Giardia lamblia, polyadenylation, and irreducible complexity
(The following is a slight adaptation of this essay. Readers may post questions and/or comments there as well as here.) As this series of essays has explained, the polyadenylation of messenger RNAs is a vital aspect of gene expression in eukaryotic cells (and a not-so-unimportant facet of RNA metabolism in other contexts). Polyadenylation is mediated by a sizeable complex that includes various RNA-binding proteins, nucleases, and other interesting activities. Genetic studies in yeast indicate that virtually every subunit of the core complex is essential - for viability and for pre-mRNA processing and polyadenylation in vitro and in vivo. (This review is freely available and serves as a good starting point for readers who wish to explore the subject further.) Biochemical and/or immunological depletion studies reveal a similar scenario in mammals, and a less-expansive set of studies suggests that a similar rule of thumb will apply in plants. The bottom line of all of this is that almost all of the subunits of the polyadenylation complex seem to be essential - remove one, and the complex cannot function. In the vernacular of a proponent of intelligent design, the polyadenylation complex would seem to be irreducibly complex.
It is in this context that the recently-completed genome of the parasitic organism Giardia lamblia enters the fray. Last year, the complete sequence of G. lamblia, some 12 million base pairs, was determined and analyzed. The authors of the study published in Science noted a number of interesting things - a preponderance of genes encoding protein kinases, evidence for substantial horizontal gene flow from bacteria and archaebacteria, and a streamlined core gene expression machinery (transcription and RNA processing). This streamlining is especially notable in the case of the polyadenylation machinery. Remarkably, of all the subunits in the yeast complex, genes for only three* can be found in G. lamblia (see the figure that follows this paragraph - adapted from Fig. 1 of Morrison et al.).
Naturally enough, one of these is the poly(A) polymerase (PAP). The other two polyadenylation-related proteins encoded by the G. lamblia genome correspond to Ysh1 and Yth1 (whose mammalian counterparts are CPSF73 and CPSF30, respective). Interestingly, as summarized here, these two subunits are the two to which nuclease activity has been ascribed. Also interestingly, the only RNA binding subunit amongst those seen is Yth1 (=CPSF30). Other subunits are missing. Thus, no other RNA binding subunits are apparent, none of the scaffolds (CPSF160/Yhh1, CstF77/Rna14, Fip1, symplekin/Pta1) are seen, and most of the subunits that have been shown to interact with the transcription complex (CPSF100/Ydh1, CstF50, and Pcf11, to name three) are absent. Indeed, entire complexes (CstF, CFmI, CFmII) appear to be missing.
What might these startling omissions mean? One possibility is that functional counterparts for most of these proteins exist, but that they have diverged so extensively as to be unrecognizable. This might be the case for some of the missing proteins, but many of these are so highly-conserved between plants and animals that this seems an unlikely explanation.
Another possibility is that mRNAs are in fact not polyadenylated in G. lamblia. This is apparently not the case, as cDNAs can be prepared using the usual methods (priming reverse transcription with oligo-dT). Moreover, these cDNAs have untemplated poly(A) tracts, and some limited sequence-gazing can identify a putative polyadenylation signal.
Neither of these possibilities seems likely. Which leaves us with the remarkable likelihood that mRNA polyadenylation in Giardia is mediated by a highly-reduced complex of but 3 proteins. This in turn brings us to some fascinating discussion, about both function and evolution.
First, about function. Absent some studies dedicated to polyadenylation mechanisms in Giardia, it's hard to make sense of the absence of so many essential components of the polyadenylation apparatus. But the fact that the Giardia complex consists of the two known endonucleases is interesting, as it suggests that the very core of the complex in eukaryotes is an endonucleolytic one. It also suggests that, as we peer ever more closely into the complex in other organisms, these two subunits will attract more attention. Other questions about RNA recognition and of links with transcription and splicing also come to mind. For example, might the RNA-binding activity of Yth1/CPSF30 play a more prominent role in polyadenylation signal recognition than has been assumed? Is there an obligatory link between transcription and polyadenylation? If so, what is the link in Giardia, and what might this suspected mechanism tell us about the analogous link in other eukaryotes? Etc., etc., etc.
Which brings us to the evolution of the complex. Giardia has gained some notoriety of sorts, having been identified at times as a very primitive, pre-mitochondrial eukaryote, or as a still-primitive eukaryote that lost its mitochondria. These two scenarios regarding the mitochondria of Giardia give us a similar set of contrasting pathways regarding the evolution of the polyadenylation complex. One scenario would be that the Giardia polyadenylation complex resembles the primordial eukaryotic complex, that the first polyadenylation apparatus consisted of little more than a nuclease and a polymerase. The complex we seen in other eukaryotes would be derived from a series of co-options, recruitments, and duplication events, all building on this simple beginning. Of course, the most exciting aspect of this scenario is that it gives us a remarkably clear link to nucleolytic activities in bacteria; this follows from the structural and functional similarities between CPSF73/Ysh1 and RNAse J (noted here).
The alternative is that the Giardia complex has lost most of the subunits that we see in other organisms. This seems unlikely, given the essential nature of most of the subunits in yeast. However, some differences in this regard exist between yeast and other eukaryotes; thus, Yth1 is essential in yeast, but its Arabidopsis counterpart is dispensible for viability. In any case, this alternative would provide us with a clear example of how extensively an irreducibly complex mechanism can evolve.
Hopefully, this essay has taught readers a thing or two. More importantly, in the best of cases, it has raised a number of questions. There may be some answers, but for many of these there await much experimentation and exploration.
Morrison, H.G., McArthur, A.G., Gillin, F.D., Aley, S.B., Adam, R.D., Olsen, G.J., Best, A.A., Cande, W.Z., Chen, F., Cipriano, M.J., Davids, B.J., Dawson, S.C., Elmendorf, H.G., Hehl, A.B., Holder, M.E., Huse, S.M., Kim, U.U., Lasek-Nesselquist, E., Manning, G., Nigam, A., Nixon, J.E., Palm, D., Passamaneck, N.E., Prabhu, A., Reich, C.I., Reiner, D.S., Samuelson, J., Svard, S.G., Sogin, M.L. (2007). Genomic Minimalism in the Early Diverging Intestinal Parasite Giardia lamblia. Science, 317(5846), 1921-1926. DOI: 10.1126/science.1143837
* - in the figure from Morrison et al., Pab1, RNA polymerase II and Glc7 are also noted as polyadenylation factor subunits. Pab1 is one of two poly(A)-binding proteins that plays roles in polyadenylation in yeast; this protein, as well as the G. lamblia protein identified in this study, is distinct in sequence and domain composition from the nuclear poly(A) binding proteins seen in mammals and plant. RNAP II is so considered because it is a scaffold of sots, upon which numerous other polyadenylation factors assemble; this function is needed for efficient polyadenylation. Glc7 is a protein that consistently purifies with the yeast polyadenylation complex. As it is not considered historically to be a core polaydenylation complex subunit, I have not elaborated on it in this essay.
[updated on Aug 30 - Carl Zimmer noted, if more briefly, the curious reduction of the Giardia polyadenylation complex in this essay. I didn't know of this until today, but thought it appropriate to mention.]
75 Comments
Laura · 29 August 2008
Wow. Impressive essay. I bewilders me that evolution can even track into the germs on everyday life. Bravo truly.
But I do have a few questions. Not necessary on this essay but on evolution in general. They are quite simple really. I claim to be neither Evolutionist nor Christian, rather I would accept the statement 'interested'. I am writing a paper on it's relationship and I need someone to answer a few questions I have. You up for it mate?
Stanton · 29 August 2008
Mike Elzinga · 29 August 2008
John Kwok · 29 August 2008
John Kwok · 29 August 2008
Dear Arthur,
This is absolutely brilliant. It is analogous to what I have seen Ken Miller describe with regards to just how "irreducibly complex" the bacterial flagellum is. Am looking forward to reading your future posts.
Regards,
John
P. S. Any chance of sending this to Ken? I think he'd be interested. You can tell him that I had suggested it.
Mike Elzinga · 29 August 2008
Mike Elzinga · 29 August 2008
Oops! Sorry about that last post. It was supposed to go to the Genome Biology thread to alert them there also. I had the wrong screen up.
David Stanton · 29 August 2008
Well once again, this proves that the concept of irreducible complexity is merely the product of a distinct lack of imagination. On the other hand, it beautifully confirms yet another prediction of what we would expect to see if the theory of evolution is true.
The most interesting issue to me is the phylogenetic position of Giardia. Is it basal in the eukaryote, thus representing an intermediate form between between prokaryote and eukaryote polyadenylation systems, or is it a degenerate eukaryote that branched off after mitochondria were acquired and subsequently lost both mitochondria and most polyadenylation genes? Quite likely, ribosomal sequences will yield the best answer to this question. It seems that the hypothesis that this represents an intermediate form would be the most parsimonous explanation.
Of course, this is also an example of the great predictive power of evolutionary thoery and the great value of comparative genomic data.
Opisthokont · 30 August 2008
fusilier · 30 August 2008
Opisthokont [#1]
Sorry to waste the bandwidth, but the first page of the Current Biology paper is blank. I'm using Acrobat Reader 8 under OSX 10.5.4, and the alert message says the error is in the document.
The balance of the paper downloads fine.
I am not a software geek, so it could well be my fault and i don't know enough to realize it.
fusilier
James 2:24
Arthur Hunt · 30 August 2008
Thanks to all for the commentary, and for the kind remarks and glowing reviews. If I may add a couple of brief comments:
John Kwok, I would expect that Ken Miller follows The Panda's Thumb. If you (or anyone else reading this) wishes to send him a heads-up, feel free.
Opisthokont, your comment is most welcome. By "notoriety", I hoped to convey that there has been considerable discussion about the place of Giardia on the tree of life. I have no wish to insinuate myself into this discussion, and I would hope that readers do not conclude from my essay that I am making any claims. For my purposes, the two possible extremes (deep-branching vs highly-reduced) are good contexts (pretexts?) for two contrasting pathways of origination of the polyadenylation complex in Giardia. Either pathway is fascinating, informative, and ultimately good fodder for a refutation of the ID use of irreducible complexity.
mafarmerga · 30 August 2008
Opisthokont's comments are well taken.
Those of us who call ourselves protistologists are slowly coming to the realization that an extant example of the earliest eukaryotes is unlikely to be found. None the less we can gain a vision of what this progenitor of all nucleated organisms was like by studying "odd-ball" organisms such as Giardia, Reclinomonas, Retortamonas, etc.
Nothing fries my lunch more than when cell biologists examine a system in yeast, mammals, and maybe some invertebrate and conclude "Ah-ha! It is nearly identical in all three and therefore MUST be very ancient"
As "Opisthokont's" name implies (brilliant choice by the way) the only thing it tells us is that yeast and animals belong to the same supergroup clade (Adl et al. 2005). Another example of our misguided animal-centric view of biology.
Science Avenger · 30 August 2008
Frank J · 30 August 2008
iml8 · 30 August 2008
David Stanton · 30 August 2008
Opisthokont,
Thanks for the great reference. It would appear that Giardia has most likely sencondarily lost mitochondria, possibly due to the parasitic life style. It could still represent a basal eukaryote lineage and thus an intermediate in the evolution of euraryotic polyadenylation systems however.
Here is another reference on the phylogenetic position of Giardia:
BMC Evolutionary Biology 8:205 (2008) Available on Science Direct
Either way, Giardia once again demonstrates that irreducible complexity is not a valid concept. There are more ways to produce a complex system then are dreamt of by Behe.
Larry Boy · 30 August 2008
Stanton · 30 August 2008
John Kwok · 30 August 2008
Dear Arthur,
Ken is probably busy preparing for the fall semester at Brown now, so there's an excellent chance he's missed it. His e-mail address is listed at www.millerandlevine.com, and you have my permission to mention me, as someone who suggested that you should contact him (I've known Ken for years. Back in the early 1980s I assisted him in his very first debate against a creationist which was held at Brown's hockey rink.).
Regards,
John
Vince · 30 August 2008
Amongst us parasitologists Beaver Fever also goes by "The Explodo Sh-ts"...
(While I've never experienced it myself I take it on good word that the fits very well...)
quantum_flux · 31 August 2008
Please tell the McCain/Palin Campaign (preferrably politely) why teaching creationism in our public schools around America is superstitious and is not in our nation's best interests. These are the feelers McCain has out there, the way in which Americans can have a voice and be heard by his campaign:
Contact his campaign directly here:
http://www.johnmccain.com/Contact/
Or go to his blogs and leave a polite message about the subject matter wherever appropriate:
http://www.johnmccain.com/blog/
Remember, McCain does a lot of things right and is a great heroic war veteran who genuinely puts his country first, but Creationism is one key area where he is completely wrong and could potentially create a major setback for American students and businesses. We can't let America fall behind foriegn countries in the departments of Science and Technology because of his superstitious beliefs.
John Kwok · 31 August 2008
Frank J · 31 August 2008
Thanks, QF. I will do that. And I would do it even if I were still a Democrat.
JGB · 31 August 2008
Tim Pawlenty went on the record this morning on Meet the Press as not understanding what actual education policy takes place in Minnesota. I don't have a transcript, but I was watching when Brokaw pushed him on Palin's support for Intelligent Design and he claimed that it was a local control issue in Minnesota which it is not. The state standards are pretty clear, and I am sure MNSCE supporters will kindly note how teaching creationism/ID has already been shown once in this state to be illegal.
John Kwok · 31 August 2008
Paul Burnett · 31 August 2008
Torbjörn Larsson, OM · 1 September 2008
John Kwok · 1 September 2008
Frank B · 1 September 2008
I am sorry for getting political, the PT is not for that, but some things need a response. Pressuring McCain about ID is worth a try. Maybe constant pressure will keep Palin defensive.
John Kwok · 2 September 2008
Dear Frank,
I intend to remind McCain of the excellent research done by Arizona biologists at both the University of Arizona and Arizona State University. If you decide to "pressure" him about ID creationism, then please do so in a respectful, quite courteous, manner.
John
Frank J · 2 September 2008
Jim Harrison · 2 September 2008
Palin's father taught science and gym. In Alaska that probably means he was simply a jock. No reason to assume he had any knowledge of science whatsoever.
Draconiz · 2 September 2008
Slightly OT, but it seems the dishonesty institute has quotemined another professor
http://www.evolutionnews.org/2008/09/prominent_atheist_professor_of.html#more
Does anyone has access to the paper in question?
Frank J · 3 September 2008
TomS · 3 September 2008
I've just taken a brief look at this paper, and it does seem to argue that ID presents an alternative to evolutionary biology, an incorrect one in his view, but still an alternative.
Draconiz · 3 September 2008
John Kwok · 3 September 2008
I just e-mailed this to Senator McCain:
Dear Senator McCain:
As a former resident of the great state of Arizona, I am delighted with your candidacy for President of the United States, recognizing that you, Senator John McCain, are the sole person who puts “Country First” among our current presidential candidates. Having been one of your constituents for a decade, I also know you possess both the great character and wisdom to become one of our great Presidents. In recognition of these admirable traits of yours, I am writing to warn you of the dangers posed by renewed advocacy of Intelligent Design and other kinds of creationism, since they represent threats to both the intellectual and economic well-being of our great nation, threatening American preeminence in science and technology, and the chance that the 21st Century will become yet another “American Century”. I am urging you to put “Country First” by rejecting demands to have creationism – especially Intelligent Design creationism – taught in American science classrooms alongside modern evolutionary theory.
It is no accident that creationism, including Intelligent Design, is repudiated by the mainstream scientific community. It is a collection of outdated ideas that were rejected soundly by science more than a century and a half ago. Creationism’s current proponents have asserted that they are persecuted by mainstream science for their beliefs, but theirs are claims that are not borne out by the real, honest truth. None have sought to present their work in the valid market of ideas known as peer-reviewed science. No papers of theirs in support of creationism have been presented in scientific meetings, and none have been submitted for publication in notable scientific journals like Nature, Science, Evolution, Ecology, Paleobiology, and Cladistics, among others, demonstrating how and why creationism is a valid scientific alternative to modern evolutionary theory. In private e-mail correspondence with two leading advocates of Intelligent Design creationism, mathematician and philosopher William Dembski and biochemist Michael Dembski, I have challenged them to explain how Intelligent Design is a better scientific alternative to modern evolutionary theory in explaining the history and structure of Planet Earth’s biodiversity. Neither one has given me an answer. Why? Because they know that Intelligent Design isn’t scientific, and therefore, that it is incapable of being such an alternative.
Back in 1973, the great evolutionary geneticist Theodosius Dobzhansky – one of the architects of modern evolutionary theory – observed, “Nothing in Biology makes sense except in the light of evolution.” His accurate comment is confirmed daily by thousands of scientists across the globe, and especially, by many great scientists who are biology professors at Arizona State University and the University of Arizona; the latter my graduate school alma mater. For example, at the University of Arizona, Regents Professor of Ecology and Evolutionary Biology Nancy Moran uses techniques from molecular biology and classical entomology to understand the evolution of symbiotic microorganisms in insects; many of those insects that she studies – such as aphids – are economically important agricultural pests. Her colleague Dr. William Schaeffer is noted for his mathematical models of the origin and spread of epidemics, relying on key principles in evolutionary biology for better understanding of public health issues. Their colleague Dr. Michael L. Rosenzweig – my graduate school mentor – is one of the most important evolutionary ecologists of our time, whose research interests have ranged from paleobiology to community ecology, and now, most recently, conservation biology. Their excellent research would be impossible if Intelligent Design or some other kind of creationism was indeed a “scientific theory”; only modern evolutionary theory has enabled them to pose the interesting questions that have led to their successful work. If we are to conquer the 21st Century challenges posed by the spread of virulent disease like HIV/AIDS, the invasions of alien species of animals and plants in North American ecosystems, and the economic damage caused by agricultural pests like aphids, then we can do so only via the science of evolutionary biology, not by invoking creationism’s scientifically discredited ideas dating from the 18th Century and before.
From a religious perspective, as a Deist, I can sympathize with your – and Governor Palin’s - difficulties in accepting modern evolutionary theory. However, great religions like Roman Catholic Christianity see no conflict between modern evolutionary theory and a belief in God. There are many religiously devout scientists, such as eminent ecologist Dr. Michael L. Rosenzweig, Professor of Ecology and Evolutionary Biology, University of Arizona, noted cell biologist Dr. Kenneth R. Miller, Professor of Biology, Brown University, and distinguished molecular biologist Dr. Francis Collins, the former director of the Human Genome Project, who see no contradiction whatsoever between their own personal devoutly held religious beliefs and their commitment to excellence in scientific research (A distinction that eludes still those like Dr. Behe and Dr. Dembski.). We should render to science, that which is science, and to religion, that which is religion, without confusing these two distinct, but important, parts of our thought. This doesn’t mean that you should sacrifice your own personal, deeply held, religious beliefs for the sake of science. Distinguished vertebrate paleobiologist Michael Novacek, Vice President and Provost, American Museum of Natural History, has stated that it is not his museum’s mission to change people’s religious views, but rather, to educate them on valid mainstream science, of which modern evolutionary theory is a most essential part.
We are engaged in a titanic struggle for America's soul, according to Brown cell biologist Kenneth R. Miller's new book, 'Only A Theory: Evolution and the Battle for America's Soul'. Sadly, I must concur with my friend Ken’s dire warning that we are in danger of losing our preeminence in science and technology – and thus our excellent economy – if creationist advocates succeed in inserting outmoded, religiously-derived ideas like Intelligent Design and other forms of creationism into American science classrooms. Our children must be taught valid mainstream science so we can meet successfully the scientific and technological challenges of the 21st Century, so we can ensure that we are “Country First” with regards to American preeminence in science and technology. I strongly encourage you and your staff to talk to distinguished evolutionary biologists like those I have cited, and to read and to reflect upon Ken Miller’s terse book and Republican Federal Judge Jones’ historic landmark ruling at the end of the 2005 Kitzmiller vs. Dover Area School District trial, in which Jones recognized that Intelligent Design is not science, but instead, a religiously derived idea promoted by those seeking to insert their religious beliefs and values into science classrooms. Again, in closing, please recognize that we must keep “Country First” by rejecting any and all attempts to inject religion into science classrooms, of which the most blatant examples are the many, still ongoing, attempts to teach Intelligent Design and other kinds of creationism.
Sincerely yours,
John Kwok
Frank J · 3 September 2008
Frank J · 3 September 2008
Draconiz · 3 September 2008
While I do appreciate John and FrankJ's effort, I still think it's wishful thinking on his part. The reason they pick Palin is to suck up to creationist theocrats, a few rationalists in the party won't change that fact.
Flint · 3 September 2008
Frank J · 3 September 2008
Flint,
The reason that I fell for "teach the controversy" had nothing to do with God or religion. At the time - 20 years into a career as a chemist - I still had many misconceptions of evolution, and knew of "creationism" only as an honest belief that life was only thousands of years old, with multiple origins of species. I naively figured that students would compare the two models see the evolution as the clear winner with minimal evidence. I had no clue at the time what a slickly crafted series of misrepresentations, mined quotes etc. anti-evolution activism was. Nor had I heard of the "don't ask, don't tell" ID approach, which shrewdly lets students infer their own alternative without critically analyzing it.
McCain and Palin have probably an even more caricatured view of evolution, with less understanding of atoms, molecules, cells, geologic time, etc. than I had. And
further disadvantaged by surrounding themselves by people who feed them the right feel-good sound bites.
While it's possible that McCain and/or Palin may be faking their support of "teach the controversy" just for votes, but if so, McCain's admission of personally accepting evolution, and not raising his hand at the debate was probably not the smartest thing to do.
Frank J · 3 September 2008
Draconiz,
McCain will surely get more "theocrat" votes with Palin, but (I can't vouch for John), I have no interest in changing their minds. It's those who are not hopeless Biblical literalists but still doubt evolution or think it's fair to teach the controversy, and do so because of reversible misconceptions (did I mention 10 times that I had some?) who might stand up and notice if a president (or candidate) said that it's wrong to misrepresent science. Saying it before the election may risk some "theocrat" votes (probably more than any gains by pro-science undecideds) but there's no reason they can't say it after (if) they win.
I'm still amazed that GWB was not demanded to elaborate on what he meant by advocating "the controversy", and what he thought of the Dover verdict just a few months later.
Torbjörn Larsson, OM · 4 September 2008
Eric · 4 September 2008
Frank J · 4 September 2008
Draconiz · 4 September 2008
Newsflash
http://www.youtube.com/watch?v=TE69ajamamo&eurl=http://scienceblogs.com/pharyngula/
Cindy Mccain is also for teaching creationism in school, good luck convincing her husband!
Frank J · 5 September 2008
Eric · 5 September 2008
Frank J · 5 September 2008
Eric,
I don't expect many people to change their vote either way even if all 3 (and Todd Palin if he thinks likewise) suddenly admit that they find Judge Jones, Ken Miller and Francis Collins more reasonable than the anti-evolution propagandists. There are just too many issues to consider. Cindy McCain's (surprising to me) admission that Roe v. Wade should not be overturned probably turned off more of the religious right than would a rejection of "creationism".
Eric · 5 September 2008
Karen S. · 5 September 2008
Check this out from Scientific American:
Science Questions for would-be presidents
Note that you can add your own questions!
iml8 · 5 September 2008
John Kwok · 5 September 2008
Hi GG and FrankJ,
Santorum has written the foreword to one of Dembski's books (Can't remember which one but it's a recently published one.). He's definitely "in" with the DI crowd. As for Judge Jones, he is disgusted with the lunatics attached to Santorum and the DI, and, I suspect, would love to take the Republican Party back from them (A sentiment I would fully endorse, as I have noted earlier.).
John
Karen S · 5 September 2008
iml8 · 5 September 2008
Frank J · 6 September 2008
Frank J · 6 September 2008
Science Avenger · 6 September 2008
RWard · 6 September 2008
I think you need to have Mom wash out your mouth with soap. And what are these children being lied to about? Are you OK?
The lie is that the creationists/intelligent design proponents are posing valid criticisms of evolutionary theory. There are lots of debates in biology. Talk to your class about group selection or stochastic processes and the differing ideas biologists have about those ideas and I have no problem. Tell your class that 'irreducible complexity' poses a serious probhlem for evolutionary theory and you're lying.
Stanton · 6 September 2008
Stanton · 7 September 2008
Science Avenger · 7 September 2008
I think you need to go fuck yourself Handjobby. Telling children there are legitimate alternate theories to evolution is the lie, moron. I'm fine, thanks for asking, but you are a lying sack of shit for implying that someone can't be disgusted with you and be OK. Half-assed snarks are all you are capable of, shit-fer-brains.
Science Avenger · 7 September 2008
What a lying sack of shit you are Handjobby. No one has ever said or implied that anyone should not be allowed to believe any fool thing they want. Do you actually think these moronic questions prove anything? Mentally masterbating in public is in very poor taste. What would the children think?
Science Avenger · 7 September 2008
Oh, and asking a question is not a criticism, moron. No wonder you have so little understanding of what we are talking about, you don't understand the vocabulary. You shouldn't have spent so much time making porn doodles in the margins of your English homework. Idiot.
Flint · 7 September 2008
"Would asking if there could be a limit to how far natural selection could take the changing of body plans from simple to complex be a valid criticism?" Why a criticism? This strikes me as the sort of question that can be investigated fruitfully. "...you do admit there is a limit to what NS can do? Or at least the limit or lack of has not been proven yet." This my understanding: there are two very different sorts of limits to what RM+NS can do. The first has to do with the requirements for life - metabolism, reproduction, etc. It's nearly tautological that life must be viable. If it's not, it dies. The second has to do with contingency. There must exist a possible path from some point A to any point B, for that point B to be reached. And so, just through the sorts of variations that just happened to be available to select from throughout biological history, lineages adopted specific evolutionary lines and structures. Just like there are possible bridge hands that have never been dealt, there are body plans that aren't physically ruled out, but simply didn't occur. None of the body plans evolution has blundered across so far have been impossible, of course. Some didn't make it for as long as others.
Flint · 7 September 2008
Apparently double spacing between paragraphs doesn't work, html tags don't work, quotations don't work...I hope my response is comprehensible. I admit defeat on trying to produce anything like a readable format.
Flint · 7 September 2008
The second has to do with contingency. There must exist a possible path from some point A to any point B, for that point B to be reached. And so, just through the sorts of variations that just happened to be available to select from throughout biological history, lineages adopted specific evolutionary lines and structures. Just like there are possible bridge hands that have never been dealt, there are body plans that aren’t physically ruled out, but simply didn’t occur. None of the body plans evolution has blundered across so far have been impossible, of course. Some didn’t make it for as long as others.
Stanton · 7 September 2008
Karen S · 7 September 2008
Flint · 7 September 2008
One way to answer is, very clearly all the body plans currently extant HAVE evolved in the time available, so clearly it's sufficient time. One might as well ask if the force of gravity is sufficient to pull a dropped brick all the way to the ground. There's no reason to ask such a question EXCEPT unless one is convinced for utterly unrelated reasons that it can't be the case, and some increment of magic must be involved.
But if that were true, what means could we conceivably use to test every known process for elements of magic? It could be lurking everywhere!
Another general observation is, when multicelled critters first occurred, there weren't any existing rules or contingencies. It was free-for-all, anything goes. So there were lots and lots of different approaches, but only slightly different from one another. In principle, each of these tiny variations represented a "different body plan" right from the start, even though evolution hadn't had nearly enough time to gradually radiate these differences.
Think of a fork in the road. Within only a few feet of that fork, there's not much distance. You can toss a stone from one fork to the other. BUT these tiny differences eventually result in the roads being very far apart, hard to believe such large differences "had time" to happen. So we look back at the Cambrian with present-colored glasses, imposing on damn-near-identical fossils entirely distinct phyla because they kinda resemble what we knew they'd become in half a billion years.
PvM · 7 September 2008
PvM · 7 September 2008
Stanton · 7 September 2008
Arthur Hunt · 7 September 2008
Please restrict any further comments to the subject of the essay.
Thanks.