The Cambrian Explosion

[achilles12604]

Besides the apologist answer that God was responsible for this phenomina by some method, does secular science have a theory as to the cause of this sudden explosion of new life all at once? (Remember I do not fall for that God of Gaps theory)

I am looking for science answer to this mystery. Anyone care to enlighten me?

[otseng]

With all due respect my friend, your logic is flawed in my view in that the rate at which changes in morphology take place makes no difference to the fact of organic evolution. Like many other time-worn creationist arguments, you are setting up a straw man argument, i.e., the definition that evolution must be 'gradualistic' when in fact the fact of evolution is not dependent upon such an assumption or claim, which is really part of one among many theories about the mechanisms of evolution.

I will admit my information comes from popular sources, rather than the latest scientific publications. All the readings I've come across state that the mainline thinking of evolution is gradualism, rather than saltationism.

In biology, saltation (from Latin, saltus, "leap") is a sudden change from one generation to the next, that is large, or very large, in comparison with the usual variation of an organism. The term is used for occasionally hypothesized, nongradual changes (especially single-step speciation) that are atypical of, or violate, standard concepts involved in neo-Darwinian evolution. The unorthodox emphasis on saltation as a means of evolutionary change is called saltationism.

Saltation is generally not held to be a method by which evolution occurs.

http://en.wikipedia.org/wiki/Saltationism

It postulates that speciation is (usually) due to the gradual accumulation of small genetic changes. This is equivalent to saying that macroevolution is simply a lot of microevolution.

http://www.talkorigins.org/faqs/modern-synthesis.html

On the theory of natural selection, we can clearly understand why she should not; for natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by the short and sure, though slow steps.

Origin of Species

Further, if I understand you correctly, are you saying that (macro) evolution could then occur at the individual level, rather than at a population level?

I have a question for you. I believe one day, ten, twenty, or perhaps fifty years from now, scientists are going to understand the regulatory genome enough to enable them to actually flip the genetic switches they have already discovered in such a manner that they will cause those changes in form that we now recognize as characterizing different phyla. If this actually should happen, then scientists would in real time be bringing about those changes in morphology that we now recognize as those changes brought about through the process of evolution.

If scientists were to be able to do this, what would then say? Upon the basis of what logic and reason would you then argue that evolution never happened? We already have the ancient genetic architectures of the master genes and their associated genetic switches, and should we one day learn to manipulate this regulatory genome to create in the labrotory changes in morphology would this not be strong evidence that what can be done in the labrotory has occurred naturally in the process of evolution?

I will say that if scientists can produce major morphological changes, then I would basically hang up my hat in debating organic evolution. But, until then, I see no strong evidence that it can happen in a short period of time.

[Goat]

[
I will say that if scientists can produce major morphological changes, then I would basically hang up my hat in debating organic evolution. But, until then, I see no strong evidence that it can happen in a short period of time.

The item I don't think you are realising is that these 'quick' morphology changes are still within 10's of millions of years. While it might be quick in geological time, when it comes to generations, it is still millions upon millions of generations of animals.

This change happened in response to a big increase in oxygen in the water. One of the articles I showed that multicellar life embroys were much more complicated even 10 million years before the 'cambrian explosian' than originally though.

Could you define 'major mophological changes'?

[Rob]

Hi Otseng,

The idea of Macromutations (earlier called Saltations) has been given new life based upon the solid scientific discoveries of Evo Devo. Contrary to the rather myopic views of some, who choose to ignore the rather vigorous debate ensuing in the field of evolutionary biology at this time, the hypothesis of Macromutations or Saltations, whichever you prefer, is alive and well. Why some dismiss this real debate based upon real evidence, is explained rather well in Gregories essay titled Macroevolution and the Genome. I have posted a lot of information in Evolutionary Developmental Biology.

The standard Panselectionist textbook orthodoxy of course is macroevolution is nothing more than microevolution writ large, which of course implies its corollary dogmas that populations rather than individuals are the targets of selection and evolution, and for some even, gradualism is the rule vs. sudden changes in morphology. Each and every one of these assumptions is now being debated based upon the new evidence forthcoming from Evo Devo. But you will hear some insist we should dismiss these questions as not worthy of real consideration. But hey, this is just proof that science is like any other human adventure; it takes all kinds of individuals, including those who become emotionally attached to what in one breath they admit are relative theories about the paths and mechanisms (not the fact of organic evolution mind you), yet in another breath insist are sacrosanct 'truth' while dismissing the very hard scientific evidence that is forthcoming from Evo Devo that raises serious questions about such assumptions. And none of these hotly debated issues for one moment casts any doubt upon the fact of evolution, and to argue these debates do is to show one has failed to grasp both the nature of the scientific evidence (facts) and the reasoning and logic of the theories (paths and mechanisms).

There is a diversity of views on these issues even within the field of Evo Devo, which is a healthy thing if science is to progress in marketplace of ideas which must compete in time with each other, ever evolving and growing as new evidence is discovered which either confirms of challenges old assumptions. In my view, it is just such new evidence that assists scientists in becoming aware of just what assumptions they hold, vs. what is really fact. New facts are discovered, interpretations change, and even old facts are viewed in a new light. Such is the nature of science. In time, we will know where these questions will lead, and what will be found true and what found incorrect. It should be a real exciting time in the field biology for the next few decades though!

I find it interesting that the historian of evolutionary theory, Peter J. Bowler, who himself is a staunch Panselectionist, says the following regarding Stephen J. Gould:

There was an episode in the 1980s when Gould did indeed seem to by toying with explicitly non-Darwinian ideas, including saltationism, but his position became much more moderate before his death in 2002. (p. 362)

(....) For a while Gould seemed to be teetering on the edge of outright anti-Darwinism, and there was much discussion about the possibility of a new direction in evolution theory.... He hinted at support for Richard Goldschmidt's saltationist theory of the hopeful monster, in which new species were produced by sudden transformations of the developmental pathway, perhaps triggered by quite small initial genetic changes. He also wondered if developmental constraints might predispose variation to take place in fixed, and not necessarily adaptive, directions.... Gould himself soon backed away from these more extreme ideas and returned to an interpretation of punctuated equilibrium theory that could be integrated with a generally Darwinian viewpoint. (p. 364)

-- Bowler, Peter J. (2003) Evolution: The History of an Idea. Univesity of California Press.

Having read Gould's final work, The Structure of Evolutionary Theory from beginning to end, I don't feel Bowler is accurately portraying Gould's position. While it is true that Gould retained allopatric speciation as the mode of speciation in his punctuated equilibrium theory, he argued strongly for a re-synthesis of evolutionary theory based upon the evidence forthcoming from Evo Devo for developmental pathways, which he characterized as imposing "constraints" upon the paths of evolution. And Gould openly states that saltation cannot be dismissed a priori without careful consideration in light of the evidence of Evo Devo, as many Panselectionists are quick to do.

If you take the time to read these links, perhaps we can discuss the issue further.

Chapter 12 Speciation and Developmental Evolution

The Genetic Basis of Species versus Larger Taxonomic Differences

It is the first issue -- the nature of the genetic differences that lead to speciation -- that has received the most attention and that has been the subject of the most controversy. The focus here is not on the genetic differences that lead to reproductive isolation, which may involve artifactual epistatic interactions, but on those genetic differences that lead to differential adaptation on the road to the formation of different species. (Wilkins 2002: 462)

The starting point of most discussions is Fisher's argument that adaptive evolution is driven by mutations of individually minute effect (Fisher, 1930) -- "micromutations," to use the term from the older literature. To the extent that species formation is accompanied by or driven by such adaptive evolution, developmental evolution in species formation would also consist of changes of individually tiny effect. This view, however, has always seemed deeply implausible to numerous developmental biologists (e.g., Goldschmidt, 1940; Gilbert et al., 1996) and patently untrue to the majority of paleontologists, based on their study of the fossil record (Valentine and Erwin, 1987; Gould, 1994; Erwin, 1999). To many members of these groups, an important role for mutations of large, or at least visible, phenotypic effect -- so-called "macromutations" -- has seemed far more probable than not. (Wilkins 2002: 462)

[Note: Gould after reviewing the new evidence of evo-devo, modified his previous views somewhat, making room for the "plausibility" of such macromutations, which were not part of his theory of punctuated equilibria, and which relied solely on allopatric speciation as the mechanism of his punctuations occuring over periods of 10 to 40 thousand years, which he termed "geological moment." The truth is that a bedding plane which represents a "geological moment" is only resolvable to 10,000 to 40,000 years, and Gould used statistical methods to determine changes in morphology over time which he associated with punctuational events. He admits in his book The Structure of Evolutionary Theory one could not differentiate between a sudden saltation and gradual allopatric speciation since both would leave the same data signature in the bedding plane. In other words, it is an assumption that the process of speciation in punctuated equilibria is due to the gradual accumulation of changes in the allels, although at an accelerated rate after a period of stasis. It is assumed that the fossil record just doesn't resolve the fine level of tiny changes assumed in this model. Yet, if these changes were to have actually have occurred via sudden saltations, the recorded data in the fossil record would be exactly the same, and one could not distinguish empirically between the two, and therefore one is left with a choice between one of two possible assumptions. The new evidence of molecular biology, genetics, and evo-devo are adding to the plausibility of sudden saltations.]

Nevertheless, the gulf between evolutionary geneticists on the one hand and developmental biologists and paleontologists on the other has narrowed in recent years, with a shift toward the position of the latter group. The new consensus is that some mutations of large phenotypic effect can and do play a part in speciation. (Wilkins 2002: 462)

(....) Phrasing this consensus in classic terms, it would seem that micromutations are not the exclusive genetic stuff upon which microevolution is built.... Admittedly, a large part of the debate about micromutations versus macromutations is hobbled by ambiguity because of vagueness of the distinction. As Arthur (1997) has pointed out, it is clear that "size" of phenotypic effect covers a large spectrum; there is no neat dichotomy between mutations causing tiny effects on the one hand and huge effects on the other.... Nevertheless, the old view that only micromutations contribute to adaptive evolution (and speciation) is clearly on the wane. (Wilkins 2002: 463)

(....) The fact that large changes in organismal structure and size appear seemingly suddenly in the fossil record had long been recognized and emphasized by paleontologists. This phenomenon was at the heart of a long estrangement between paleontology and new-Darwinian evolutionary biology. Simpson, who justly deserves credit for ending (or, at least, substantially ameliorating) the estrangement, wsa the first person to study these rate differences quantitatively. His survey was first presened in his book Tempo and Mode in Evolution (Simpson, 1944). His conclusions were that these rate differences are real; that four different patterns of evolution, each characterized by its own tempo, can be discerned in the fossil record; and that these patterns give clues to the existence of different "modes" of evolutionary change. (He did not try to specify precisely what those modes [mechanisms] were.) (Wilkins 2002: 465)

Qualitatively similar conclusions have subsequently been presented and argued by others (Valentine and Erwin, 1987; Gould 1994; Erwin 1999). Even though the genetic substratum of macroevolutionary change may be the same as that of microevolution, the dynamics of this process are not always constant. At the very least, much of the "invention" of new developmental processes that has accompanied the major radiations of new forms (the Cambrian explosion, the radiation of land plants in the Devonian and Carboniferous periods, the proliferation of mammalian and avian phylogenetic groups at the start of the Cenozoic era) has differed dramatically in tempo, if not in genetic character, from that of microevolution. (Wilkins 2002: 465)

-- Wilkins, Adam S. The Evolution of Developmental Pathways. Massachusetts: Sinaur Associates; 2002; pp. 462-465.

Chapter 4: Development and Evolution

Constraint and Saltation

Developmental biology has long been a focus for evolutionary theory (Bonner 1982; de Beer 1958; Garstang 1922; Goldschmidt 1938; Gould 1977; Haeckel 1866; Raff 1996; Raff and Kaufman 1983; Waddington 1940). Evolution can be seen as a change in developmental programs that elaborate the phenotype. The effects of genes and the range of genetic variation would best be investigated on a mechanistic basis, yet until the 1990s, we had only a very small window on this enormously important developmental landscape.

Once we can understand the nature of development and how it constructs the phenotype, we confront anew some of the age-old questions of evolutionary biology. Development is legendary for its organization, sometimes appearing to be remarkably automatic and even self-organizing. The strong integration of the developmental process might not easily be breached by a mutant, which would disrupt fundamental and tightly integrated cellular and molecular processes. This would suggest a force for conservatism in evolution. On the other hand, the tremendous organization of developmental processes suggest to many that simple genetic changes might beget enormous salutatory evolutionary change.

The Janus-headed coin of development is illustrated well by the evolutionary change of the tail in ascidian tadpole larva, which has been lost in evolution several times independently (Jeffery 1997). (....) This major switch in morphology is associated with a mundane larval adaptation for reduced dispersal by the tail-less form. (Tadpole larvae are not brilliant dispersers, either.) Tail-less development results from the abbreviation of developmental programs owing to maternal message and gene regulation in the zygote. The zinc-finger gene Manx is expressed in tailed species but is downregulated in tail-less species, which suggests a simple mechanism for a momentous developmental reorganization, dropping some of the lynchpins of the chordate anatomical plan (Swalla and Jeffery 1996).

The message told by the Manx gene is not clear, despite teh elegant experimental results. On the one hand, it tells us that it is rather easy to lose the tail and a host of associated developmental trajectories (e.g., notochord, tail, otolith, and muscle cells). (....) If it is that easy, why is it so uncommon? Again, we face teh two faces of constraint and possibility for major change.

Time and again, the concepts of constraint and saltation have been formulated in terms of development. Developmental constraints are nonrandom channelizations of evolutionary direction due to limitations imposed by complex interactions of gene expression and epigentic interactions, such as tissue inductions, in the developing organism. The disruption of such interactions may strongly influence fitness and therefore restrict evolutionary change. In the context of development, saltations are rapid evolutionary fixations of phenotypic discontinuities guided by developmental controls, which do not permit continuity of form in polymorphic populations.

The Holy Grail: Connecting an Understanding of Genes and Development

(....) We are now at the threshold of a completely new period, in which development and genetics are being connected in great detail. At first, this became apparent from the emerging understanding of a widespread homeobox sequence that united all of the triploblastic animals at least. Now, modern methods of gene sequencing, manipulation of gene expression, and tracing of spatial patterns of gene expression have resulted in an explosion of information that is not leading, as yet, to many useful evolutionary rules. So far, we are seeing the same errors promulgated in lionizing past laws of ontongeny and phylogeny. Beliefs in major genetic revolutions, master switch genes, and other universals are beginning to form a modern version of the ontogentic laws of old, with little consideration for the possibilities of convergence in developmental gene function. Nevertheless, the new tools allow us to better peak through the curtains, and the early flush of enthusiasm will likely be followed by substantial advances in development evolution.

Phylogeneticists and Developmentalists

(….) The developmentalists claim that “the diversity of structures that have been formed through the process of evolution is constrained by the rules which govern pattern formation during development” (Stock and Bryant 1981, p. 432). As such, evolutionary change of necessity is the evolution of developmental sequences. The individual, therefore, is treated in terms of its entire ontogeny, and development is therefore both the constraint and target of selection. There is a developmental toolbox, and certain tools may be used in many contexts, but this does impose a possibly limited set of alternative developmental pathways.

(….) [M]orphological structures often come as complete structures or not at all. Of equal interest is the importance of localization in development. Embryos develop only as the result of a complex series of timing events that bring different cells into contact or place cells or molecules of restricted developmental potency in a proper environment for induction. The spatial position of cell groups seems crucial in the generation of morphological patterns, owing to

· Localized intercellular movement and regional movement of dissolved substances that often set gene expression in motion (Garcia-Bellido, Rippoil, and Morata 1973; Summerbell 1981; Turing 1952; Wolpert 1969)
· Transcellular electric fields (Jaffe and Stern 1979; Nuccitelli 1983)
· Mechanochemical interactions (e.g., Odell, Oster, Alberch, and Burnside 1981; Oster, Murray, and Harris 1983)
· Specialized cell adhesion molecules (Edelman 1986)

Must these not influence the direction of evolution? These two phenomena integrity of structure and topological restriction of development suggest that an embryo can be transformed in only limited number of directions during the process of development and evolution. That is the fundamental message about form that Richard Goldschmidt’s pioneering book Physiological Genetics (1938), derived from Spémann (1938), underscored so well.

Some examples of developmental mutants show the discontinuous and often spectacular nature of possible structural change. Consider the cyclops mutant (Bowen, Hanson, Dowling, and Poon 1966) of brine shrimp males. After the fourth instar, the lateral eyes move forward and fuse together, forming a single large compound eye by the ninth instar. During this fusion, the ganglia and nerves of the two optic stalks fuse; the resultant eye resembles the normal medial eye of the cladoceran Leptodora. Thus, a quirk of development has caused a structure to change from that characteristic of one taxonomic order to another! The development of the vertebrate limb shows similar quantum steps.



(….) A developmental notion of macromutation springs from the nature of development described above. If a simple transplant places toes on wings or replaces scales with feathers, why couldn’t evolution occur in major steps? Some have seen such discontinuities in development as a vehicle for major evolutionary jumps (Goldschmidt 1940; Gould 1980a; Lovtrup 1974; Maderson et al. 1982; Schinderwolf 1936, 1950), or at least see them as possible stuff of major saltations (Alberch, Gould, Oster, and Wake 1979; Frazzetta 1970)

-- Levinton, Jeffrey S. Genetics, Paleontology, and Macroevolution. Cambridge: Cambridge University Press; 2001; pp. 157-162.

Abstract:

In the cyclopean mutant of the anostracan branchiopod Artemia franciscana, paired stalked eyes are replaced by a single, median, sessile, eye resembling that found in certain monocular branchiopod orders. This eye, its nerve supply, and skeletal support, comprise a perfect unit which appears to be a spontaneous atavism. However, according to recent calculations this cannot be so. These suggest that while re-activation of long-silent genes, on which atavisms depend, can occur after a lapse of up to 6 million years (My), this is impossible after 10 My unless the gene is maintained by active selection, which cannot apply here. However, the Anostraca is an old group, and the atavism (if such it be) is clearly very ancient. Efficient DNA repair, not considered in the calculations, offers a possible explanation of how silent genes may survive for longer than the suggested period of viability. Particularly intriguing is that a binocular condition is primitive and the cyclopean derived, which has remarkable evolutionary implications. It suggests two reversals during the history of the Anostraca from paired sessile eyes to a long-extinct monocular condition such as prevails in certain other branchiopods, later to paired stalked eyes. Other ancient atavisms also challenge the claim that silent genes have short life spans. This problem, which has fundamental biological implications, is still sub-judice.

-- Fryer, Geoffrey. The case of the one-eyed brine shrimp: are ancient atavisms possible? Journal of Natural History . 1999 Jun 1; 33(6):791-798.

Waddington and Schmalhausen, two of the few biologists actually interested in connecting development and evolution during the height of influence of the modern synthesis, introduced the first phenomenon, what Waddington (1942) termed “canalization.” (….) The evolution of discrete traits has been a problem in evolutionary theory since its inception, especially because these traits often have a complex genetic architecture so that single mutations are unlikely to produce the necessary change (but see the next section for a situation in which this could occur). Goldschmidt’s notion of “hopeful monsters” has been universally panned, but several lines of recent evidence suggest that saltational change may have played a role in evolution (Dietrich, 2003). Probably most important for reviving at least some acceptability for the notion of sudden change was the discovery of the genetic basis of change in the numbers or arrangements of segmental or meristic traits. The classic instance was the finding that Hox gene mutation could produce extra wings or leg-antennal substitutions I flies, examples of traits considered long ago in this context by William Bateson (Bateson, 1894). Change in segment number can be brought about by mutation in a single gene. Though the change is often not completely viable or competitively “fit,” it shows that small genetic change can lead to organized morphological change, and some of this resembles evolutionary changes in body plan (Carroll et al., 2001)

-- Hallgrimsson, Benedikt and Hall Brian. Variation: A Central Concept in Biology. Amsterdam: Elsevier Academic Press; 2005; p. 511.

Where, then, is the scientists' fallacy in explaining macroevolution within the context of creationism-evolution discussions? It lies in the pretense that we have a full answer when at most we have a few (tantalizing) clues. (Pigliucci 2002: 339)

(....) Perhaps the mother of all macroevolutionary mysteries is the Cambrian explosion... As we saw ..., there is nothing to indicate that the relatively rapid origin of many new body plans in animals is a mystery requiring anything more than our current understanding of evolutionary and ecological problems. [Hardly true in light of the ongoing debate.] Yet the clues currently available are so few that it is not intellectually honest to say that we "understand" the Cambrian explosion. We are studying it, and we are making progress. One cannot ask more from science. (Pigliucci 2002: 240)

-- Pigliucci, Massimo. Denying Evolution: Creationism, Scientism, and the Nature of Science. Massachusetts: Sinauer Associates; 2002; p. 339-240.

[otseng]

If you take the time to read these links, perhaps we can discuss the issue further.

I'll try to read through it and get back to you. Though it might take some time.

But, in the meantime, 50 tokens for you for an excellent post.

[Jose]

I will say that if scientists can produce major morphological changes, then I would basically hang up my hat in debating organic evolution. But, until then, I see no strong evidence that it can happen in a short period of time.

This is precisely the reason that the anti-evolutionists make so much noise about Ed Lewis's 4-winged fly and all the other homeotic mutants out there. They say "this is not a new species, so it doesn't count." The fact remains, however, that it demonstrates beyond a shadow of a doubt how major morphological changes occur: by mutations in the regulation of developmental-control genes.

I think it is an obfuscation to equate this genetically- and molecularly-understood process with the vague term, "saltation." Saltation was invented to fill a knowledge gap, using the concept of sudden, major changes in form (mechanism unknown). Now that we know the mechanism for changes in form, and know the genes involved and how they work, we know that the reality doesn't match that old concept.

Of course, scientists can't "produce" major morphological changes on purpose, given that mutations occur at random. Even in searching explicitly for new Antp mutations, for example, one is at the mercy of probability distributions. I can expose flies to X-rays, but I can't tell the X-rays what to do. So, scientists can create collections of animals with different mutations, and perform crosses to move multiple mutations into the same strain--which is what Ed did to create the 4-winged fly--but they can't purposely set out to make a new form of animal with pre-planned characteristics.

The 4-winged fly is also undervalued by anti-evolutionists because it essentially is a reversion to an older evolutionary state, rather than a "progression" to a new state. In flies, the hind wings are modified into halteres (aka "balancers"); Ed's mutations turn off the modification pathway, resulting in a form similar to the ancestral condition (e.g. as in dragonflies). Even so, this is important information, as it provides additional evidence of the species' evolutionary history. Along these lines, there are similar mutations in humans. They tend to produce people with 2 or 3 pairs nipples, instead of the normal 1 pair. By analogy to the fly mutations, this is most easily interpreted as a reversion to an ancestral condition, revealing our relationship to other mammals.

As Rob has pointed out, there are now well-understood mechanisms for morphological change, involving the "regulatory genome" and "master switch genes" (i.e. changes in regulation of the genes that determine how development proceeds). These mechanisms solve the "problem" of morphological change, and simultaneously eliminate the need for terms like "macromutation" and "saltation." These terms are now seen to refer to mutations (and their effects) in regulatory genes.

I might note, by the way, that stating this fact is not falling into the false ideology of panselectionism. Panselectionism claims that selection is the primary, if not exclusive, means of sorting among mutations and converting genetic diversity into evolutionary change. Describing the mutations and their effects makes no claim of how they become fixed in populations. Often, selection has little to do with it--there are founder effects (e.g. one individual is the founder of an isolated population), or genetic drift (e.g. a few individuals survive a flood). It's actually rather amusing to listen to discussions among evolutionary biologists about the relative contributions of selection vs other mechanisms of fixing alleles in populations. Things can become quite heated, with people calling each other names (like "you rabid selectionist!"). I think the short answer is that we don't know the relative contributions. On the other hand, we hear a lot more about selection, because it's conceptually pleasing; it's hard to teach students about genetic drift and founder effects and keep them awake.

[otseng]

Sorry for the delay, it's been quite busy at the otseng household lately (and with the holidays approaching, I don't think it'll get much better).

Rob, I've read through the information you provided (including the Evolutionary Developmental Biology thread). Though most of it went over my head, hopefully I'll be able to absorb more of it over time. But, I did enjoy reading them and learned a lot from them.

The starting point of most discussions is Fisher's argument that adaptive evolution is driven by mutations of individually minute effect (Fisher, 1930) -- "micromutations," to use the term from the older literature. To the extent that species formation is accompanied by or driven by such adaptive evolution, developmental evolution in species formation would also consist of changes of individually tiny effect. This view, however, has always seemed deeply implausible to numerous developmental biologists (e.g., Goldschmidt, 1940; Gilbert et al., 1996) and patently untrue to the majority of paleontologists, based on their study of the fossil record (Valentine and Erwin, 1987; Gould, 1994; Erwin, 1999). To many members of these groups, an important role for mutations of large, or at least visible, phenotypic effect -- so-called "macromutations" -- has seemed far more probable than not. (Wilkins 2002: 462)

And in this thread, this is primarily the angle I'm coming from, the paleontological viewpoint, and in particular, the Cambrian Explosion viewpoint. I would think macromutation/saltationism would be the only way to help explain the Cambrian Explosion. And evo devo could probably be the mechanism behind it.

I think the biggest question in my mind is the concept of individual (macro)evolution. So, with evo devo, is this concept brought back to the table?

Could you define 'major mophological changes'?

Examples of a major morphological feature would include:
- asexual to sexual reproduction
- scales to feathers
- wingless to wings
- eyeless to eyes

The key factor for me would be the introduction of something new. So, I would not consider having extra fingers to be a major morphological change.

it's hard to teach students about genetic drift and founder effects and keep them awake.

I think if you say "let's make it into a debate and whichever side wins will get 500 tokens", I think more might remain awake.

[Goat]

Could you define 'major mophological changes'?

Examples of a major morphological feature would include:
- asexual to sexual reproduction
- scales to feathers
- wingless to wings
- eyeless to eyes

The key factor for me would be the introduction of something new. So, I would not consider having extra fingers to be a major morphological change.

it's hard to teach students about genetic drift and founder effects and keep them awake.

I think if you say "let's make it into a debate and whichever side wins will get 500 tokens", I think more might remain awake.

Well, there are articles about each and every one of those. For example,
The Morphogenesis of Feathers shows how feathers developed from scales. The actual PDF of the article can be found at http://www-hsc.usc.edu/~cmchuong/nature.pdf . It gets rather involved technically, so I don't want to cut/paste the article here (other than to point out that the problem is not a black box).

Eye Evolution is pretty well docuemented also. Although we don't have
fossils of eyes, we have examples of eyes in various stages of development throughout species.

From http://www.pbs.org/wgbh/evolution/libra ... 11_01.html

Through natural selection, different types of eyes have emerged in evolutionary history -- and the human eye isn't even the best one, from some standpoints. Because blood vessels run across the surface of the retina instead of beneath it, it's easy for the vessels to proliferate or leak and impair vision. So, the evolution theorists say, the anti-evolution argument that life was created by an "intelligent designer" doesn't hold water: If God or some other omnipotent force was responsible for the human eye, it was something of a botched design.

Biologists use the range of less complex light sensitive structures that exist in living species today to hypothesize the various evolutionary stages eyes may have gone through.

Here's how some scientists think some eyes may have evolved: The simple light-sensitive spot on the skin of some ancestral creature gave it some tiny survival advantage, perhaps allowing it to evade a predator. Random changes then created a depression in the light-sensitive patch, a deepening pit that made "vision" a little sharper. At the same time, the pit's opening gradually narrowed, so light entered through a small aperture, like a pinhole camera.

Every change had to confer a survival advantage, no matter how slight. Eventually, the light-sensitive spot evolved into a retina, the layer of cells and pigment at the back of the human eye. Over time a lens formed at the front of the eye. It could have arisen as a double-layered transparent tissue containing increasing amounts of liquid that gave it the convex curvature of the human eye.

In fact, eyes corresponding to every stage in this sequence have been found in existing living species. The existence of this range of less complex light-sensitive structures supports scientists' hypotheses about how complex eyes like ours could evolve. The first animals with anything resembling an eye lived about 550 million years ago. And, according to one scientist's calculations, only 364,000 years would have been needed for a camera-like eye to evolve from a light-sensitive patch.

In other words, we start with a light sensative patch (examples which are around today), and through small successive changes, this 'eye patch' became more complex and more efficient. Each step is not something you would consider a 'morphology' change by your definition, but evolution is accumulative. The various minor modifications add up to one much greater modification over many thousands of generations.

As for asexual to sexual reproduction, we have examples of species that switch over from one to another still today. A sample of this would be
Dugesia ryukyuensis. From http://icb.oxfordjournals.org/cgi/conte ... t/43/2/242

Many metazoans convert the reproductive modes presumably depending upon the environmental conditions and/or the phase of life cycle, but the mechanisms underlying the switching from asexual to sexual reproduction, and vice versa, remain unknown. We established an experimental system, using an integrative biology approach, to analyze the mechanism in the planarian, Dugesia ryukyuensis (Kobayashi et al., 1999). Worms of exclusively asexual clone (OH strain) of the species gradually develop ovaries, testes and other sexual organs, then copulate and eventually lay cocoons filled with fertilized eggs, if they are fed with sexually mature worms of Bdellocephala brunnea (an exclusively oviparous species). This suggests the existence of a sexualizing substance(s) in sexually mature worms. Random inbreeding of experimentally sexualized worms (acquired sexuals) produces an F1 population of spontaneous sexuals (innate sexuals) and asexuals in a ratio of approximately 2:1. All regenerants from various portions of innate sexuals become sexuals. In the case of acquired sexuals, head fragments without sexual organs regenerated into asexuals though regenerants from other portions became sexuals. Thus, we conclude that neoblasts, the totipotent stem cells in the planarians, of acquired sexuals remain "asexual" and the worms require external supply of a sexualizing substance for the differentiation of sexual organs and gametes. On the other hand, some, if not all, neoblasts in innate sexuals are somehow "sexual" and do not require external supply of a sexualizing substance for the eventual differentiation of themselves and/or other neoblasts into sexual organs and gametes. It is also shown that sexuality in acquired sexuals is maintained by the putative sexualizing substance(s) of their own. The sexualization is closely coupled with cessation of fission, and the worms seem to have an unknown way of controlling the karyotype. Our integrative approach integrates multiple fields of study, including classic breeding, regeneration, and genetics experiments, as well as karyotyping, and biochemical and molecular biological analyses; none of which would have revealed much about the intricate mechanisms that regulate sex and fission in these animals.

[/url]

[Jose]

The starting point of most discussions is Fisher's argument that adaptive evolution is driven by mutations of individually minute effect (Fisher, 1930) -- "micromutations," to use the term from the older literature. To the extent that species formation is accompanied by or driven by such adaptive evolution, developmental evolution in species formation would also consist of changes of individually tiny effect. This view, however, has always seemed deeply implausible to numerous developmental biologists (e.g., Goldschmidt, 1940; Gilbert et al., 1996) and patently untrue to the majority of paleontologists, based on their study of the fossil record (Valentine and Erwin, 1987; Gould, 1994; Erwin, 1999). To many members of these groups, an important role for mutations of large, or at least visible, phenotypic effect -- so-called "macromutations" -- has seemed far more probable than not. (Wilkins 2002: 462)

And in this thread, this is primarily the angle I'm coming from, the paleontological viewpoint, and in particular, the Cambrian Explosion viewpoint. I would think macromutation/saltationism would be the only way to help explain the Cambrian Explosion. And evo devo could probably be the mechanism behind it.

Remember, evo devo is not, in itself, anything special. It's simply the overlap of evolutionary biology and developmental biology, made possible by our understanding of the genes that are responsible for the embryological events that produce morphology. I think it's an obfuscation to bring in Fisher's and even Goldschmidt's thoughts about things that were then unknown. The only difference between a "micromutation" and a "macromutation," as defined by evo devo, is which genes the mutations are in. So-called micromutations are in genes that don't control morphological characteristics (like hair color, or in the case of Peppered Moths, wing color). So-called macromutations are in genes that do control morphological characteristics.

But even then, whether we call such a mutation a "macromutation" or not depends on personal taste. Polydactyly is such a mutation, but we don't see it as anything special because of our personal viewpoint. So, non-evolutionists tend not to call it a macromutation, even though it is.

The kind of "macromutation" that The Ancients imagined when they coined and used the term was for the imagined process of "saltation" in which a Whole Bunch of Things change at once, and presto-chango, there's a new form. This is, as you suggest, the way we'd have to think of the Cambrian "explosion," if we believed in such a thing any more, and if we had no knowledge of the metameric nature of the preceding animals (ie, they were segmented). Now that we do know some things about the preceding animals, why bother to postulate a magic solution when ordinary genetics will do the job nicely?

I think the biggest question in my mind is the concept of individual (macro)evolution. So, with evo devo, is this concept brought back to the table?

Nope. We're still stuck with biology as we know it. There is no data to support any other form of life, with unpredictable Lamarckian inheritance patterns. We're pretty much constrained to proposing mechanisms that involve normal genetics.

Individuals cannot evolve. The only way to produce offspring that are different is by having one or more mutations occur in their sex cells, and of course, the reshuffling of existing alleles through meiosis. So, if a mutant offspring is born, he's either similar enough to the others that he can mate with them and pass on his mutations, or he's a dead end--he can't be a new species. He might have some morphological differences, due to mutations in regulatory genes, but if they make him look too different, potential mates won't recognize him. Of course, these "rules" might not apply to self-fertilizing hermaphrodites, like C. elegans.

Could you define 'major mophological changes'?

Examples of a major morphological feature would include:
- asexual to sexual reproduction
- scales to feathers
- wingless to wings
- eyeless to eyes

The key factor for me would be the introduction of something new. For example, I would not consider having extra fingers to be a major morphological change.

Indeed, polydactyly is not a major morphological change--but would loss of digits count? After all, birds reduced their digit number from 5 to 3 (and, for that matter, the first tetrapods had 8 digits). That would make wingless to wings (for birds) be merely a matter of turning scales to feathers. And even that isn't considered a "major morphological change" in the pigeon- and chicken-fanciers' world. Look at those guys whose leg scales have been transformed into flight feathers, or the ones whose overall covering is all down-feathers. The genetic programs of scales and feathers seem to be pretty close.



It might be a bigger question for insects, many of which have no dorsal appendage at all. The thinking there, according to one paper from some time ago, was that an extant group of insects might give a clue: these guys don't have wings, but do have dorsal thingies that they can raise upward. These act like sails, and help them blow rapidly across water. Once you've developed that characteristic, it's only "minor modifications" to control the angle of the sails for steering, and from there to holding them sideways rather than straight up. Voila--lift.

Hmmm....I wonder if "major morphological change" is merely a matter of perspective. If chickens consider feathers vs scales to be minor, and if the number of digits is minor, and if using limbs for walking vs paddling is minor, then.... ....at the time that these transitions are happening, nothing is a major change. It's only in retrospect that they seem like major changes. And then, the major changes are typically seen through the lens of comparative anatomy: comparing current species to each other. But, comparing current species isn't historically accurate. We have to compare individuals within a lineage, from species A to species B to species C.

it's hard to teach students about genetic drift and founder effects and keep them awake.

I think if you say "let's make it into a debate and whichever side wins will get 500 tokens", I think more might remain awake.

They might indeed!

[otseng]

The Morphogenesis of Feathers shows how feathers developed from scales. The actual PDF of the article can be found at http://www-hsc.usc.edu/~cmchuong/nature.pdf . It gets rather involved technically, so I don't want to cut/paste the article here (other than to point out that the problem is not a black box).

I think you're overstating things in regards to the article. The article does not go about to show "how feathers developed from scales". But from my interpretation of the technical jargon, it provides "insights on the possible developmental mechanisms in the evolution" of "feathers from cylindrical epithelia to the hierarchical branched structures".

Eye Evolution is pretty well docuemented also. Although we don't have
fossils of eyes, we have examples of eyes in various stages of development throughout species.

From http://www.pbs.org/wgbh/evolution/libra ... 11_01.html

And this is not a correct assessment either.

Zoologist Dan-Erik Nilsson demonstrates how the complex human eye could have evolved through natural selection acting on small variations. Starting with a simple patch of light sensitive cells, Nilsson's model "evolves" until a clear image is produced.

Nilsson uses modeling, rather than demonstrating the animal pathway of eye evolution. So, though he does propose how the eye could've evolved, I would not describe it as "eye evolution is pretty well documented."

But, no matter, the context of what I was talking about with major morphological changes was in regards to Rob's question.

I have a question for you. I believe one day, ten, twenty, or perhaps fifty years from now, scientists are going to understand the regulatory genome enough to enable them to actually flip the genetic switches they have already discovered in such a manner that they will cause those changes in form that we now recognize as characterizing different phyla. If this actually should happen, then scientists would in real time be bringing about those changes in morphology that we now recognize as those changes brought about through the process of evolution.

If scientists would be able to "flip a switch" and produce the major morphological changes that I described, the empirical evidence would sway me to believe that saltationism is possible, and even common descent would be probable.

I think the biggest question in my mind is the concept of individual (macro)evolution. So, with evo devo, is this concept brought back to the table?

Nope. We're still stuck with biology as we know it. There is no data to support any other form of life, with unpredictable Lamarckian inheritance patterns. We're pretty much constrained to proposing mechanisms that involve normal genetics.

I think we can agree that Lamarckism has been discredited. However, I don't think it has any relationship with evo devo.

Individuals cannot evolve. The only way to produce offspring that are different is by having one or more mutations occur in their sex cells, and of course, the reshuffling of existing alleles through meiosis. So, if a mutant offspring is born, he's either similar enough to the others that he can mate with them and pass on his mutations, or he's a dead end--he can't be a new species. He might have some morphological differences, due to mutations in regulatory genes, but if they make him look too different, potential mates won't recognize him. Of course, these "rules" might not apply to self-fertilizing hermaphrodites, like C. elegans.

This is how I perceive to be the common view of evolution. That is, individuals do not evolve, but populations do. And that evolution occurs gradually. Thus, this is why I've stated that this is at odds with the evidence of the Cambrian Explosion.

Species evolution must've occured at an accelerated rate during the Cambrian Explosion since all the (extant and extinct) phyla came about during this time period. Yet, the problem is that relatively few species are found in the Cambrian layer. With the accelerated evolution during this time, one would expect to find a huge number of different species if evolution is incremental.

"A central paradox of early life: How could so much disparity in body plans evolve in the apparent absence of substantial diversity in number of species?"
Gould, Wonderful Life, 1989.

So, in my mind, the only evolutionary way to account for this would be saltationism. That is, evolution did not occur through "slight successive variations" over many generations. But, it had to produce "hopeful monsters" in which there would be large jumps from one species to another.

But, of course, Jose is right in that hopeful monsters would most likely be a dead end. It might be a super wonder animal with amazing powers. But, if doesn't have another hopeful monster of the opposite gender at the same location at the same time, then it doesn't have any hope of passing its genes on.

[Rob]

Eye Evolution is pretty well docuemented also. Although we don't have fossils of eyes, we have examples of eyes in various stages of development throughout species.

Evo-Devo has shed substantial light upon the underlying genetic pathways of eye evolution, and what is most interesting is the realization that these pathways are based upon anciently conserved genetic pathways and core moleculer and developmental patterns found in the regulatory genome:

No case has received more attention, generated more surprise, rested upon firmer data, or so altered previous "certainties," than the discovery of an important and clearly homologous developmental pathway underlying the ubiquitous and venerable paradigm of convergence in our textbooks: the independent evolution of image-forming lens eyes in several phyla, with the stunning anatomical similarities of single-lens eyes in cephalopods and vertebrates as the most salient illustration. As Tomarev et al. (1997, p. 2421) write: "The complex eyes of cephalopod mollusks and vertebrates have been considered a classical example of convergent evolution." (....)

PARALLELISM IN THE LARGE: PAX-6 AND THE HOMOLOGY OF DEVELOPMENTAL PATHWAYS IN HOMOPLASTIC EYES OF SEVERAL PHYLA

DATA AND DISCOVERY. Salvini-Plawen and Mayer (1977), in a classical article nearly always cited in this context, argued that photoreceptors of some form have evolved independently some 40 to 60 times among animals, with six phyla developing complex image-forming eyes, ranging from cubomedusoids among the Cnidaria, through annelids, onychophores, arthropods and mollusks to vertebrates along the conventional chain of life. In the early 1990s, using Drosophila probes, researchers cloned a family of mammalian Pax genes, most notably Pax-6, which includes both a paired box and homeobox (Walther and Gruss, 1991). (....) The similar function of these Pax-6 homologs in different phyla was then dramatically affirmed by expressing the mouse gene in Drosophila (Halder et al., 1995), and finding that the mammalian version could still induce the formation of normal fly eyes. (....) [T]he Pax-6 story has now furnished an important homological basis in underlying developmental pathways for generating complex eyes in cephalopods and vertebrates. Thus, a channel of inherited internal constraint has strongly facilitated the resulting, nearly identical solution in two phyla, and evolutionists can no longer argue that such similar eyes originated along entirely separate routes, directed only by natural selection, and without benefit of any common channel of shared developmental architecture. But just as the advocates of pure convergence erred in claiming exlclusive rights of explanation, the discovery of Pax-6 homologies does not permit a complete flip to exclusive explanation by constraint. (Gould 2002: 1123-1128)

-- Gould, Stephen J. The Structure of Evolutionary Theory. Cambridge: Harvard University Press; 2002; pp. 1123-1128.

Many years ago, from a detailed phylogenetic survey of the distribution of eye structures within the Metazoa, Salvini-Plawen and Mayr (1977) argued that eyes had evolved independently, perhaps as often as 20 times, in different metazoan phyla. Thus, in their reconstruction, there was no single ancestral eye form in the Metazoa. Furthermore, in their scheme, continuity of appearance of structures was a cardinal test of homology. In cephalopods, for instance, it seems certain that ancestral molluscan forms did not possess eyes. Therefore, the eyes of cephalopods must have originated in lineages derived from forms lacking eye development (Salvini-Plawen and Mayr, 1977). In addition, there might not even have been a common ancestral metazoan photoreceptive field. The phylogenetic analysis of Salvini-Plawen and Mayr indicates that photoreceptor cells themselves may have evolved independently between 40 and 65 times during metazoan evolution. (Wilkins 2002: 166)

The only, or at least the simplest, way to reconcile this pattern with the equally unambiguous general role of Pax6 and its associated genes in eye development throughout the Bilateria is to posit shared genetic potential for development of eyes, even when the potential is not expressed. From this perspective, both the multiple independent occurrence of certain traits in different related lineages (cases of "parallelism") and the reacquisition of a particular trait in a lineage that had seemingly lost it (a "reversal") can, in principle, be accommodated. The key is the retention of genes and genetic architectures within those lineages and their suppression or revocation through certain genetic changes (Simpson, 1983; Wake, 1991; Butler and Saidel, 2000). Loss-of-function mutations that abrogate the operation of networks are easy to imagine. Similarly, suppression of the effects of such mutations, leading to a restoration of the network’ functions, can also be readily envisaged. In this conceptual framework, the widespread usage of Pax6 and its associated network of genes is a significant fact, while the polyphyletic pattern of eye development drawn by Salvini-Plawen and Mayr is not dismissed, but can be interpreted as reflecting losses and gains of the use of this network. (Wilkins 2002: 167)


Altogether, these considerations require some changes in the ways in which we view, and use, the term "homology." The basic concept of shared possession of a trait through common descent remains intact, but the idea that the "same" trait in two different organisms may actually exhibit more points of visible difference than of discernible identity seems counterintuitive, to put it mildly. Nevertheless, the idea that homologous morphological traits and genes need not share tight, invariant relationships had been anticipated long ago by de Beer (1938, 1958, 1971) …. In effect, a set of decoupled relationships would involve the sharing of part of the same regulatory circuitry, but without visible (morphological) homology being involved. As Fernald (2000) has expressed it:

Recently, the discovery of conservation of many of the genes used during ontogeny of the eye, particularly Pax6, has led to the proposal that all eyes are monophyletic -- that is, they arose from an "Ur" eye. However, our current level of understanding of the genetic control of eye development does not support this conclusion. Instead, there appears to be a continuity of genetic information that regulates the development of similar but nonhomologous eyes.

Perhaps, however, it is the traditional framework that needs reevaluation. In cladistic terms, a homologous trait is a synapomorphy. If morphological similarity is not a reliable guide to homologous relationships, what precisely would such a snyapomorphy consist of? In light of the material presented here, one can suggest that it would be a combination of shared key genes (one or more) plus a shared biological or developmental function for which those genes are crucial. This is a rather radical revision of the concept of homology, which for 150 years has been tied to visible similarity, and which has specifically disavowed shared function as a criterion of homology. The formulation put forward here, however, allows one to incorporate the observations on conserved key regulatory genes without invoking convergent evolution, and it preserves the basic idea of homology as "continuity of [genetic] information." Davidson (2001, p. 201) has reached a similar position. In his words, when it comes to assessing homologous relationships, "seeing is not [necessarily] believing." (Wilkins 2002: 167)

From this persepective, the eyes of insects and vertebrates are homologous, even though they look different from each other, develop differently, and may have arisen independently in separate lineages from ancestors lacking eyes. What they share is the inherited regulatory machinary and the ancestral use (function) of that machinery -- dating to early in metazoan evolutionary history -- for light sensing or some rudimentary form of vision. (Wilkins 2002: 168)


-- Wilkins, Adam S. The Evolution of Developmental Pathways. Massachusetts: Sinaur Associates; 2002; pp. 166-168.

Ernest Mayr's (1963, p. 586) epitome of Darwinism as preached by the Modern Synthesis: "All evolution is due to the accumulation of small genetic changes, guided by natural selection ..., and that transpecific evolution ... is nothing but an extrapolation and magnification of the events that take place within populations and species." (Gould 2002: 160)

Throughout Mayr's 1963 book [Animal Species and Evolution, Cambridge MA: Harvard University Press] -- with a cadence that sounds, at times, almost like a morality play -- phenomenon after phenomenon falls to the explanatory unity of adaptation, as the light of nature's truth expands into previous darkness: non-genetic variation (p. 139), homeostasis (pp. 57, 61), prevention of hybridization (p. 109). Former standard bearers of the opposition fall into disarray, finally succumbing to defeat almost by definition: "It is now evident that the term 'drift' was ill-chosen and that all or virtually all of the cases listed in the literature as 'evolutionary change due to genetic drift' are to be interpreted in terms of selection" (p. 214). All particular Goliaths have been slain (although later genetic studies would revivify this particular old warrior): "The human blood-group genes have in the past been held up as an exemplary case of 'neutral-genes,' that is, genes of no selective significance. This assumption has now been thoroughly disproved." (p. 161). (Gould 2002: 539)

However, Mayr's most interesting expression of movement towards a hardened adaptationism occurs not so much in these explicit claims for near ubiquity, but even more forcefully in the subtle redefinition of all evolutionary problems as issues of adaptation. The very meaning of terms, questions, groupings and weights of phenomena, now enter evolutionary discourse under adaptationist presumptions. Not only have alternatives to adaptation been routed on an objective playing field, Mayr claims in 1963, but the conceptual space of evolutionary inquiry has also become so reconfigured that hardly any room (or even language) remains for considering, or even formulating, a potential way to consider answers outside an adaptationist framework. (Gould 2002: 539)

Major subjects, the origin of evolutionary novelty for example, now reside exclusively within an adaptationist framework by purely functional definition: "We may begin by defining evolutionary novelty as any newly acquired structure or property that permits the performance of a new function, which, in turn, will open a new adaptive zone" (p. 602). In a world of rapid and precise adaptation, morphological similarity between distantly related groups can only arise through convergence imposed by similar adaptive regimes upon fundamentally different genetic material. The older, internalist view (constraint-based and potentially nonadaptationist) -- the claim that we might attribute such similarities to parallelism produced by homologous genes -- is dismissed as both old-fashioned and wrong headed. (In modern hindsight, this claim provides a particularly compelling example of how hardened adaptationism can suppress interesting questions -- for such homologues have now been found in abundance. Their discovery ranks as one of the most important events in modern evolutionary science -- see Chapter 10, p. 1092, where we will revisit this particular Mayrian claim): "In the early days of Mendalism there was much search for homologous genes that would account for such similarities. Much that has been learned about gene physiology makes it evident that the search for homologous genes is quite futile except in very close relatives" (1963 [Animal Species and Evolution, Cambridge MA: Havard University Press], p. 609). (Gould 2002: 539)

(....) All potential anomalies yield to a more complex selectionist scenario, often presented as a "just-so-story." Why did the crown height of molars increase slowly, if hypsodontry became so advantageous once horses shifted to vegetational regimes of newly-evolved grasses with high silica content? Mayr devises a story -- sensible, though empirically wrong in this case -- and regards such a hypothetical claim for plausibility as an adequate reason to affirm a selectionist cause. (The average increase may have been as small as the figure cited by Mayr, but horses did not change in anagenetic continuity at constant rates. Horses probably evolved predominantly by punctuated equilibrium -- see Prothero and Shubin, 1989). The average of a millimeter per million years represents a meaningless amalgam of geological moments of rapid change during speciation mixed with long periods of stasis: "An increase in tooth length (hysodontry) was a selective advantage to primitive horses shifting from browsing to grazing in an increasingly arid environment. However, such a change in feeding habits required a larger jaw and stronger jaw muscle, hence a bigger and heavier skull supported by heavier neck muscles, as well as shifts in the intestinal tract. Too rapid an increase in tooth length was consequently opposed by selection, and indeed the increase averaged only about 1 millimeter per million years" (1963, p. 238) (Gould 2002: 540)

(….) [T]he synthesis can no longer assert full sufficiency to explain evolution at all scales…. I advance this opinion only with respect to a particular, but … quite authoritative, definition of the synthesis…. The definition begins Mayr’s chapter on "species and transspecific evolution" from his 1963 classic… Mayr wrote …: "The proponents of the synthetic theory maintain that all evolution is due to the accumulation of small genetic changes, guided by natural selection, and that transspecific evolution is nothing but an extrapolation and magnification of the events that take place within populations and species." (Gould 2002: 1003)

(....) The discovery [evo-devo] that has so discombobulated the confident expectations of orthodox theory can be stated briefly and baldly: the extensive "deep homology" now documented in both the genetic structure and developmental architecture of phyla separated at least since the Cambrian explosion (ca. 530 million years ago) should not, and cannot, exist under conventional concepts of natural selection as the dominant cause of evolutionary change. (Gould 2002: 1065)

(....) Darwinian biology attributes the origin of shared homologous characters to ordinary adaptation by natural selection in a common ancestor. Moreover, homologous characters not only continue to express their adaptive origin, but also remain fully subject to further adpative change -- even to the point of losing their ready identity as homologies -- if they become inadaptive in the environment of any descendent lineage. Homological similarity in related taxa living in different environments therefore indicates a lack of selective pressure for alteration, not a limitation upon the power of selection to generate such changes. (At the Chicago Macroevolution meeting in 1980, for example, Maynard Smith acknowledged the allometric basis of many homologies, but stated that the attribution of such similarity to "developmental constraint" would represent what he proposed to christen as the "Gould-Lewontin fallacy" -- for natural selection can unlock any inherited developmental correlation if adaptation to immediate environment favors such an alteration.) (Gould 2002: 1065-1066)

(....) Second, homological holds must be limited in taxonomic and structural extent to close relatives of similar Bauplan and functional design. The basic architectural building blocks of life -- the DNA code, or the biomolecular structure of fundamental organic compounds for example -- may be widely shared by homology among phyla. But the particular blueprints of actual designs and the pathways of their construction ... must be limited to clades of closer relationship. (Gould 2002: 1066)

(....) Any wider hold of homology [which has already occurred] would have to inspire suspicions that the central tenet of orthodox Darwinism can no longer be sustained: the control of rates and directions of evolutionary change by the functional force of natural selection. In a particularly revealing quote within the greatest summary document of the Modern Synthesis, for example, Mayr ... formulated the issue in a forthright manner. After all, he argued, more than 500 million years of independent evolution must erase any extensive genetic homology among phyla if natural selection holds such power to generate favorable change [novelty]. Adaptive evolution, over these long intervals, must have crafted and recrafted every genetic locus, indeed every nucleotide position, time and time again to meet the constantly changing selective requirements of continually varying environments. At this degree of cladistic separation, any independently evolved phenotypic similarity in basic adaptive architecture must represent the selective power of separate shaping by convergence, and cannot record conserved influence of retained genetic sequences, or common generation by parallelism: "In the early days of Mendelism there was much search for homologous genes that would account for such similarities. Much that has been learned about gene physiology makes it evident that the search for homologous genes is quite futile except in very close relatives." (Gould 2002: 1066)

But we now know that extensive genetic homology for fundamental features of development does hold across the most disparate animal phyla. For an orthodox Darwinian functionalist, only one fallback position remains viable in this new and undeniable light... One can admit the high frequency and great importance of such genetic constraints (and also designate their discovery as stunningly unexpected), while continuing to claim that natural selection holds exclusive sway over evolutionary change because deep homology only imposes limits upon styles and ranges of developmental pathways, but cannot power any particular phyletic alteration. Natural selection can still reign supreme as the pool cue of actual evolutionary motion. (Gould 2002: 1066-1067)

But a formalist defender of positive constraint will reply that such unanticipated deep homology also channels change in positive ways -- and that the key to this central argument resides in an old distinction that, unfortunately, cannot be matched for both conceptual and terminological confusion, and for consequent failure of most evolutionists to engage in the issue seriously: namely, the differences in causal meaning (not just in geometric pattern) between parallelism and convergence. (Gould 2002: 1067)

(....) But -- and now we come to the nub of the issue, and to the central role of positive developmental constraint as a major challenge to selectionist orthodoxy -- the attribution of similar evolutionary changes in independent lineages to internal constraint of homologous genes and developmental pathways, and not only to an external impetus of common selective pressures, must be limited to very close relatives still capable of maintaining substantial genetic identity as a consequence of recent common ancestry. Mayr's characterization of selectionist orthodoxy comes again to mind: distantly related lineages cannot be subject to such internal limitation or channeling because the pervasive scrutiny and ruthless efficiency of natural selection, operating on every feature over countless generations in geological immensity, must have fractured any homological hold by underlying genes and developmental pathways over the freedom of phenotypes to follow wherever selection leads. (Gould 2002: 1067)

Darwin’s famous words, so often quoted, haunt the background of this discussion (1859, p. 84): "It may be said that natural selection is daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad, preserving and adding up all that is good; silently and insensibly working, whenever and wherever opportunity offers, at the improvement of each organic being in relation to its organic and inorganic conditions of life." (Gould 2002: 1068)

Therefore, an uncannily detailed phenotypic similarity evolved between distantly related groups must arise by convergence from substrates of non-homologous genotypes thus affirming our usual view of selection’s overarching power, especially if common function for the two similar forms can validate the hypothesis of generation within a comparable adaptational matrix. (Note the logical danger of circularity that intrudes upon the argument at this point, for this extent of detailed similarity the very datum that, in an unbiased approach, would lead one to entertain parallelism based upon common internal constraint as a viable alternative to convergence based on similar adaptive needs now becomes an a priori affirmation of selection’s power, the hypothesis supposedly under test.) (Gould 2002: 1068)

For this reason, such detailed functional and structural similarities, evolved independently in distantly related lineages, have become "poster boy" examples of convergence itself the "poster boy" phenomenon and general concept for showcasing selection’s dominant sway precisely because similarities evolved in this mode cannot, by Mayr’s argument, be ascribed to parallelism based on positive constraint imposed by homologous genetic and developmental pathways. With internal channeling thus theoretically barred as a potential source of impressive similarity, convergence becomes the favored explanation by default. The argument, surely "tight" in logic and principle, seems incontrovertible. (Gould 2002: 1068)

(....) [O]ne of the major discoveries of evo-devo has revealed a deep genetic homology underlying and promoting the separate evolution of lens eyes in cephalopods and vertebrates.... oth phyla share key underlying genes and developmental pathways as homologies, and the example [of 'convergence'] has lost its former status as the principle textbook case of natural selection's power to craft stunning similarities from utterly disparate raw materials. (Gould 2002: 1069)

With this "one liner" of maximal force evo-devo has reinterpreted several textbook examples of convergence as consequences of substantial parallelism we can encapsulate the depth of theoretical disturbance introduced by this subject into the heart of Darwinian theory. Our former best exaples of full efficacy for the functional force of natural selection only exist because internal constraints of homologous genes and developmental pathways have kept fruitful channels of change open and parallel, even in the most disparate and most genealogically distant bilaterian phyla. The homological hold of historical constraint channels change at all levels, even for the broadest patterning of morphospace, and not only for details of parallel evolution in very closely related groups. (Gould 2002: 1069)

(....) [P]arallelism marks the formal influence of internal constraint, while convergence reflects the functional operation of natural selection upon two substrates different enough to exclude internal factors as influences upon the resulting similarity. This recognition of internal channeling as the root cause of parallelism -- the principle basis for ascribing evolutionary change, and not only limitation, to historical constraint -- lies at the heart of evo-devo's theoretical novelty and importance to the Darwinian worldview. (Gould 2002: 1075)

* * *

I began this "symphony" of evo-devo with a quotation from one of the great architects of the Modern Synthesis -- Mayr's statement, based on adaptationist premises then both reasonable and conventional, that any search for genetic homology between distantly-related animal phyla would be doomed a priori and in theory by selection's controlling power, a mechanism that would surely recycle every nucleotide position (often several times) during so long a period of independent evolution between two lines. The new data of evo-devo have falsified this claim and revised our basic theory to admit a great, and often controlling, power for historical constraints based on conserved developmental patterns coed by the very genetic homologies that Mayr had deemed impossible. (Gould 2002: 1175)

* * *

(....) The argument that structural and morphological archetypes underlie, and actively generate, a basic and common architecture in taxonomically distant groups defines -- both as a fact of our profession's actual history and as a dictate of the logic of our explanatory theories -- the strongest kind of claim for developmental constraint as a major factor in patterns of evolutionary change and the occupation of morphospace. I suspect that the depth of this challenge has always been recognized, but the empirical case for such constraining archetypes has remained weak, since the heyday of Geoffroy and Own some 150 years ago, that the issue simply didn't generate much serious concern -- and rightly so.

The concept of interphylum archetypes, deemed too bizarre to warrant active refutation, experienced the curt and derisive dismissal reserved for crackpot ideas in science. (Goldschmidt's saltational apostasy, on the other hand, inspired voluminous and impassioned denial because his ideas did seem sufficiently and dangerously plausible to the Modern Synthesis -- see pp. 451-466). Indeed, the notion of interphylum archetypes struck most biologists as so inconceivable in theory that empirical counterclaims hardly seemed necessary. After all, the notion required extensive genetic homology among phyla, and the power of natural selection, working on different paths for minimum of 530 million years since the origin of distinct phyla in the Cambrian explosion, seemed to guarantee such thorough change at effectively every nucleotide position that the requisite common foundation could not possibly have been maintained (see Mayr, 1963, p. 609, as previously discussed on pp. 539 and 1066).

-- Gould, Stephen J. The Structure of Evolutionary Theory. Cambridge: Harvard University Press; 2002; p. 539; 540; 1003; 1065-1069; 1075.