|
Transcript from February 5, 2003 9:00-10:15 PM Eastern
Copyright © by International Society for Complexity, Information, and Design 2003. ISCID Moderator
Our guest speaker today is Paul Nelson. Dr. Nelson is a philosopher of biology, specializing in evo-devo and developmental biology. He is also a fellow of the International Society for Complexity, Information and Design. Dr. Nelson received his Ph.D. from the University of Chicago Department of Philosophy. His thesis critiques aspects of macroevolutionary theory in light of recent developments in embryology and developmental biology. Entitled "On Common Descent", it will be published as volume sixteen in the University of Chicago Department of Ecology and Evolution's "Evolutionary Monographs" series (and the first in this prestigious series to critique neo-Darwinism). ISCID Moderator
Dr. Nelson has written several articles on the philosophical aspects of evolutionary biology including one recently published in Biology and Philosophy. He edits the journal Origins & Design. ISCID Moderator
Everyone is encouraged to take a glance at the discussion paper that Dr. Nelson put together for the chat. It can be viewed at the following URL: http://www.iscid.org/nelsonchat.pdf ISCID Moderator
I am now going to hand the talk over to Dr. Nelson. Participants can start sending in questions. Paul Nelson
I apologize for not making the paper available sooner (I had hoped to have it up on the ISCID page by Feb 1, but family illnesses and other matters intervened). Paul Nelson
Sadly, I've earned a well-deserved reputation for being probably the pokiest design theorist on the planet. :-( Paul Nelson
Anyway -- I hope tonight's discussion sparks your thinking about how best to explain the origin of animals. Paul Nelson
Here's the format I think would work well. I'll ramble on for a while, laying out some ideas, and then will type BIG SIGH or something like that. Paul Nelson
BIG SIGH is your clue that I've gone on long enough (doubtless more than long enough), and it's time to ask questions. Paul Nelson
Truth in advertising note: Paul Nelson
This ISCID informal discussion material represents work in progress that I am undertaking in collaboration with Marcus Ross, a paleontology graduate student in the Department of Geosciences, University of Rhode Island (317 Woodward Hall, 9 East Alumni Avenue, Kingston, RI, 02881-2019; E-mail: mros1106@postoffice.uri.edu). Paul Nelson
Marcus, who is the paleontological half of the project, presented the first part of our joint work as a poster, "Ontogenetic Depth as a Complexity Metric for the Cambrian Explosion," Paper No. 187-34, at the 2002 Annual Meeting of the Geological Society of America (30 October 2002). Paul Nelson
We will be submitting a follow-up paper on the concept of ontogenetic depth to the 62nd (2003) Annual Meeting of the Society of Developmental Biology, to be held July 30 to August 3 in Boston, MA (where, incidentally, the poster Jonathan Wells and I had planned to present at the 2002 SDB meeting in Madison will also be available as a paper for anyone who is interested). Paul Nelson
Practical consequence of truth-in-advertising note: I may well have to punt on paleontology questions. Sorry! Paul Nelson
Marcus and I plan to have a comprehensive paper ready to submit to SDB by the end of March. Maybe we can put a pre-print up at ISCID. Paul Nelson
Before I get to the main course (love those food metaphors), let me give you a couple of philosophical/sociological appetizers. Paul Nelson
The longer I work as a senior fellow at the Discovery Institute -- writing, lecturing, going to meetings (both design and regular science) -- the more I value evolutionary biologists. Paul Nelson
" Opposition is true friendship," wrote William Blake (1757-1827). Blake loved a good paradox, and this one fits the bill. Paul Nelson
If competition ("opposition") is good for selling computers or the political process, it's also good for science and scientific discovery. Paul Nelson
What's the fastest way to find out what's wrong with your ideas? Run them by someone who thinks you are dead wrong. Paul Nelson
And he or she will tell you! If they're right, you can abandon that idea. (Hurts, of course.) Paul Nelson
If they throw their best criticisms at your ideas, however, and the ideas stumble through, still alive, you may be onto something. Paul Nelson
I'm hoping that people (tonight) can suggest aspects of the problem of the origin of animals that I've overlooked or missed. Paul Nelson
But there is another respect in which I value evolutionary (and evo-devo) biologists. For their own candor. Paul Nelson
Here's an example, where I cannot name the scientist in question (yet, anyway). This is directly relevant to tonight's discussion. Paul Nelson
This happened at the now-notorious 1999 China meeting on the origin of animal body plans. Paul Nelson
(The meeting has become notorious because design theorists were involved in its planning, and presented papers, much to the dismay of the American and British scientists present.) Paul Nelson
(The European and Chinese scientists didn't really seem to care -- but that's another story.) Paul Nelson
One day, I was sitting outside the dining hall, waiting for lunch to be served. Paul Nelson
I had with me (on top of my conference files) an overhead transparency, showing the complex regulatory sequence of an invertebrate developmental gene. Paul Nelson
And up walked the author of that very paper (a visiting American biologist), from which I had borrowed the figure. Paul Nelson
Of course he spotted the diagram right away. "What do you plan to do with that?" he asked me. Paul Nelson
" I thought I might use this in my talk," I said, "as a quick illustration of the complexity of embryonic regulation." Paul Nelson
The biologist smiled. Knowing that he was something of a critic of neo-Darwinism, I asked him what historical process he thought had assembled the complex regulatory sequence. Paul Nelson
His answer really surprised me. "I don't know," he said, "but I do know that ordinary mutation and selection won't do it." Paul Nelson
He went on to say that he thought our (that is, the biological community's) understanding of evolution lagged way behind its other knowledge. Paul Nelson
And that brings me to ontogenetic depth. Paul Nelson
Let me describe our motivating puzzle. ("Our" refers to Marcus Ross and me.) Paul Nelson
No scientist sets out (consciously, anyway) to become the butt of jokes in the future. Thus, when we now read Ernst Haeckel's statement that a cell is a "simple little lump of albuminous combination of carbon," we smile to ourselves - perhaps saving the passage for a humorous Powerpoint interlude - but we may also add, "Well, actually, in the late 19th century - could Haeckel or anyone else have foreseen just how complicated the cell would turn out to be?" The guy got it wrong. Paul Nelson
The deeper point, of course, is that one cannot explain the origin of something when one does not understand what that thing really is. Haeckel failed to explain the origin of cells because he profoundly misunderstood, or mischaracterized, his explanatory target. Paul Nelson
As the historian and philosopher of science Harmke Kamminga (1986) has observed, "At the heart of the origin-of-life problem lies a fundamental question: What is it that we are trying to explain the origin of?" In 2003, we know that the ultimate target of abiogenesis research - the object whose origin we are trying to explain - is not an "albuminous combination of carbon." Paul Nelson
Therefore any historical explanation that aims to generate "simple lumps," instead of a real cell, will miss the mark by a long distance. Paul Nelson
The same problem of accurately characterizing the explanatory target arises later in the history of life, with the origin of the bilaterian animals. The origin of the animals has remained a puzzle in historical biology from Darwin's time to the present. Paul Nelson
As with any scientific problem, understanding what needs to be explained stands as the first task. The motivating question can be framed as follows: What sort of biological event does the geological first appearance of forms such as arthropods (e.g., Anomalocaris) or molluscs (e.g., Scenella) represent? Paul Nelson
As with any scientific problem, understanding what needs to be explained stands as the first task. The motivating question can be framed as follows: What sort of biological event does the geological first appearance of forms such as arthropods (e.g., Anomalocaris) or molluscs (e.g., Scenella) represent? Paul Nelson
Various measures have been proposed to quantify complexity increases in evolution, notable among them genome size (Britten and Davidson 1969), gene number, and cell type (Valentine 1994). Paul Nelson
But genome size is vulnerable to the so-called "C-value paradox," i.e., the lack of correlation between genome size (measured as DNA content) and apparent morphological complexity. Paul Nelson
Gene number estimates can vary widely (see, e.g., Ewing and Green 2000 versus Liang et al. 2000, whose estimates for gene number in humans differ by a factor of 4), and cell type counts may be skewed by the use of intensively studied model taxa, possibly leading to higher counts (McShea 1996, 483). Paul Nelson
These difficulties suggest that a more comprehensive measure, relating more of the data of interest - body plans, organ systems, cell and tissue types, etc. - may be needed. Paul Nelson
Valentine (1994, 406) notes that "the ultimate measure of body-plan complexity would presumably be one that reflects the information required to specify the entire body, involving both gene number and the organization of gene expression." Paul Nelson
We suggest that a measure of *ontogenetic depth* may bring together many (if not most) of the key biological parameters, and help investigators focus on what really needs to be explained in such events as the Cambrian Explosion. Paul Nelson
So here is our proposal. Paul Nelson
Consider Figure 1 [which should be available sometime this week from ISCID] which shows several of the salient biological levels employed in assessing the complexity increases exhibited by the Cambrian Explosion. Paul Nelson
Gene number is the sum of all functional sequences in a taxon's genome (whether those loci are classical protein-coding genes or regulatory sequences). Paul Nelson
Cell number is the total count of discrete cells, of any type, possessed by an adult organism capable of reproduction. ISCID Moderator
Figure 1 is available now: http://www.iscid.org/nelsonchat.pdf Paul Nelson
Cell type describes the total number of histologically differentiated cellular morphologies (e.g., gut epithelium, nerve, muscle, blood cell). Paul Nelson
You're amazing, Mod! Paul Nelson
Tissue type describes the organization of cell types into functional units such as sheets or epithelia, connective materials, skeletal parts, and so on. Paul Nelson
Organ systems are the higher-level anatomical relationships responsible for major organismal functions (e.g., sensory, locomotory, digestive, reproductive). Paul Nelson
Body plans represent the major architectural features characteristic of groups such as Arthropoda, Mollusca, Brachiopoda, and the other bilaterian phyla. Paul Nelson
Now, it might seem that the natural way to illuminate the relationship between these levels would begin "bottom up," with the genes. Paul Nelson
We argue, however, that for the problem of the origin of the phyla, the concept of an ontogenetic network best integrates these levels (see Figure 2). Paul Nelson
An example of one aspect of an ontogenetic network can be seen in Figure 3, depicting the beginning of the cell lineage of the nematode Caenorhabditis elegans. Paul Nelson
Ontogenetic networks in all animals commence with a single cell, the fertilized egg. Then an unfolding arborescence of developmental decisions begins, whose complexity and overall architecture varies by taxon. Paul Nelson
In all animals, however, a point in the adult phenotype arrives when reproduction - the generation of gametes capable of fertilization - is possible. Paul Nelson
This distance, from the egg to the adult capable of reproduction, is what we term ontogenetic depth (see Figure 4). Paul Nelson
Somewhat more formally, ontogenetic depth may be defined as the distance, in terms of cell division and differentiation, between a unicellular condition and a macroscopic adult metazoan able to reproduce itself (i.e., generate gametes). Paul Nelson
The ontogenetic depth of a handful of extant animals (from the model systems of developmental biology) is known with precision. Paul Nelson
In the nematode Caenorhabditis elegans, for instance, a relatively small animal only 1.5 mm in length, 7 to 9 rounds of cell division lie between the fertilized egg and any cell in the adult: 959 somatic cells in the hermaphrodite (with a variable number of germ cells), and 1031 cells in the male (with its distinctive tale). Paul Nelson
For larger metazoans, of course, such as the dipteran Drosophila melanogaster, ontogenetic depth is much greater, as total cell number, degree of cellular differentiation, and time to reproductive capability increase accordingly. Paul Nelson
The value of ontogenetic depth as a complexity metric lies in its relationship to all the parameters listed in Figures 1 and 2. Paul Nelson
Of course, the ontogenetic depth of any extinct organism cannot be determined with complete exactitude. Paul Nelson
However, it should be possible, using modern analogues for fossil taxa - e.g., the extant monoplacophoran Neopilina for the extinct mollusc Scenella - to obtain good estimates on the ontogenetic depth requirements of many Cambrian forms. Paul Nelson
This is research we are now conducting. It is likely that reasonable estimates of the ontogenetic networks, and depth, required to specify such extinct organisms as Anomalocaris or Opabinia, will require no less complexity than that of modern animals. Paul Nelson
All right, you say. So what? What's the significance of this idea? Paul Nelson
That's where we come to what I've been calling "the marching band problem." I first used this term at the China meeting (Nelson 1999). Paul Nelson
I realize "marching band problem" may seem tangential (at best) to the discussion, but bear with me. Paul Nelson
To a skeptic, the concept of ontogenetic depth may look to be little more than a roundabout way of expressing the already-familiar problem of how animals originally evolved from unicellular or colonial ancestors. Paul Nelson
We think, however, that focusing on ontogenetic depth helps to illuminate the central challenge that standard (neo-Darwinian) evolutionary theory faces when confronted with phenomena such as the geological first appearance of forms like Anomalocaris. Paul Nelson
As noted earlier, the cells of an adult metazoan are specialized for particular functional roles (as gametes, nerves, gut epithelia, skin, skeleton or exoskeleton, sensory organs, and so on). ISCID Moderator
Figures for the marching band problem can be found here: http://www.iscid.org/nelsonchat.pdf Paul Nelson
" The production of [these] differentiated cell types," writes Carl Schlichting (2003), "is a hallmark of multicellular organisms." The production process itself is an ontogenetic network, commencing with the fertilized egg. Paul Nelson
" A function [one might say *the* function] of developmental processes," notes Strathmann (2000), "is putting the right kind of cells in the right places at the right times. The criterion for 'right" is survival and reproduction." Paul Nelson
Or what we're calling "reproductive capability." Quick (but very important) note: differences in reproductive output are the only conditions on which natural selection can act (see John Endler's 1986 monograph, Natural Selection in the Wild). Paul Nelson
One can conceive this process of differentiation (or cellular specialization) very much on the model of an American university marching band (see Figure 5, where a 105 member marching band is depicted as orange dots, arrayed at the sideline of a football field). Paul Nelson
Mea culpa: the discussion paper says the band has 140 members. Obviously I can't do 3rd grade multiplication! :-( Paul Nelson
In one sense, of course, any marching band is strongly disanalogous to a developing animal. Paul Nelson
A nematode or fruit fly commences its existence as a single cell (the fertilized egg), and will then construct its cell populations during development, whereas the marching band begins its maneuvers with all of its members already present. Paul Nelson
But in another sense - the one that we'll focus on - the two processes share many parallels. Paul Nelson
The band will move, through a series of intermediate maneuvers, toward its functional endpoint - say, spelling "CAL STATE" on the field (see Figure 6). Paul Nelson
In its development, an animal also moves from the fertilized egg, through a series of intermediate "maneuvers," towards its functional endpoint, namely, an organism capable of reproduction. Paul Nelson
The latter process, of course, is vastly more complex: Paul Nelson
" This temporally ordered sequence of morphological heterogeneities that we call development," writes Arthur (1997), "generates adult tissue patterns that, in some taxa, can be highly complex, involving very precise and repeatable arrangements of billions, even trillions, of cells." Paul Nelson
Now, if the band is going to spell "CAL STATE" successfully, it should be intuitively obvious that the members must have their instructions in place before they venture onto the field. Paul Nelson
The trumpet player, for instance, standing in the front row on the sideline, who will eventually become the tip of the serif at the bottom of the letter "L" (see Figure 7), must know how to execute the series of turns and motions that will carry him to his endpoint on the field. Paul Nelson
The same is the case with a developing organism. "Development is possible," writes Arthur (2000), "only if cells 'know' what to do in all these respects," i.e., assign their planes of division, tendencies to move, directions and rates of movement, modes of differentiation into particular cell types, and cell death (apoptosis). Paul Nelson
" So the key question," Arthur continues, "becomes 'how do they know?', and the whole of developmental biology could be regarded as an attempt to answer this question." Paul Nelson
If the question "How do cells know?" is to be answered by developmental biology, its sister (and far more difficult) question "How did cells learn what they know?" must be addressed by evolutionary (or historical) biology. Paul Nelson
And here serious, and currently unanswered, questions arise. Paul Nelson
" How cell types of multicellular organisms came to be differentiated," notes Schlichting (2003), "is still an open issue...the origins of differentiation remain unclear." Paul Nelson
Given that the origin of animals - organisms defined by differentiated structures - is thought by most scientists to have been a problem solved, at least in outline, by Charles Darwin, this is not a minor difficulty. Paul Nelson
Some authors have recently noted this explicitly, e.g., Davidson 2001. He writes: Paul Nelson
" ...classical Darwinian evolution could not have provided an explanation, in a mechanistically relevant way, of how the diverse forms of animal life actually arose during evolution, because it matured before molecular biology provided explanations of the developmental process." Paul Nelson
" To be very brief, the evolutionary theory that grew up before the advent of regulatory molecular biology dealt with the problem of the origin of novel organismal structures in two ways." Paul Nelson
" The first has been to treat the mechanisms generating novel morphological structures as a black box. New forms were considered to arise 'because' the environment changed." Paul Nelson
" But while changes in Precambrian or Ordovician weather, continental shifts, or temperature may have contributed crucial selective forces, they do not generate heads or appendicular forms; only genes do that." Paul Nelson
[Side comment from Paul: Or, we might say, genes *plus* (the three-dimensional localization of their protein products, et cetera - nucleic acid alone an organism never made).] Paul Nelson
Davidson goes on to argue that "stepwise, adaptive changes in protein sequence...is probably largely irrelevant to the evolution of any salient features of animal morphology," but we will focus on a more general difficulty, involving the process of natural selection itself, and its (probable) impotence for constructing ontogenetic networks. Paul Nelson
Suppose we interrupt a marching band midway through its maneuvers, at some stage before "CAL STATE" appears on the field. Paul Nelson
Suppose, furthermore, that we cause this interruption at a marching band competition where "success" is defined (at least in part) by actually reaching the endpoint where the name of the band's home institution is spelled. Paul Nelson
It should again be intuitively obvious that the functional reason for the band's intermediate maneuvers is not the maneuvers themselves, but rather the distant endpoint that those maneuvers enable or bring about. Paul Nelson
Now look again at Figure 3, showing the early cell lineage of C. elegans. One cannot interrupt this canonical cell division pattern and obtain a viable organism. Paul Nelson
Viability, and, in particular, reproductive capability - the only outcome "visible" to natural selection - lie in the distance, after several rounds of cell division and differentiation. Paul Nelson
How then did natural selection construct the ontogenetic network of C. elegans? Paul Nelson
Figure 8 represents this problem in schematic form, using a very shallow network to make the point. Paul Nelson
Reproductive capability arises only in the square on the right, when its five cells are in place. Paul Nelson
But the cells must be put there by a specific developmental process. What constructed that process? Paul Nelson
OK. BIG SIGH. Paul Nelson
Oops -- before the Q & A, here are the references: Paul Nelson
Arthur, Wallace. 1997. The Origin of Animal Body Plans: A Study in Evolutionary Developmental Biology. Cambridge: Cambridge University Press. Paul Nelson
Britten, Roy and Eric Davidson. 1969. Gene Regulation for Higher Cells: A Theory. Science 165:349-357. Paul Nelson
Davidson, Eric. 2001. Genomic Regulatory Systems: Development and Evolution. New York: Academic Press. Paul Nelson
Ewing, Brent and Phil Green. 2000. Analysis of expressed sequence tags indicates 35,000 human genes. Nature Genetics 25:232-234. Paul Nelson
Kamminga, Harmke. Protoplasm and the Gene. In A.G. Cairns-Smith and H. Hartman, eds., Clay Minerals and the Origin of Life. Cambridge: Cambridge University Press, pp. 1-10. Paul Nelson
Liang, Feng et al. 2000. Gene Index analysis of the human genome estimates approximately 120,000 genes. Nature Genetics 25:239-240. Paul Nelson
McShea, Daniel. 1996. Metazoan Complexity and Evolution: Is There a Trend? Evolution 50:477-492. Paul Nelson
Nelson, Paul. 1999. Generative Entrenchment and Body Plans. Lecture presented at the International Symposium on the Origins of Animal Body Plans and Their Fossil Records, eds. J.Y. Chen, P.K. Chien, D.J. Bottjer, G.X. Li, and F. Gao, Early Life Research Center, Kunming, People's Republic of China, 21-25 June. Paul Nelson
Schlichting, Carl D. 2003. Origins of differentiation via phenotypic plasticity. Evolution and Development 5:98-105. Paul Nelson
Schnabel, Ralf. 1997. Why does a nematode have an invariant cell lineage? Seminars in Cell & Developmental Biology 8:341-349. Paul Nelson
Strathmann, Richard. 2000. Functional design in the evolution of embryos and larvae. Seminars in Cell and Developmental Biology 11:395-402. Paul Nelson
Valentine, James W. 1994. The Cambrian Explosion. In S. Bengston, ed., Early Life on Earth (New York: Columbia University Press), Nobel Symposium No. 84; pp. 401-411. Paul Nelson
Valentine, James et al. 1994. Morphological Complexity Increase in Metazoans. Paleobiology 20:131-142. Lydia
Paul, I know some ID theorists have questioned the idea that "genes are everything" and have pointed to issues of embryonic development in their criticisms. I'm actually thinking especially of Jon Wells, here. To what extent does the problem raised in this paper turn on the issue of whether or not "genes are everything" in embryonic development? If genes could explain it all, would that help much in the development of an evolutionary explanation in Darwinian terms? Paul Nelson
I guess we're raising a different issue. First of all, however, genes can't possibly explain everything, if by "genes" one means nucleic acid (DNA and RNA). The best corrective for that view is simply to look at a beaker full of DNA. Looks like thick syrup. It ain't an organism, even by a long stretch. phil
With extensive gene duplication, does it make any difference which sequence is activated? If, yes, how are they chosen? Paul Nelson
Yup -- sorry. Let me continue. When a fertilized egg begins to divide, it has to *go* somewhere -- that is, the cell lineages that arise must head off in particular directions. Paul Nelson
Oops -- sorry, Phil, I was still answering Lydia. The "directions" are endpoint (terminally differentiated cell types) in the adult, and these cell types perform particular functions. They are arranged in relation to each other in 3-dimensional patterns. Paul Nelson
Our question asks whether natural selection, in principle, can construct these programs of differentiation. Genes enter into that question, but there's a lot more to it. Paul Nelson
Phil, can you clarify what you mean by "gene duplication" in relation to development? Paul Nelson
Lydia, did my answer scratch your itch (or not)? phil
I am supposing there is a relation, but you can answer better than I can. Carry the thread if it has relevance. Lydia
Yes, because I think the answer really implies that the insufficiency of genes is an important _part_ of the issue in this paper, since one can't just say, "Serendipitous mutations are how evolution could do it." Apparently serendipitous mutations wouldn't do it _anyway_, because there is so much more involved. At least, I think I have that right?? Paul Nelson
Yes, I think you do. Funny thing about developmental mutations: they're marching band wreckers (if I may stick with that metaphor). When I was writing my dissertation, I asked one of my advisors if he knew of any cases of the heritable modication of early development . He couldn't think of a single example (other than a change from left-to-right shell coiling, or back again, in snails -- the exception that proves the rule). Donald M
Wouldn't an evolutionary biologist just say that the instructions for the perceived ontogenetic depth were written one line at a time into the genetic code as each new adaptive change occured in the evolutionary process and simply remained in the DNA with each successful reproduction of an organism? Paul Nelson
Don -- yes, that's what an evolutionary biologist should say, I suppose. What's interesting is how few do say that, however, because the answer doesn't make much biological sense. A mass of undifferentiated cells still needs to make a living. Who (which cells) are going to be feeding cells, and which reproductive? Whenever one tries to give any *functional specificity* to the "add one cell at a time" story, one immediately runs into difficulties. itzhak-n
Paul. What actually happens when the development of C. Elegans is disrupted. I it instant death? A malformed organism? Or What? And does it make a difference at what place in the development sequence the disruption occurs? The effects of disruption may be an index of the irreducible complexity of the process, an idea that may be worth pursuing. Paul Nelson
Itzhak -- it depends when the disruption occurs, and in what cell lineages. Some disruptions kill the embryo. Others it can tolerate (i.e., it's viable for a while), but eventually the organism dies (e.g., its gut is malformed). I fully agree that exploring the range of disruptions one allow one toassign relative degrees of importance. END Tristan Abbey
Question for Paul: I'd just like to make sure I understand ontogenetic depth. This sounds very similar to the generative entrenchment argument you have raised in the past. Let's suppose Organism A has 10 hypothetical developmental stages. Mutations at stages 1-9, hypothetically, would result in non-viable organisms. Now, in your ontogenetic depth argument, you are asking how stages 1-9 even _arose_, in that organisms with developmental endpoints at stages 1-9 would be non-viable? Paul Nelson
That's right. Burgess Guy
Paul- since developmental geneticists use loss of function mutants to determine the role that a particular gene plays in development, it is hardly surprising that developmental mutations are "trainwreckers". After all, that is what the geneticists were looking for. Paul Nelson
Burgess -- yes...but where are the viable mutants? When this question was put to Eric Wieschaus in the early 1980s, shortly after he published his saturation mutagenesis experiments in Drosophila, he shrugged and said (in effect), "Heck, I don't know. You'd think we would have seen *some*." (He made this remark at the 1982 AAAS meeting, in a memorable talk on the evolutionary implications of his experiments.) Paul Nelson
There's another aspect to this question to consider. The Metazoa differ fundamentally in their overall developmental architectures (compare, for instance, C. elegans and Drosophila). Yet mutations to these early stages are invariably profoundly deleterious. So how did the fundamental early differences arise? How where the train wrecks avoided? Why couldn't my advisor (a very smart guy ) give me a single example of a heritable change in cleavage patterns, other than coiling in gastropods? Lydia
Paul--Tim is wondering what the evolutionary biologists have to say about this? For example, are they still holding out for the possibility that some sort of mutation-plus-selection scenario will ultimately arise, or do they have some entirely novel theory, or what? Is it a promissory note, or a refusal to consider the issue, or what? Paul Nelson
Lydia -- you can tell Tim that, for nearly all evolutionary biologists of my acquaintance, when the alternative is magic (design), the problem has to stay on the "Unsolved Difficulties" shelf. END phil
Mine was a more basic general question. I am puzzled by the presence of multiple copies of a gene and wondered how they were sorted to do the right thing at the right time and place. Paul Nelson
Phil, that's a good question that I don't think I can answer properly, within the constraints of this discussion. I'm sorry. charleybrown
Why isn't the concept of "ontogenetic depth" just another way of applying the god-of-the-gaps approach to something we don't really understand very well? Paul Nelson
Charley -- our hope is that ontogenetic depth will find its place in the evo-devo literature, as a description of biological reality, first of all. It's hardly a g-o-t-g to say that C. elegans goes through 7 to 9 rounds of cell division from egg to adult. That's just what the worms do. Paul Nelson
But, as I said at the beginning, the start of any scientific answer begins with correctly understanding the problem. Ontogenetic depth helps to do that. This is what any candidate theory of animal origins has to explain. it is not itself an explanation, but a description. Bern
If levels of developement are simply added, should cleavage in the developing embryo not happen in the same manner for all organisms with extra stages added? Or what controls the framework for that process? Paul Nelson
Bern -- the problem you describe (variations in early development) has long puzzled evolutionary biologists. I think differing cleavage patterns actually point to historical discontinuities among groups. Paul Nelson
Charley -- one final comment about God-of-the-gaps. You wouldn't say we have a "gap" in our understanding of basic physics, if someone wanted to build a real-time communication system between Earth and Mars. Paul Nelson
Rather, given what we know about electromagnetic transmissions (that carry a signal), it will take about 3.5 minutes for a signal to travel from Earth to mars (and another 3.5 for a reply to come back). Paul Nelson
That's how the world really works. In a parallel sense, if the developmental variation of animals is fundamentally constrained, then it would be a mistake to try to explain how it *isn't* constrained (and evolved from a common ancestor). In short: not every "gap" is waiting to be filled with knowledge. Burgess Guy
Paul- How would you apply your ontogenetic depth thinking to situations such as direct development in closely related species, such as found in sea urchins? Paul Nelson
Burgess -- let me explain, for the others, what you're talking about. Paul Nelson
In the sea urchin genus Heliocidaris, two modes of development are present. Paul Nelson
One species goes through a pluteus (free-living) larval stage. The other, however, develops directly into the adult. These ontogenies are remarkably different from each other, yet the adult sea urchins are more or less indistinguishable. Paul Nelson
Rudy Raff, a developmental and evolutionary biologist at the University of Indiana, has argued that Heliocidaris provides a model for the (radical) evolution of early development. Paul Nelson
And it may. But here are some puzzles to think about. First, the endpoint of development in both species is pretty much the same (really only experts can distinguish the two species in the genus). Paul Nelson
Second, to my knowledge, no one has ever successfully induced mutations in either genus to switch its mode of development. Paul Nelson
Third, although Raff has achieved hybridization (artificially) between the species, the hybrid offspring are badly malformed and do not live beyond one month or so. They are incapable of leaving offspring. Paul Nelson
So, while I find Heliocidaris fascinating, I don't consider it a genuine counterexample to my argument. ISCID Moderator
Alright, that wraps up tonight's chat. ISCID is grateful to Dr. Nelson for his stimulating presentation and discussion. (We're grateful for the thoughtful questions from the audience as well!) Copyright © by International Society for Complexity, Information, and Design 2003. |