Posted 07 October 2006 - 06:07 PM
Stuff about Darwinism and comments by noted bioloigists
AN EPISTEMOLOGICAL BLIND SPOT: DARWIN’S ASSERTION OF GRADUALISM COUNTERED BY THE PROBLEM OF THE GAPS IN THE FOSSIL RECORD
One of Lonergan’s criticisms of Darwin is that his notion of scientific explanation depends on a perceptual rather than an intellectual epistemology. So, his notion of the basic unit as the species (even if changing all the time), rather than the species in interaction with the environment, and his focusing on a gradual accretion of minor changes as an insight into macroevolutionary development (that is, development from one species to another, rather than minor changes within a species), seem to depend on a notion of scientific knowledge as taking a good look at what’s happening rather than employing a notion of explanation in terms of correlations between the data: for relations between various species over time, there’s no question of looking for visible, gradual changes.
Voegelin, in his 1964 Hitler and the Germans lectures (144f), comments on this:
So the expression, ‘survival of the fittest,’ which he willingly uses in place of his earlier expression ‘natural selection’ (in contrast to artificial selection by man), comes from Spencer and the context of English early liberalism. It thus contains precisely this social element that is later asserted to have been illegitimately pinned on Darwin. Darwin continues:
[M]an by selection can certainly produce great results...But Natural Selection...is a power incessantly ready for action, and is as immeasurably superior to man’s feeble efforts, as the works of Nature are to those of Art. (OS, 77)
And now, how does nature carry out its selection?
A struggle for existence inevitably follows from the high rate at which all organic beings tend to increase... It is the doctrine of Malthus applied with manifold force to the whole animal and vegetable kingdoms. (OS, 79)
That is…in order to strengthen his theory of evolution, Darwin uses the categories of English early liberalism from the politics of Spencer and of Malthus. He hasn’t observed anything at all. Indeed, after he developed his theory, he had to make yet another great restriction. In the evolutionary theories of the Darwinian type (there are also other evolutionary theories—there should be no doubt about evolution—but now we are speaking about the specifically Darwinian evolutionary theory) it is established, on the basis of observation of plant and animal breeding, that there are what later were called mutations, slight changes, producing new varieties within a species. Darwin now extrapolates from this, saying that also the species that are clearly differentiated, have changed from one to the other through similar little steps, like those one observes in artificial and natural breeding within the species. But from this it would follow that between the species recognizable today—which are rather far away from each other—there must be an unlimited profusion of infinitesimal transitions, if not from one to another, then from the different species available at present back to common original species from which they have developed. And Darwin now makes this supposition too. Evolution took place through this infinitely small change of variations, from which the species then arose. But the question then arises: Why then do we have only these clearly distinguishable species, and what has happened to all these links in between?
And Darwin…replies now to this, in chapter 10, entitled ‘On the Imperfection of the Geological Record.’ He speaks of the principal objection to his theory as:
the distinctness of specific forms, and their not being blended together by innumerable transitional links...Geology assuredly does not reveal any such finely graduated organic chain; and this, perhaps, is the most obvious and serious objection which can be urged against the theory. The explanation [which all these missing links, that must have been innumerable, does not produce] lies, as I believe, in the extreme imperfection of the geological record. (OS, 412-3)
…Precisely this part of the theory, which for Darwin is the most decisive, has no empirical basis, and has had none up to now. Rather, this part of the theory is an application to geological phenomena of the liberal politics of competition and the selection of the best in competition with each other, without a trace of an empirical observation.
Long after the 1859 publication of the Origin, Darwin continued to be aware of particular gaps in the fossil record.
For example, he knew of a difficulty about 120m years ago with the sudden appearance of the angiosperms (the flowering plants: flowers, cereals, grasses, all trees from beech to palm, except conifers, most vegetables). The angiosperms have no ancestors in the fossil record, nor can any intermediate links be traced between them and the conifers or gymnosperms, which they largely replaced. Darwin wrote in an 1881 letter to Hooker: ‘Nothing is more extraordinary in the history of the Vegetable Kingdom, as it seems to me, than the apparently very sudden or abrupt development of the higher plants.’
Another type of difficulty has to do with the assertion that sophisticated physiological organs have occurred gradually over time. Darwin himself was also fully aware of this problem, writing to Asa Gray in 1861: ‘The eye to this day gives me a cold shudder.’ (Quoted in Denton, Evolution, 326) It is precisely these difficulties that have been raised within paleontology and biology especially since the early 1970s.
What we’re saying here does not disparage Darwin’s tremendous capacity for detailed observation, his lifetime of collecting specimens, experimentation with plants, and small animals, and so on. And, as Lonergan noted, in fact Darwin’s notion of chance variation can be seen as an instance of what we called the principle of material emergence.
Still, there are a number of basic biological difficulties with the theory at the level of empirical data. Thomas Huxley pointed out one of the main problems to Darwin. Darwin had written (quoted in Johnson, Darwin on Trial, 33):
‘[S]o will natural selection, if it be a true principle, banish the belief of the continued creation of new organic beings, or of any great and sudden modification in their structure.’
Huxley warned Darwin that, ‘You have loaded yourself with an unnecessary difficulty in adopting natura non facit saltum [nature does not make jumps] so unreservedly.’ (Johnson, 33)
Darwin should have differentiated philosophical questions involving efficient causality (or extrinsic causation) from the kind of interrelational explanation (or intrinsic causation) proper to the natural sciences. If he had done so, what seems most original in his theory, its magnificent attempt to deal at the level of an explanatory biology with the unity and diversity of all living reality, would not have run into so many problems.
DARWIN COULD EXPLAIN ‘MICRO-EVOLUTION’ BUT NOT ‘MACRO-EVOLUTION’
As a fairly typical example of dissent from within biology itself, Denton quotes Steven Stanley’s Macroevolution:
The known fossil record fails to document a single example of phyletic (gradual) evolution accomplishing a major morphological transition and hence offers no evidence that the gradualistic model can be valid. (Quoted in Denton, 164)
And Darwinian evolutionist Stephen Gould berates what he calls ‘the false iconography of the march of progress.’ (Gould, Wonderful Life, 31) He criticizes not only what he calls ‘the great warhorse of tradition’—the familiar illustration of the various stages in the evolution of the horse—as quite fallacious, but the even more widespread iconography of human evolution, from monkey through apes through hominids to man. (Gould, Wonderful Life, 37; 27–36)
While few paleontologists would say that there are no examples in the fossil record of gradual sequences that could be used as evidence for Darwinism, nonetheless, within the Darwinian fold itself, the general lack of fossil evidence has been pointed out rather bluntly, for example, by Niles Eldredge, in his Reinventing Darwin: The Great Evolutionary Debate, 3; 94–95:
[I]n the 1960s...I tried to document examples of the kind of slow, steady, directional change we all thought ought to be there, ever since Darwin told us that natural selection should leave precisely such a telltale signal as we collect our fossils up cliff faces. I found instead that once species appear in the fossil record, they tend not to change very much at all. Species remain imperturbably...resistant to change as a matter of course—often for millions of years...species are fundamentally stable entities—a phenomenon that Stephen Jay Gould and I dubbed ‘stasis’ in our 1972 paper elaborating the evolutionary notion of ‘punctuated equilibria.’
[N]ew species...tend to show up abruptly in the fossil record as the overwhelming rule...No wonder paleontologists shied away from evolution for so long. It seems never to happen...When we do see the introduction of evolutionary novelty, it usually shows up with a bang, and often with no firm evidence that the organisms did not evolve elsewhere!
Niles Eldredge’s earlier comment on the behaviour of palaeontologists since the late 1940s is instructive:
We have proffered a collective tacit acceptance of the story of gradual adaptive change, a story that strengthened and became even more entrenched as the [neo-Darwinian] synthesis took hold. We palaeontologists have said that the history of life supports this interpretation, all the while really knowing that it does not. [Time Frames: The Rethinking of Darwinian Evolution and the Theory of Punctuated Equilibria, 144.]
Robert Augros and George Stanciu, who have quoted Eldredge, add that ‘Something is gravely wrong with a theory that forces us to deny or ignore the data of an entire science.’ [The New Biology, 175.]
What I think is an ideological rather than scientific attempt to tailor the data to a scientistic rather than scientific theory can be found in the attempt to prevent questions of existence arising by what we can call the Great Wall of Gradualism.
We’re ruling out here that a sequence where minor modifications of a Darwinian type occur could gradually bring about major ones, which has been characterized as micro-evolution. But it’s helpful to distinguish micro-evolution from macro-evolution—which is what Darwin considered he was explaining:
There is no question that evolution of the Darwinian kind occurs, in the sense that types of living organisms have a certain capacity for variation. This is a process commonly called microevolution, and it accounts for such things as the variant characteristics of plants and animals that have been transported to an isolated island environment. The problem is that there is no evidence for, and very much evidence against, the Darwinian assumption that some similar process of step-by-step gradual change produced the basic body plans of plants and animals in the first place or brought about the existence of complex organs like wings and eyes. (Phillip Johnson, Reason in the Balance,14).
Johnson is referring to changes in finch beaks in the Galapagos Islands described by Peter and Rosemary Grant in The Beak of the Finch: A Story of Evolution in Our Time, 1994, where finch beaks grew about 5% longer after the drought year of 1977, which were better shaped for opening the last tough seeds remaining. Then, after the floods of 1983, the first postflood generation of finches again had smaller beaks, more suited to the many tiny seeds that had become available—so a cycle of beak size, from smaller to larger and back to smaller, due to environmental changes, was observed.
Johnson also comments on the effect of industrial melanism in the peppered moth: H. B. D. Kettlewell observed in 1959 how the moth’s wings changed from light-coloured to dark to provide the species with better camouflage from birds when it found itself in industrially polluted areas: ‘The example does not illustrate moths in the process of changing to something else, or even changing in color. It illustrates an essentially stable population that can vary cyclically to adjust to conditions.’ (Ibid., 73n) (Unfortunately, Kettlewell’s observations, frequently put forward as a convincing argument for evolution in general, including macro-evolution, have been shown to be seriously flawed, if not downright fraudulent: cf. Judith Hooper’s 2002 investigation, Of Moths and Men: Intrigue, Tragedy & the Peppered Moth.)
Later, we can ask ourselves what all the fuss was about, since we’ll be agreeing with another group of molecular biologists from the 1990s on who seem to have indicated how macro-mutations can occur without any gradual processes. It’s just that it seems as if, to prevent questions that a philosopher would see to be questions about existence from being raised at all, Darwin tried with all his might not to allow any ‘chink’ in the armour of gradualism.
ALTERNATIVES TO A GRADUALIST THEORY OF EVOLUTION:
i) Berg-Goldschmidt-Groves
Australian primatologist and evolutionary theorist Colin P. Groves, in his A Theory of Human and Primate Evolution sees ‘neoteny’ as the ‘law’ behind evolution. He quotes Stephen Jay Gould as writing that ‘neoteny’ [the persistence of some infant- or childlike-characteristics in later development] requires that ‘aspects of shape are retarded with respect to the rest of development.’
The vast potential for evolutionary change opened up by heterochrony [that is, development of different parts of an organism at varying speeds] has perhaps not been appreciated by most biologists; in the last analysis, it seems possible to ascribe a major proportion of evolutionary change to changes in rates of development. Extreme dissociation of developmental rates of different organ systems is what the Krassilov Effect is all about. I have no intention of insisting that “heterochrony” is a synonym for “evolution.” But I would like to stress the uninvestigated explanatory potential embodied in the concept. (Groves, 57–8)
Groves: ‘The first of the “complete theories” of evolution developed in modern times was the “nomogenesis” of Berg (Lev S. Berg, Nomogenesis or evolution determined by law, (1922) tr. J. N. Rostovson, MIT Press, Cambridge, Mass., 1969]). ‘…Berg’s consideration of the Darwinian model led him to the conclusion that it was incompatible, at least as the major mechanism, with what he knew of the pattern of living organisms and their evolution. His conclusions are as follows (1969, 406–7):
1. Evolution is based not on chance variations but on laws (hence the title “nomogenesis” [genesis or development according to law, Greek nomos])
4. Evolution proceeds not by slow continuous changes but by jumps.
5. Hereditary variations (mutations) are restricted in number and are directional, not numerous and random.
6. Natural selection is primarily an agency of conservation, not of change.
7. New species arise by “mutations” (meaning sharply distinct changes).
8. Much of evolution is the unfolding of pre-existing rudiments, rather than the formation of new characters as such.’ (Groves, 60)
And Michael Denton, in his Nature’s Destiny: How the Laws of Biology reveal Purpose in the Universe (1998) notes that Berg ‘believed that the variation of characters in an evolutionary lineage is confined within certain limits, that it follows a “definite course, like an electric current moving along a wire.”’ Denton continues, quoting examples of nomogenesis given by Berg: ‘[H]e cited the gradual ossification [bone-growth] of the vertebral [spinal] column, a reduction in the number of bones in the skull, and the transformation of a two-chambered heart into a three- and four-chambered organ associated with a corresponding increase in the complexity of the circulatory system.’ (273)
Colin Groves also mentions the work of the German zoologist Richard Goldschmidt, which while much criticized in the 1940s, has been rediscovered as relevant in the light of continual difficulties with Darwinian gradualism:
Goldschmidt could not believe that natural selection, with all its slowness and the slightness of its effects, could produce anything but microevolutionary changes. In particular, he found it difficult to accept that macroevolutionary changes are simply an accumulation of microevolutionary changes as required by the [neo-Darwinian] synthesis [of Darwin’s theory with genetics]. From time to time, he proposed, a major mutation occurs producing a radically different form, which must then be tested against pre-existing forms.
In many respects, the punctuated equilibrium model is a simple resurrection and elaboration of the hopeful monster [=Goldschmidt’s] theory. (Groves, 62)
Groves thinks that, while more is needed for a complete theory of evolution, ‘the accumulating evidence does suggest that some version of the Goldschmidt theory is by now inescapable.’ (Groves, 62)
‘Gould’s interim model [see below on punctuated equilibrium], which is really all that it’s intended to be, is largely descriptive, with little attempt to elucidate mechanisms…it needs to be elaborated by some attempt to explain processes, and certainly it badly needs the concept of directionality. That is why the most satisfactory evolutionary model is, I believe, best described by Berg’s term, nomogenesis.’ (Groves, 62–3)
Simon Conway-Morris, in his 2003 Life’s Solution: Inevitable Humans in a Lonely Universe, quotes Denton & Marshall: ‘underlying all the diversity of life is a finite set of natural forms that will recur over and over again anywhere in the cosmos where there is carbon-based life.’ (11). And Denton, Marshall and Legge in their essay on ‘The Protein Folds as Platonic Forms: New Support for the Pre-Darwinian Conception of Evolution by Natural Law,’ (2002) suggest that ‘natural law may have played a far greater role in the origin and evolution of life than is currently assumed.’ (325) This is on the basis of the extremely limited number of protein folds (‘the basic building blocks of proteins and therefore of the cell…’ (330)
Again and again they question the priority of function over form in modern evolutionary thinking. They refer to Aristotle’s Parts of Animals, ‘in the famous analogy, where he envisages the pre-existing plan [or form] of a house acting as an “attractor” molding the material constituents, the bricks and stones, during the building of the house into conformity with its pre-existing plan: “Now the order of things in the process of formation is the reverse of their real and essential order…bricks and stone come chronologically before the house…but logically the real essence and the Form of the thing [the pre-existing plan of the house or the pre-existing Form of the protein fold] comes first.”’ (333)
Further, Conway-Morris notes how biologists’ use of ‘surprising,’ ‘amazing,’ etc. regarding convergences between different organisms—means they ‘sense the ghost of teleology looking over their shoulders.’ For example, mosquito hearing by amplification is amazingly equivalent to vertebrates, 191, with a deeper auditory similarity called transduction [conversion of a mechanical force, for example, sound, into electrical signals, in quite different phyla [major animal groups], insects and vertebrates, 193.
He discusses ant and bee capacities, quoting an author on army ant colonies where we see ‘the emergence of flexible problem-solving far exceeding the capacity of the individual’ and presupposing effective communication, so that ‘intelligence, natural or artificial, is an emergent property of collective communication…This is exactly what happens when army ants pass information from individual to individual through the “writing” and “reading” of symbols, often in the form of chemical messengers or trail pheromones, which act as stimuIi for changing behavior patterns.’ 203
Again, Conway-Morris notes (403nn43–44) the convergent evolutionary emergence of eusocial [that is, in favour of the whole society] behavior in different types of insects, ants, wasps, stingless and other bees, and sees it as expression of inevitable constraints [including climate change] on the evolution process. He also notes how hummingbirds, songbirds and parrots have evolved different solutions to the same problem of vocal learning.
Conway-Morris makes an important concession on ‘inevitable’: ‘…evolution is labile, it does show reversals, but the point still remains that the emergence of various biological properties is in response to adaptation and is governed by selection…Convergence simply tells us that the evolution of various biological properties is certainly highly probable, and in many cases highly predictable.’ LS, 223.
ii) Eldredge-Gould: Punctuated Equilibrium
While it’s not possible here to go into the dramatic shift from non-perceptual life to life which is perceptually organized, it’s no secret that the gaps in the fossil record Darwin presumed would soon be filled have remained. As a result, Niles Eldredge and Stephen Jay Gould in 1972 proposed what they saw was an important modification to gradualist Darwinian evolution, in their ground-breaking article, ‘Punctuated Equilibria: An alternative to Phyletic Gradualism.’ [Reprinted in Niles Eldredge, Time Frames: The Rethinking of Darwinian Evolution and the Theory of Punctuated Equilibria, 193–223]
Succinctly, Eldredge explains:
[I]f evolutionary change doesn’t simply accumulate over the course of time, the question becomes, When and under what conditions does evolutionary change occur?...new species...tend to show up abruptly in the fossil record as the overwhelming rule...Punctuated equilibria is a combination of empirical pattern (stasis interrupted by brief bursts of evolutionary change) coupled with preexisting biological theory. [Reinventing Darwin: The Great Evolutionary Debate, 94, 104.]
The supporters of classical Darwinism, represented most vocally by Richard Dawkins in England and Daniel Dennett in America, regarded punctuated equilibrium as less a revision than a heretical betrayal of Darwin. Eldredge and Gould were subjected to vitriolic attack, even though Dawkins was quite aware that they wished to be regarded as Darwinian biologists.
Simon Conway Morris’s approach in Life’s Solution is not too far from Lonergan’s—who understands biological and zoological species as successful answers to certain questions of survival in a given habitat. Conway Morris too sees evolutionary changes as varying solutions to the problem of living in a given environment. Writing, for example, of different plant ‘solutions’ to surviving in different desert environments, he notes that: ‘there may be more than one adaptive solution to a problem’ 365n138. He provides interesting data on cross-phyletic mechanisms—where vertebrates, insects and mollusks (slugs, 390n215)—have the same odor mechanisms (389n208). For Conway Morris, these have evolved separately, since the organisms preceding them lack these mechanisms. They also have the same underlying molecular background. In addition, he writes about the similar molecular pathways underlying both visual and smell mechanisms (indicating what a philosopher would see as a shared material basis for Aristotle’s (180–81), the common or organizing perceptual integration of the various sensory systems in animals. Conway Morris also notes the same capacity for echo-location in bats, dolphins and some birds, 181–82.
DESPITE THESE PERSPECTIVES ON THE COSMOLOGICAL-BIOLOGICAL WHOLE, THERE REMAIN QUESTIONS ABOUT THE ORIGINS OF LIFE
Conway Morris, in Life’s Solution, observes that ‘how inanimate became animate has proved stubbornly recalcitrant’ (xiv); still, in a very short time—maybe as little as 200m yrs, what could be the best possible genetic code emerges on earth. He is immensely sceptical regarding hypotheses about the origin of life. At the same time, and perhaps this is a contradiction, Conway Morris is convinced that life must have begun through a step-like process, (53), although he rejects Graham Cairns-Smith’s hypothesis on clay as the origin of life as without the reality check of experimentation. (342n33) It’s beyond my competence to pronounce on this matter, still, some biologists come close to denying any gradual emergence of life from non-living material. For example, in 1953, the same year as Stanley Miller and Harold Urey tried to produce life experimentally, James Watson’s and Francis Crick’s discovery of the role of the DNA molecule in all living things indicated an extraordinary complexity in living cells, making a chance emergence of living organisms from chemicals appear less likely. Nobel prizewinning biologist Jacques Monod remarked that:
the simplest cells available to us for study have nothing ‘primitive’ about them....the major problem is the origin of the genetic code and of its transitional mechanism. Indeed it is not so much a ‘problem’ as a veritable enigma. The code is meaningless unless translated. The modern cell’s translating machinery consists of at least fifty macro-molecular components which are themselves coded in DNA: the code cannot be translated except by products of translation. It is the modern expression of omne vivum ex ovo [all that’s living comes from an egg]. When and how did this circle become closed? It is exceedlingly difficult to imagine. [Jacques Monod, Chance and Necessity: An Essay on the Natural Philosophy of Modern Biology, 134f.] [Author’s emphasis].
And Francis Crick, co-discoverer of the structure of DNA noted that:
An honest man, armed with all the knowledge available to us now, could only state that in some sense, the origin of life appears at the moment to be almost a miracle, so many are the conditions which would have had to have been satisfied to get it going. [Quoted in: Michael Denton, Evolution: A Theory in Crisis, 268.]
Conway Morris in Life’s Solution, reflects that despite fact that most [scientists] presume there’s ‘unbroken continuity’ between prebiotic ‘soup, clay, clouds…’ and the origin of life, such a hypothesis presumes there’d be aeons of time for life to emerge—but ‘there probably were not.’ He goes on in chapter 5 to suggest that life’s emergence is very rare, or maybe unique to our Solar System.
Daniel Dennett quotes Stephen Gould somewhat dismissively for recognizing the intrinsic limitations of biology (even if those limitations can hardly be overcome by passing them on to chemistry):
Evolution, in fact, is not the study of origins at all. Even the more restricted (and scientifically permissible) question of life’s origin on our earth lies outside its domain. (This interesting problem, I suspect, falls primarily within the purview of chemistry and the physics of self-organizing systems.) Evolution studies the pathways and mechanisms of organic change following the origin of life. (in Darwin’s Dangerous Idea, 310)
Few biologists, in fact, are more aware of the contingency of their subject-matter than Gould—the final chapter of Wonderful Life is a reflection on the contingency of the biological world. Yet he does not seem inclined to move beyond contingency. But contingent existence is existence by definition requiring an existing, non-contingent ground. And while the heat in Dennett’s attack on Gould’s ‘punctuated equilibrium’ revision conveys something of the inquisitor’s fire, as if Gould had been guilty of lèse majesté for daring to criticize Darwin’s theory at all, what’s even more striking is that neither Dennett nor Gould seem to advert to the philosophical question of existence as such. (Ibid., 282–312. Gould’s own riposte to what he calls the ‘fundamentalism’ of Dawkins and Dennett is to be found in his essays, ‘Darwinian Fundamentalism’ (New York Review of Books, June 12, 34–37; ‘Evolution: Pleasures of Pluralism’ (NYRB, June 26, 47–52.)
Since Gould has been fiercely attacked by those he would be driven to call ‘Darwinian Fundamentalists,’ the reason why a gradualist as opposed to a saltatory approach to evolution was adopted, is perhaps, as Thomas Nagel seems to think, due to a ressentiment not only towards a first cause, but towards philosophy. Such a ressentiment ill serves biology as a natural science:
My guess is that this cosmic authority problem is not a rare condition and that it is responsible for much of the scientism and reductionism of our time. One of the tendencies it supports is the ludicrous overuse of evolutionary biology to explain everything about life, including everything about the human mind. Darwin enabled modern secular culture to heave a collective sigh of relief, by apparently providing a way to eliminate purpose, meaning and design as fundamental features of the world. Instead they became epiphenomena, generated incidentally by a process that can be entirely explained by the operation of nonteleological laws of physics on the material of which we and our environments are all composed. There might still be thought to be a religious threat in the existence of the laws of physics themselves, and indeed the existence of anything at all—but it seems to be less alarming to most atheists. [The Last Word (1997), 131]
Voegelin too, in his Hitler and the Germans lectures (1999), 144f., commented on the non-observational core of Darwin’s evolutionary theory. Voegelin’s reference, from the viewpoint of the natural sciences, to the givenness of ‘the epiphany of structures in reality’ accompanied by a pneumopathological fear of the underlying mystery, may be one of the factors making any debate on biological or zoological emergence such a heated one in our culture.
However, as we’ll see, Neal C. Gillespie in his Charles Darwin and the Problem of Creation amply discusses the anti-intellectualism Darwin had to deal with in certain religious circles, along with his own internal religious struggle—a struggle perhaps more cultural than religious in fact. The whole issue of closure to scientific inquiry by a school of apologists in the name of a literalizing reading of Scripture makes one sympathetic to writers like Dennett’s and Dawkins’ exasperated desire to fend them off, however polemically..
iii) Evolutionary-Developmental (‘evo-devo’) Theory
The more recent breakthrough in the early ’90s, called ‘evolutionary-developmental’ or ‘evo-devo,’ seems in many ways to correspond at the molecular level to Eldredge and Gould’s punctuated equilibrium hypothesis, which in turn is supplemented by Berg and Groves’ insight into nomogenesis as underlying the macro-evolutionary shifts Eldredge and Gould were trying to deal with. Gould’s last great work, The Structure of Evolutionary Theory (Cambridge, Mass.: Harvard University Press, 2002) is a massive attempt to marry both his revisionary Darwinism with evo-devo.
The basic discovery, made in the early 1990s, was that the sudden emergence of the 35 phyla or major zoological groups (chordates, crustaceans, mollusks, etc.), around 550m years ago showed a common deep genetic structure. Each phylum had the same genetic instructions for its top/bottom axis, front/back polarity, head, and sensory organs. To get some of the flavor of what evo-devo involves, I’ll quote from Raff’s The Shape of Life:
Higher taxonomic groups, most notably phyla, possess suites of anatomical features that distinguish them from other groups. Such an underlying anatomical arrangement is called a body plan…no new phyla appear to have originated since the Cambrian….(xiv)
Wallace Arthur in his The Origin of Animal Body Plans: A Study in Evolutionary Developmental Biology (Cambridge: Cambridge University Press, 2000), 81], gives his opinion that:
there was no multicellular animal life prior to 600 my ago; there was an explosion of body plans in Ediacaran times, with many becoming extinct, and a second body-plan explosion in the early Cambrian; evolution in Vendian and Cambrian times was much more ‘experimental’ than it is now; and internal factors such as developmental constraint (or early lack of it) are important in evolution as well as considerations about niche space and external adaptation.
So it’s no surprise that Sean Carroll discusses this Cambrian event—in one of its most famous expressions, the Burgess Shale in Alberta, of 510m years ago (along with the related Ediacaran in South Australia [545m years ago] and the recently discovered Chengjian fauna in China [520 m years ago]) in chapter 6 of his Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom (New York: Norton, 2005)—as ‘The Big Bang of Animal Evolution.’ He asks, ‘What ignited the Explosion? Why did large and complex animals first appear at this time? Why did these particular forms succeed?’ (138)
What’s amazing are the jellyfish, or cnidarians, belonging to a 36th phylum which may have originated with the first Ediacaran fauna originating 50m years earlier without the two-sided body-plans of the other 35 phyla: although they lack a brain, they still seem to have the same genetic plan for eyes that they share with the other phyla.
The most primitive animals with eyes are the cnidarians. Some have simple eyes lacking lenses, but other medusae [type of jellyfish] have well-developed eyes on the edges of their bells. The Cubomedusae (box jellies), whose highly toxic stings are such a notorious threat to swimmers on Australia’s north coast, have up to 24 eyes that are linked to the nerve net and enable them to orient accurately in light. These eyes are complex, with an epidermal cornea, a spherical lens, a multilayered retina, and a region of nerve fibers. There are about 1,000 sensory cells in each eye. As [these jellyfish] are the deepest [=earliest] branch of the metazoan tree, the complexity of their eyes is surprising. If cnidarians were indeed part of the Ediacaran fauna, it suggests that eyes long predate the Cambrian radiation of bilaterian animals. [Rudolf A. Raff, The Shape of Life: Genes, Development, and the Evolution of Animal Form (Chicago: University of Chicago Press, 1996), 376–77.
This early origin of eyes, or at least organs sensitive to light are mentioned by Simon Conway Morris, in Life’s Solution, 165f. He writes of eukaryotic visual sensation: ‘Despite the absence of a nervous system,’ its eye-spot is ‘strikingly convergent on the animal eye.’ So he argues for evolutionary co-option and redeployment of these pre-existing capacities rather than a sudden emergence of the eye. He points out that even bacteria have eye-spots, though this doesn’t mean sight, along with later protistan [mobile eukaryotic cells] eye spots. (171f; cf 381–82nn101–111) The time for the evolution from eye-spot to eye, would then be less than 1m yrs. (387n182)
Raff notes that:
‘If externally applied natural selection is the only force required to produce evolutionary change, then developmental processes don’t matter except as features upon which selection can act. If internal organization and processes govern modes of change, then development must be incorporated into any complete theory of evolution.’ (xviii) [We can suggest a melding of both approaches, with ‘external’ including the properly zoological in interaction with environment.]
‘The major concern of this book is to establish the connection between the developmental processes that produce body structure in each generation and the evolutionary processes that produce new anatomical features and novel animal body shapes.’
‘Because metazoans [earliest living multicellular animals] are monophyletic [belonging to a single phylum, or major group of animals], all animal body plans arose as a result of transformations of a single ancestral body plan. Further, because all bilaterian [= two-sided] animals share a common ancestor, they arose from a common bilaterian body plan. The interesting question then becomes whether there is a set of genetic rules that bilaterian animals share. If there is, the diverse body plans of bilaterian phyla have been built upon shared developmental genetic themes, which might constitute a conserved genetic body plan. Slack and co-workers have called this hypothetical Hox gene-centered genetic body plan for most animal phyla the “zootype”…. [Wallace Arthur has an important correction to this statement, in his The Origin of Animal Body Plans: ‘…neither the “zootype” of Slack et al. (1993) nor any other pattern of gene activity represents a body plan; rather such patterns may form part of the explanation of body plans.’ (28)]
‘The central problem for evolutionary biologists interested in development has been how morphology is transformed in evolution.’ (23) He writes that in 1933, Joseph Needham ‘suggested the idea of dissociability of elements of developmental machinery. He pointed out that it is possible experimentally to separate differentiation from growth or cell division, biochemical differentiation from morphogenesis, and some aspects of morphogenesis from one another. The implication of this idea is enormous: developmental processes could be dissociated in evolution to produce novel ontogenies out of existing processes, as long as an integrated developmental program and organismal function could be maintained.’ (23) [This seems close to what Lonergan has to say on the non-systematic that calls forth correspondence, and we’ve already mentioned heterochrony as a possible agent in evolution] Raff speaks of ‘heterochrony’ as ‘an evolutionary dissociation in timing.’ (24) But Raff warns against it becoming the dominant explanation at developmental level for evolutionary changes in morphology. (24)
‘Because we now have the ability to clone regulatory genes, purely developmental questions have quickly become issues in evolutionary biology as well. The most prominent of the gene domains that has been studied…is the homeobox. This is a sequence that encodes a 60-amino-acid-long protein domain rich in basic amino acids that has been shown…to fold into a helix-turn-helix structure that binds DNA in a highly specific manner. The homeobox was discovered…in homeotic genes of Drosophila. Homeoboxes are highly conserved in evolution and…were very quickly found…to be present in other phyla as well. The surprise was that although homeoboxes are associated with genes that control external body segmentation in Drosophila, they are also present in nonsegmented phyla such as echinoderms and nematodes, as well as in the vertebrates, which exhibit some internal segmental features. In all organisms in which they have been studied, homeobox-containing genes homologous to those of the homeotic genes of Drosophila function in some aspect of developmental specification of aspects of the body axis.’ (26)
‘The phylogenetic distribution of homeobox-containing genes indicates a pattern of gene duplication and divergence as well as profound conservation. It is also clear that the roles homeobox-containing genes play in development have undergone significant modification. Homeobox-containing genes in vertebrates establish axial polarity in the central nervous system, but do not set up epidermal segmental patterns as in insects. Additional roles can be shown for them in aspects of vertebrate development and co-option for new functions, such as in neural crest cell patterning…Other regulatory gene families, such as the steroid receptor family…show analogous patterns of evolutionary expansion and co-option to provide genetic raw material for regulatory innovations in the evolution of development. It has happened again and again in evolution within numerous families of regulators. [emergent probability]
The revelation of wide-ranging conservation and co-option of regulatory genes is a new and crucial one for two reasons. First, it presents us with the linkage between an evolutionary continuity of gene structure and the changing function of those genes in the evolution of development. Second, the underlying genomic similarity in the development of highly disparate animals reveals one of the things that makes the evolution of complex forms possible at all.
If each new species required the reinvention of control elements, there would not be time enough for much evolution at all, let alone the spectacularly rapid evolution of novel features observed in the phylogenetic record. There is a kind of tinkering at work, in which the same regulatory elements are recombined into new developmental machines. Dissociability of processes requires the dissociability of molecular components and their reassembly.’ (27)
‘In contemplating the rapid radiation of animal body plans during the Cambrian and their subsequent stability, both Gould and Jacobs have suggested that a form of internal genetic constraint may act as a brake on body plan evolution. They argue that an originally flexible developmental regulatory system allowed experimentation with very basic patterns of development. Once patterns were established, genetic regulatory system became rigidly fixed. After that, a powerful genetic constraint limited evolution to changes within existing body plans.’ (309
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