybrids between direct- and indirect-developers have been made (see, e.g., Mark G. Nielsen et al., "Novel gene expression patterns in hybrid embryos between species with different modes of development," Evolution & Development 2 [2000]:133-144). The important question is what do those hybrids signify?
Seems to me that hybrids signify that two species with different development patterns evolved from a common ancestor in the very recent (evolutionary) past. Even the strictest young-earth creationists consider hybridization (even if only partially successful) good evidence of common descent of two species within a "kind". These developmental differences of which you are making so much are -- to coin a phrase -- mere microevolutionary variation within a type.
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Tristan Abbey
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posted 08. February 2003 03:02
quote:
Seems to me that hybrids signify that two species with different development patterns evolved from a common ancestor in the very recent (evolutionary) past.
The hybrids don't explain how different developmental patterns even arise.
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yersinia
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posted 08. February 2003 06:01
quote:
The hybrids don't explain how different developmental patterns even arise.
Not exactly, but they give us a Big Hint that this particular difference came about by a natural process. In short, there ain't no gap! The durn things can interbreed and produce offspring that undergo a significant degree of development even if they don't make it to reproductive maturity. This is something that only closely related species can do.
As for exactly how these changes occurred, it would be nice if we knew what all the relevant genes and proteins did during the process of development, in both of the parent species and the hybrid. This would give us some basis to discuss what kinds of mutations might have occurred and been selected. Until that point is reached however we're stuck with more general arguments.
Anyone care to suggest an ID model for how the differing development of these two closely-related species came to be? Perhaps ID-intervention is not restricted to the mists of billions of years ago after all?
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yersinia
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posted 08. February 2003 06:20
PS: Looks like RA Raff & co. have already made significant progress along the lines I suggested above:
quote:
Evol Dev 2001 Jan-Feb;3(1):24-33
Wnt gene expression in sea urchin development: heterochronies associated with the evolution of developmental mode.
Ferkowicz MJ, Raff RA.
Indiana Molecular Biology Institute and Department of Biology, Indiana University, Bloomington 47405, USA.
The Wnt genes encode a large family of conserved secreted proteins that are widely involved in animal development. The variety and ubiquity of this ancient family suggest that Wnt genes may have been important in the evolution of animal development, including early development. To test this hypothesis, we have characterized the expression of several Wnt genes in closely related sea urchins that exhibit radically different modes of early development. Wnt-1, -4, and -5 genes exhibit several conserved molecular and developmental characteristics, both within sea urchins and with Wnt genes examined in other animals (Ferkowicz et al. 1998). Here, we demonstrate that sea urchin Wnt-5 transcripts are specifically detected by in situ hybridization in discrete embryonic, larval, and developing adult tissues and processes: (1) in a band of vegetal ectoderm in mesenchyme blastula stage embryos, (2) in the larval ciliary bands, (3) in tissues that form the early adult rudiment (left coelomic pouch and overlying vestibular ectoderm), and (4) in the developing adult radial nervous system. We find that the sites of Wnt-5 transcript accumulation are conserved in species exhibiting either indirect- or direct-developmental modes, suggesting that Wnt-5 function(s) have been conserved in sea urchin development. However, dramatic heterochronic changes in Wnt-5 gene expression have occurred in the direct-developing species that parallel the accelerated morphological changes that occur during direct development. These results suggest that heterochronic changes in the expression of conserved developmental regulatory genes, such as the Wnt family members, are agents of evolutionary change in animal development.
In other words, same old genes, different timing. Perhaps evolving direct development from indirect development is in part just a matter of speeding things up?
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Paul A. Nelson
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posted 08. February 2003 10:42
Replies to various.
Emma wrote:
quote:
But if you work the problem from the other end, the problem seems to disappear altogether: Every time an experimenter subjects a population of animals to selection, creating two distinct populations, you end up with populations that are different in some way. At some point in an individual animals' development, they diverge from that of members of the other population - even if it's just microevolution. And yet population changes due to novel mutations & subsequent selection have happened countless times in the lab & in the wild.
Yes -- developmental pathways and processes do change. Look simply at the phenotypic differences among breeds of dogs. But does development change to the degree that the theory of common descent requires?
That's a different question entirely, as is the other question posed by the marching band problem: How can natural selection build ontogenetic networks from scratch? Carl Schlichting of the University of Connecticut wasn't indulging in hyperbole when he wrote (this year), "How cell types of multicellular organisms came to be differentiated" -- i.e., the marching band problem -- "is still an open issue" (2003, 98).
The marching band problem thus has two dimensions:
1. Building ontogenetic networks de novo; and
2. Deeply modifying those networks, once they have been built.
Attempts to solve problem (2) comprise a large literature. Neo-Darwinian geneticists warned that tinkering with body plans was likely to lead to disaster. The geneticist Bruce Wallace, for instance, a student of Dobzhansky's at Columbia, wrote:
quote:
The Bauplan [body plan] of an organism...can be thought of as the arrangement of genetic switches that control the course of embyronic and subsequent development of the individual; such control must operate properly both in time generally and sequentially in the separately differentiated tissues. Selection, both natural and artificial, that leads to morphological change and other developmental modification does so by altering the settings and triggerings of these switches....The extreme difficulty encountered when attempting to transform one organism into another but still functional one lies in the difficulty in resetting a number of the many controlling switches in a manner that still allows for the individual's orderly (somatic) development. (1984, 70)
But animals do develop in very different ways, from the earliest stages forward. Thus, if common descent is true [remember that assumption], early development must vary heritably somehow. But how?
Evidence that early developmental stages vary heritably is almost always comparative. That is, an author assumes that two or more groups share a common ancestor (see my comments on Heliocidaris, below); those groups exhibit early embryonic differences; therefore, variation or mutation in early development is heritable.
Consider, for instance, the following passage from Raff and colleagues (1991, 189):
quote:
...early development is often held to be particularly subject to constraint, and thus to be highly conserved in evolution.
There's plenty of experimental support for this, and that support was the main reason classical neo-Darwinism avoided postulating macromutations.
But Raff et al. continue:
quote:
Yet early development does evolve, and sometimes dramatically.
OK -- what's the evidential basis for this claim? Well, it's not experimental. Developing organisms typically respond to early-acting mutations by dying (embryonic lethals).
The evidential basis is the assumption of common descent -- an assumption that here, as elsewhere in historical biology, is not up for grabs.
Yersinia wrote:
quote:
Seems to me that hybrids signify that two species with different development patterns evolved from a common ancestor in the very recent (evolutionary) past.
Possibly. I'm certainly very interested in discovering if there's a natural pathway from indirect (H. tuberculata) to direct (H. erythrogramma) development in Heliocidaris (assuming that indirect development is ancestral), and keep that as a live possibility.
But the hybrids do not demonstrate that pathway, even when one assumes the very point at issue, namely, the common descent of different developmental patterns. Furthermore, if the hybridization experiments had failed, Raff and colleagues would not have said that a natural pathway did not exist (only that it is now lost to experimental reconstruction). One needs always to bear in mind what one has assumed versus learned from experiment.
Yersinia wrote:
quote:
The durn things can interbreed and produce offspring that undergo a significant degree of development even if they don't make it to reproductive maturity.
Careful. The "interbreeding" is entirely artificial, and only the H. erythrogramma (egg) x H. tuberculata (sperm) cross gets beyond the gastrula stage. The reciprocal cross, Ht (egg) x He (sperm), "did not even complete embryogenesis, but instead arrested at gastrulation" (E.C. Raff et al 1999, 1941-42).
What's striking is the lethal cross is the only one that happens without extensive investigator intervention: "H. tuberculata eggs can be readily fertilized by H. erythrogramma sperm in the laboratory" (Raff et al. 1999, 1938). "In contrast, H. erythrogramma eggs with an intact jelly coat cannot be fertilized by H. tuberculata sperm," meaning that Raff and colleagues had to remove the He egg jelly coat first, artificially.
Bottom line: Heliocidaris hybrids do not demonstrate that radical modifications to early development are heritable, as the theory of common descent requires. The "novel ontogeny" exhibited by the He x Ht hybrids goes no further than the F1 generation.
One should of course keep open the possibility that H. erythrogramma and H. tuberculata share a common ancestor. But that was (and remains) a given for Raff and colleagues, held without question from their earliest publications on Heliocidaris until today. If the hybridization experiments had failed completely, they'd still hold to common descent.
Refs
Raff, E.C. et al., 1999. A novel ontogenetic pathway in hybrid embryos between species with different modes of development. Development 126:1937-1945.
Raff, Rudolf A. et al. 1991. Implications of Radical Evolutionay Changes in Early Development for Concepts of Developmental Constraint. In New Perspectives in Evolution, New York (Wiley-Liss), pp. 189-207.
Schlichting, Carl D. 2003. Origins of differentiation via phenotypic plasticity. Evolution & Development 5:98-105.
Wallace, Bruce. 1984. Adaptation, Neo-Darwinian Tautology, and Population Fitness: A Reply. Evolutionary Biology 17:59-71.
[ 08. February 2003, 11:27: Message edited by: Paul A. Nelson ]
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Frances
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posted 08. February 2003 21:01
Paul,
The marching band problem may be too narrowly defined since you suggest that it requires building ontogenetic networks de novo but you seem to forget to include gene duplication for instance as a pathway to new ontogenetic networks or the reuse of existing ontogenetic networks with different timing for instance.
You also seem to doubt that common descent is true. I find this somewhat puzzling since the evidence supporting common descent is vaste and imho quite conclusive.
In fact the idea addressed by Yersinia above namely the timing of certain events, seems to go back to the ideas of the late Stephen Gould.
Bauplans themselves or hox genes seem to quite conserved although they still show quite some evidence of their evolutionary history. Hox genes suggest that variations on the same theme may have been responsible for the various bauplans.
You object to Raff's findings wrt the novel ontogenetic pathway by correctly pointing out that they did not have any off spring. But there is where you miss the point namely that developmental pathways are harldy as inflexible as you seem to want to suggest.
A good review article on the timing of embryonic development is "TEMPORAL PATTERN FORMATION
BY HETEROCHRONIC GENES" by Frank Slack and Gary Ruvkun. They show that heterochronic variation is wide spread and suggests that variation in the gene loci which determine the developmental timing may play important roles in evolutionary change. They study heterochronic mutations in C. elegans and show that the control of the timing is analogous to Drosophila. Neoteny is another interesting concept in which the retarded expression of (previously) juvenile characteristics in the adult stage can lead to morphological changes.
Or as Gunther J. Eble argues in _Multivariate approaches to development and evolution_ "Heterochrony is a powerful mechanism of evolutionary change in development."
To come back to the sea urchin, "Transcripts from all three sea urchin in _Phylogenetic Relationships and Developmental Expression of
Three Sea Urchin Wnt Genes_ Ferkowicz et al argue that "Wnts are detected at various developmental stages of both indirect- and direct-developing species. These data support the view that sea urchin Wnt genes exhibit many conserved aspects and at least three orthologs are developmentally regulated in both indirect- and direct-developing sea urchin embryos."
It seems to me that timing itself can play significant roles in creating novel morphologies.
Exciting new research has shown for instance 'The
spatial expression patterns of the Hox genes illuminate the evolutionary process by which the pentameral echinoderm body plan emerged from a bilateral ancestor.' as argued in _Spatial expression of Hoxcluster genes in the ontogeny of a sea urchin_. Co-option of regulatory genes seems to have been a major process in this clade.
Haag et al in _FROM MUTANTS TO MECHANISMS? ASSESSING THE CANDIDATE GENE PARADIGM IN EVOLUTIONARY BIOLOGY_ look at more extreme mutations (Goldsmiths Hopeful monster ??)
It may be that neo-Darwinism may have avoided macromutations ala Goldsmith but recent work suggests that this may have been a mistake.
Btw common ancestry were not a given for Raff as much as what the evidence showed.
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yersinia
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posted 09. February 2003 00:25
Paul writes, in part,
quote:
But animals do develop in very different ways, from the earliest stages forward. Thus, if common descent is true [remember that assumption], early development must vary heritably somehow. But how?
Evidence that early developmental stages vary heritably is almost always comparative. That is, an author assumes that two or more groups share a common ancestor (see my comments on Heliocidaris, below); those groups exhibit early embryonic differences; therefore, variation or mutation in early development is heritable.
[...]
The evidential basis is the assumption of common descent -- an assumption that here, as elsewhere in historical biology, is not up for grabs.
[...]
One should of course keep open the possibility that H. erythrogramma and H. tuberculata share a common ancestor. But that was (and remains) a given for Raff and colleagues, held without question from their earliest publications on Heliocidaris until today. If the hybridization experiments had failed completely, they'd still hold to common descent.
Paul appears to be attempting to save his argument by questioning the common descent of Heliocidaris erythrogramma and Heliocidaris tuberculata.
But the common descent of these two species from a common ancestor is no mere assumption that can be brushed aside! Based on the hypothesis that the two species share a close common ancestor, we can predict:
1) Close similarity in DNA and amino acid sequences, which will group next to each other, to the exclusion of everything outside the genus, to a high degree of statistical confidence.
2) Close similarity in chromosome number, structure, and gene arrangement on the chromosomes.
3) Essentially all genes will be shared by both species.
4) The two species will have shared mistakes in nonfunctional pseudogenes
...and many more. Even if all of these have not been fufilled yet (we haven't gotten around to sequencing their genomes), based on almost no firsthand knowledge of echinoderms, precious little investigation of the literature, and of course the modern theory of evolution, I can make these predictions with a high degree of confidence. Score one for the predictive power of common descent.
And of course, even on the criteria of the highly skeptical antievolutionary baraminologists, the partial hybridization evidence would score these two species as descended from a common ancestor.
Thus the common descent of Heliocidaris species is not an assumption but a well-documented fact which is readily susceptible to even further testing by well-known methods. Those who dispute this should propose an alternative hypothesis for the origin of these two species that better explains the available evidence and which is similarly testable.
I encourage everyone to now go back up to the quote and read the bits where Paul bases his counterargument on calling the common descent of these two species an assumption. If it's not an assumption but a fact, then he's stuck based on his own arguments.
In light of this, Heliocidaris should count as a piece of evidence that developmental pathways are, at least sometimes, rather more labile than Paul has been arguing.
yersinia
PS: Scientists are not bereft of information on how the changes in Heliocidaris development occurred, either, please follow the link I posted with the article in my previous post. Any serious attempt to argue that the changes were not possible by natural processes would have to wrestle with this literature as well as the fact of the common descent of these two species.
[edits for correction, and addition of PS]
[ 09. February 2003, 00:36: Message edited by: yersinia ]
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Paul A. Nelson
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posted 09. February 2003 15:11
Frances and Yersinia,
If H. erythrogramma and H. tuberculata were not related by common descent, how would you know it? Give the question some thought.
Yersinia -- some of the comparative work you mention has already been done. See, e.g., M.J. Smith et al., "Single-Copy DNA Distance between Two Congeneric Species Exhibiting Radically Different Modes of Development," Molecular Biology and Evolution 7 (1990):315-326. You can probably download the .pdf of this paper for free.
I'll have more to say later today or early tomorrow. There's an interesting story concerning Raff's arguments about Heliocidaris, relative to my dissertation work on the Univ. of Chicago, that fits in here.
[ 09. February 2003, 15:12: Message edited by: Paul A. Nelson ]
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Frances
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posted 09. February 2003 16:07
Hi Paul
How do we infer common descent between H. erythrogramma and H. tuberculata and how would we rule out common descent you ask.
First of all common descent by itself is a well accepted interpretation of the evidence provided to us by the fossil record, phylogenetic data etc. If you are questioning the fact of common descent then we need to go back to the basics, but assuming that you accept common descent (one or multiple common ancestors) one may ask how are the two related. Various studies have looked at the phylogenetic evidence that relates H. erythrogramma and H. tuberculata.
First the History of Echinoidea which looks at the fossil evidence. Since echinoids have a hard skeleton, fossil evidence is quite helpful in infering the historical evidence.
So now the relationship between the two Heliocidaris:
In _Phylogeny, Rates of Evolution, and Patterns of Codon Usage among Sea Urchin Retroviral-Like Elements, with Implications for the Recognition of Horizontal Transfer_ Mark Springer et al look at retroposons in the echinoids to establish a phylogenetic tree.
First of all there is the data from fossil record and molecular data. The data indicate that the H. erythrogramma and H. tuberculata diverged 20-30 million years ago. Based on reverse transcriptase , H3 and H4 histones, ribosomal RNA the estimate is less than 20 million years.
SURL elements again place the two heliocidaris sharing a common ancestor. Common ancestry, divergence all rely on a multitude of data. In _Single-Copy DNA Distance between Two Congeneric Sea Urchin Species Exhibiting Radically Different Modes of Development_ Smith et al estimate 10-13 million years for the divergence.
Charlotte Jeffery is a paleontologist who looks into echinoids and their diversification.
All in all fossil data as well as genetic data are very helpful in understanding the echinoids in space and time.
A great website THE ECHINOID DIRECTORY
There are many succesful applications of phylogenetic and fossil comparisons such as _Phylogenetic relationships among extant classes of echinoderms, as inferred from sequences of 18S rDNA, coincide with relationships deduced from the fossil record._ by Wada et al.
[ 09. February 2003, 16:21: Message edited by: Frances ]
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Argon
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posted 09. February 2003 18:34
Francis wrote (to Paul Nelson):
"How do we infer common descent between H. erythrogramma and H. tuberculata and how would we rule out common descent you ask."
William Dembski previously wrote here:
quote:
Let me spell out what's needed for a detailed Darwinian pathway to explain an IC system. The problem is similar to that of connecting two organisms in the fossil record. Given organisms X and Y, if they are related by common descent via the Darwinian mechanism, there must be a continuous chain of organisms X(0) --> X(1) --> ...--> X(m) = X and Y(0) --> Y(1) --> ...--> Y(n) = Y where X(0) = Y(0) is a common ancestor. In the best circumstance, each such X(i) and Y(j) must be explicitly exhibited and any arrows of causation connecting two organisms must produce small incremental changes that are highly probable on the basis of the Darwinian selection mechanism. The more intermediates that are missing from this picture and the more handwaving and just-so story-telling to describe the arrows of causation, the more problematic the evolutionary explanation.
As for how one rules out common descent (or at least, how one rules out a particular phylogenetic reconstruction), I believe that there are published methods in the literature. In the absence of a good fossil record, most involve comparisons of traits. Accumulated data seems to have convinced ID biochemists like Michael Behe and Mike Gene, as well as others who frequent this board, that common descent has some pretty good merits.
While I respect Paul Nelson's ability to discuss matters in a fairly neutral manner, to be honest, I don't think one could convince Paul that dogs had a common ancestor with wolves if he thought the Bible said otherwise, and so the matter of common descent wrt H. erythrogramma & H. tuberculata probably isn't worth pursuing in a scientific discussion (granted, the Bible is mute about the origins of these two species). Whether one could rule out ID or "natural" mechanisms as having roles in evolution via common descent with divergence, is probably the real issue worth discussing.
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posted 09. February 2003 19:14
Argon,
Have you noticed that the discussion has thus far focused on scientific issues. Why introduce epistemological issues regarding "belief" when everyone else is managing just fine critiquing Paul on scientific grounds?
I view such comments as fouls (such as blatant jabs to the groin in a boxing match). There is no reason to go low, when a perfectly good conversation can take place on even ground.
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yersinia
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posted 09. February 2003 19:23
quote:
Frances and Yersinia,
If H. erythrogramma and H. tuberculata were not related by common descent, how would you know it? Give the question some thought.
Well, if the sequences of the two species failed to group together against other randomly chosen species (e.g., one species groups with humans, another with liverworts), that would be a pretty good indication.
If echinoderm junk DNA encoded "made by Rael, 5 million B.C.", that too would be a good indication.
Many further hypotheticals along these lines could be suggested. This isn't very hard; should I be having difficulty?
The other possibility that would indicate that H. erythrogramma and H. tuberculata were not related by common descent would be an alternative hypothesis that explained the available facts better and which was similarly subject to tests. We're still waiting on that one.
quote:
Yersinia -- some of the comparative work you mention has already been done. See, e.g., M.J. Smith et al., "Single-Copy DNA Distance between Two Congeneric Species Exhibiting Radically Different Modes of Development," Molecular Biology and Evolution 7 (1990):315-326. You can probably download the .pdf of this paper for free.
Thanks for the tip! Looking it up reveals:
quote:
Mol Biol Evol 1990 Jul;7(4):315-26
Single-copy DNA distance between two congeneric sea urchin species exhibiting radically different modes of development.
Smith MJ, Boom JD, Raff RA.
Department of Biological Sciences, Simon Fraser University, Burnaby, British Columbia, Canada.
We have investigated the differences between nuclear genomes of two purportedly congeneric species of sea urchin that differ radically in early development. Heliocidaris tuberculata develops by means of a typical pluteus larva, whereas H. erythrogramma develops directly from an egg that is 100-fold the volume of the H. tuberculata egg. Reassociation kinetic analysis shows that the kinetic components of the genomic DNA from the two species are essentially the same. No single repeat component explains the 30% difference between the H. erythrogramma and H. tuberculata genomes. Reciprocal hybridization of tracer-labeled single-copy DNA fractions between these species indicates that approximately 50% of the single-copy DNA is sufficiently similar to form hybrids at standard hybridization criterion. Thermal denaturation profiles of the hybridized single-copy DNA sequence yields median (T50H) values of 13.8 degrees-16.5 degrees C. This result suggests a divergence time of 10-13 Mya, which is comparable to divergence times between congeneric sea urchin species in other genera that do not differ significantly in development. Radical differences in early developmental processes can evolve rapidly between closely related forms.
It looks to me like Raff et al. have tested the hypothesis of common descent of these two species for themselves.
Here is the link to the article, which I do recommend:
http://mbe.oupjournals.org/cgi/reprint/7/4/315.pdf
Some background points from the paper:
1) These two species (the only ones in their genus) are both found only in the "Australian region" (should have put a biogeography prediction in my original list)
2) The fossil record for the genus goes back 30 million years (this is not in the paper but I'd bet my boots that the fossils are in the Australian region also)
3) This paper uses a fairly crude DNA hybridization technique, and concludes in favor of a close relationship after about a ~10 million year divergence although the date is uncertain)
4) Direct development is found in about 20% of the 1000 or so sea urchin species, in widely scattered groups, indicating that this change has occurred multiple times
5) Direct development in sea urchins is associated with (1) much much larger eggs (the larva are non-feeders in the extreme case; I think the figure was an egg 200x the size of an indirect developer egg) (2) ~30% higher amounts of total DNA in the genome (nicely matching up with the widespread observation that C-value (total DNA amount) correlates with cell volume, perhaps because larger cells need bigger "junk" DNA spacers to make the genes more accessible for more rapid production of proteins -- and yes, if the pattern holds, then most of the difference in DNA amount is due to differing amounts of noncoding DNA).
6) I can't believe this one hasn't come up in this thread yet, but an important fact from the article that was news to me is that there are many sea urchins displaying varying degrees of specialization in the direct-vs.-indirect development continuum. This would seem to grossly contradict the idea that intermediates would be inviable.
7) The conclusions based on DNA hybridization from this paper are supported by other sequence data, e.g. they write, "The close relationship between the two Heliocidaris species and of both of them to Strongylocentrotus is borne out by 18S rRNA sequence data (Raff et al. 1988)."
So, Raff himself did at least two molecular studies to confirm close relationship more than 10 years ago; how then can this be called a mere "assumption" by Paul Nelson? How can molecular sequence similarities be considered good evidence of common descent in court paternity cases, forensic investigations (think Bill Clinton for instance), studies of human migration, and detection of HIV or anthrax, but not in this case?
Does there not come a point at which it becomes perverse to withhold assent from a particular hypothesis, and haven't we reached that point with the hypothesis that the two Heliocidaris species descended from a recent common ancestor?
[ 09. February 2003, 19:31: Message edited by: yersinia ]
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Frances
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posted 09. February 2003 19:30
The following Link may be of help understanding Paul's arguments wrt common descent.
The existence of small variants on the genetic code in no means undermine common descent. neither is the existence of horizontal gene transfer evidence against common descent although it does tend to muddle the roots a little bit.
Paul's final parting comments quote:
If common descent cannot be dislodged by the "evidence," then how should we go about evaluating it?
seems to be a strawman since one surely could formulate evidence that would disprove common descent.
Btw Darwin himself did not consider the primordial ancestor to be a single species. Non-universal genetic code is very well linked evolutonary so should not be considered as evidence against common descent.
Common descent seems to be the best explanation for the genetic and fossil data. A good FAQ can be found on the talkorigins website
What may help understand Paul's argument wrt developmental genes and the fact that many of them have been highly conserved is the following statement "According to philosopher of biology Paul Nelson, "This is a remarkable, remarkable result." If it turns out that these unique genes have important functions and can’t be altered without harming the organism, this would be a major blow to Darwinism. "
Source
It may be helpful to hear how this finding would be a major blow to Darwinism? The suggestion that these genes may not have precursors is of course imho a non sequitur.
Paul's dissertation seems to argue against common ancestry as well although it is not clear to me if he provides for a better explanation of the data.
[ 09. February 2003, 19:58: Message edited by: Frances ]
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yersinia
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posted 09. February 2003 19:55
Hi Francis,
It might be wise to limit ourselves to Heliocidaris common descent and development in this thread and save the genetic code, LGT etc. for another thread, just to keep things focused.
PS: I just realized that on page 1 you quoted the paper I just discussed in my previous post which Paul Nelson suggested. Apologies for forgetting...
quote:
A continuum of modifications of the pluteus has evolved, ranging from obligately feeding pluteus to loss of all larval features. These modifications include facultative planktotrophy (Emlet 1986), nonfeeding plutei (Okazaki 1975; Olsen et al., accepted), armless nonfeeding larvae, and brooded embryos that lack a larval stage. The changes resulting in direct development are expressed as a pattern of heterochronies, in which many features of indirect larval development are deleted
or abbreviated, and features of adult development are initiated early and are accelerated in rate. The evolution of direct development is accompanied by changes in oogenesis.
Species with typical feeding plutei produce small eggs (diameter - 100 pm), whereas
the nonfeeding forms have very large eggs (diameter 300-1,200 pm) (Emlet et
al. 1987). [I left out some citations]
This passage is crucial to the whole question in the thread.
[ 09. February 2003, 19:56: Message edited by: yersinia ]
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Argon
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posted 09. February 2003 22:59
Moderator 1 writes:
"Argon,
Have you noticed that the discussion has thus far focused on scientific issues. Why introduce epistemological issues regarding "belief" when everyone else is managing just fine critiquing Paul on scientific grounds?
I view such comments as fouls (such as blatant jabs to the groin in a boxing match). There is no reason to go low, when a perfectly good conversation can take place on even ground."
Understood. Thank you for leaving my post intact. I appreciate that.
I suppose my frustration is with the idea that common descent, particularly in the case of obviously closely related species, would somehow need additional support at this period of time (What a difference 150 years makes). As if the data had not already convinced most biologists, including those who support ID such as Behe, Mike Gene and Denton. If this doesn't impress others, I don't know what will -- At least not within the current capabilities of science to provide.
In most cases, common descent is no longer a question. What remains is determining the mechanism[s] behind it. To put it bluntly, this is the direction where ID is clearly proceeding as a potentially viable scientific enterprise. Mike Gene and Behe have suggested front-loading, perhaps with the first cell, as part of the explanation. Denton starts even earlier by proposing front-loading at the time of the big bang. William Dembski has suggested a combination of front-loading and tiny, probability-rigging events at the atomic level. All these map just fine to common descent and they all differ from "natural" (i.e. non-direct ID) mechanisms. H. erythrogramma and H. tuberculata are related, pure and simple, as are humans and chimps. Now let's move on.
Are there "Darwinian" pathways that can link H. erythrogramma and H. tuberculata, or were specific ID "events" required in particular stages in the evolution of these species? As I suggested earlier, that is the type of question that's worth debating.
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