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Canadia spinosa one of many types of organism unique to the Cambrian

Punctuated Equilibrium: Darwin survives as the debate evolves

Scientific American Mar 90

In 1972 two paleontologists, Niles Eldredge of the American Museum lof Natural History and Stephen Jay Gould of Harvard University, startled-and in some cases dismayed biologists by suggesting that the view of evolution most of them held was an "insufficient picture." Eldredge and Gould maintained that life was no stately unfolding of gradually changing forms that slowly divided to create new species. Rather, they believed, species formed relatively quickly as a result of rapid bursts of evolutionary change. Eldredge and Gould thought their colleagues were wrong to blame the rarity of intermediate fossil sequences on gaps in the fossil record. Instead they proposed that such sequences were rare because evolution did not happen that way. Eldredge and Gould considered it more likely that new species evolve within a few thousand years, the mere blink of an eye in geologic time. Once a species has evolved, they argued, it enters a period of stasis, remaining unchanged for possibly millions of years. The two scientists called their theory punctuated equilibrium. Eldredge and Gould did more than offer a new picture of evolution's progress. They also suggested that Darwin's theory of natural selection acting on individuals was inadequate, because it predicted gradual evolution of species in response to environmental change rather than a "punctuated" pattern. In order to reconcile punctuated equilibrium with the long-term trends seen in fossil lineages-such as the progression from smaller to larger species of mammals-Eldredge and Gould therefore proposed a variant of Darwin's theory. They suggested that natural selection not only acts on the individual level but must also be acting at the species level, an idea since termed species selection According to this idea, lineages thrive if they either resist extinction or produce frequent daughter species. Gould even wrote that the "synthetic theory"-Darwin's theory of natural selection combined with genetics-was "effectively dead" as an explanation for long-term evolutionary trends. Not surprisingly, these ideas attracted attention. Popular accounts fostered the impression that Darwin's theory of evolution was wrong. Creationists gleefully played up the argument in their effort to reshape biology teaching, despite Eldredge and Gould's protests that their theory was not meant to question the basic tenets of evolution. How has punctuated equilibrium fared since it was first proposed almost 20 years ago? Many biologists say Eldredge and Gould attacked a straw theory: evolution has never been viewed as exclusively slow and gradual, they argue, and natural selection is quite able to account for the formation of species over thousands of years, as required by punctuated equilibrium. Ernst Mayr of Harvard University makes the point by quoting Darwin, who wrote in later editions of On the Origin of Species that "the periods during which species have been undergoing modification ... have probably been short in comparison with the periods during which these same species remained without undergoing any change." Mayr, who is often regarded as the dean of evolutionary biology, proposed in 1954 that "genetic revolutions" in small, isolated populations may give rise to new species, an idea that Eldredge and Gould have acknowledged as inspirational. Mayr says that Eldredge and Gould have sometimes made "outrageous claims,' but he believes they were correct to draw attention to stasis in fossil lineages. Mayr theorizes that such periods-some of which might last for tens of millons of years-are caused by genetic and developmental constraints that limit change. Jeffrey S. Levinton of the State University of New York at Stony Brook is one of the strongest critics of punctuated equilibrium. He maintains there are "literally scores" of good examples of gradual change. He says Eldredge and Gould remind him of a Scandinavian aphorism: "People come crashing through open doors." How, for example, can the theory be tested when the formation of species cannot be clearly recognized in the fossil record? Levinton concludes that "the totality of the evidence makes it a theory not worth following up." Antoni Hoffman of the Institute of Paleobiology in Warsaw, who has written a book aimed at refuting punctuated equflibriurn. accuses proponents of the theory of creating a moving target. According to Hoffman's analysis, some versions of the theory claim merely that evolution varies in speed-which is undisputecl Later versions claim that gradual change is nonexistent or negligible; that, Hoffman maintains, is "blatantly false.' In 1987 Peter R. Sheldon then at Trinity College in Dublin, seemed to score a direct hit against punctuated equilibrium when he reported finding that eight types of trilobite had each evolved gradually over a three-million year interval during the Ordovician period, more than 440 mimon years ago. The finding prompted John Maynard Sniith of the University of Sussex to comment that "we can forget about new paradigms and the death of neo-Darwinism." But the arguments persist and each side continues to collect evidence. For example, Adrian M. Lister of the University of Cambridge reported in Nature recently that red deer on the island of Jersey underwent a sixfold reduction in their body weight in less than 6,000 years, suggesting that rapid evolution can indeed occur under some conditions. Gould thinks Sheldon's interpretation of the trilobite record is weak, yet he maintains that paleontologists can discern the formation of new species in the fossil record through comparisons with living species. He admits that the evidence is not yet in and suggests that the technical difficulty of finding adequate data, in the form of well-preserved fossils from undisturbed sedimentary beds, has made for slow progress. While conceding that rapid change is not universal, Gould declares that he and Eldredge will be proved right if rapid species formation and stasis do tum out to dominate in the fossil record. Where does that leave species selection? Most evolutionists seem to agree with Hoffman that it cannot explain features of individuals, although it might in principle explain some of the long-term trends in evolution. Yet there are no proven examples, and the idea may be, as Hoffman puts it, "an explanation in search of phenomena to explain." Montgomery W. Slatkin of the University of Cahfomia at Berkeley argues that even if punctuated equilibrium does tum out to be common in evolution, it may nonetheless be driven by natural selection acting on individuals. Species selection has even been criticized by a scientist whom Gould lionizes in his latest book, Wonderful Life. Gould recounts the story of the Burgess Shale in British Columbia, which contains the remains of bizarre soft-bodied animals that appeared on the earth some S30 million years ago, soon after the first emergence of multicellular creatures at the start of the Cambrian period. Work in recent decades has shown that not only are the Burgess fossils surprisingly complex, but few of them fit into known taxonomic classes. One of the heroes of Gould's tale is Simon Conway Morris of the University of Cambridge, who painstakingly reconstructed some of the specimens. In a recent article in Science, Conway Morris writes that he sees no need for special evolutionary mechanisms (such as species selection) to explain the stunning diversity of the Burgess Shale. He adds that "there is no reason to think that any species did not arise by natural selection" Gould responds that he finds it hard to imagine natural selection plays no part in the formation of species, but he emphasizes the role of chance in creating the genetic rearrangements that may generate them. Indeed, the importance of chance is the main lesson Gould draws from the Burgess Shale fossils. He maintains it would have been impossible for any biologist to predict which of the myriad Burgess Shale animals would give rise to later groups: he sees the survival of species as a lottery controlled by historical contingency. On that point, Gould and his critics agree for once. The implications of a starring role for Lady Luck are profound. Were evolution's drama to be 'replayed," the earth's fauna and flora would be radically different, and the human species-that "improbable and fragile entity"-would presumably not exist. -Tim Beardsley

Arthur Koestler's Doppelgangers A Marsupial and placental jerboa, B Marsupial flying phalanger and placental flying squirrel C Skull of marsupial Tasmanian wolf compared to skull of placental wolf. (The Ghost in the Machine).

Darwin's Third Force: The Shape of Life New Scientist 22 Jun 96
Brian Goodwin is in the deparment of biology at the Open University, Milton Keynes.

F. SCOTT FITZGERALD believed that an "intelligent person is one who can hold two contradictory ideas in mind simultaneously". Using this criterion, Rudolf Raff scores highly in his book The Shape of Life. Data on evolutionary patterns has been flowing in at an accelerating rate from research as disparate as gene sequencing, interpretation of fossil records, and comparative embryology, and anyone trying to make sense of it needs to be able to cope with paradoxes. As Raff says: "An accounting for human evolution that began as ineluctable has ended up as ineffable.' Acceptance of apparent impasses on the way to future enlighterunent is a great strength of the book. The perspective that informs every aspect of this work is Darwinism, that is, biology as an essentially historical science whose main aim is to redescribe evolution in terms of a few scientifically understood processes and forces. Raff aims to extend the Darwinian synthesis, particularly by reintegrating developmental processes into the evolutionary narrative. In doing so, he maintains a historical perspective that is well informed and informative, recognising the fundamental contributions of the 19th-century pioneers who articulated the problems with which we are still struggling-Georges Cuvier, Etienne Geoffroy, Karl von Baer and Richard Owen. He also takes on board the 20th-century biologists such as Thomas Morgan, Gavin de Beer and Conrad Waddington who made significant attempts at a synthesis of genetics with development and evolution. What is new is the data. What, for ex ample, are we to make of the fossil images revealed in the sandstone at Ediacara in the Flinders Range of South Australia? They were part of a worldwide distribution of soft-bodied animals about 600 million years ago whose nature still defies description in relation to known species. Were they segmented bottom-dwellers like flattened, fan-like worms? Or were they frond-like symbionts containing photosynthetic microorganisms, attached by their "heads' to the seafloor and looking like algae? Then there was the extraordinary diversification of animals during the Cambrian period between 550 and 500 million years ago, an "explosion" in geological time, but long enough for a stunning variety of bewildering body plans to emerge. The mystery arises from a comparison of this diversity of basic body plans, or phyla, with the pattem of later evolution, where diversification was confined to the few surviving phyla. All modem phyla derive from the S per cent of species that survived the dramatic extinctions during the Permian between 300 and 250 million years ago. Instead of repeating the Cambrian patcern, the great variety of species that emerged as life bounced back from the brink was 'Restricted to the few types of organism that we know today, such as sponges, corals and jellyfish, unsegmented and segmented worms, molluscs, arthropods, starfish and sea urchins, fishes, amphibians and higher vertebrates, including us. What caused this sudden constraint on possibilities? Raff is gnomic about this: "Profound change within conserved body plan points obscurely, but powerfully, to a richer role for the interplay between development and evolution than we have suspected." We need, he says, "to understand the integration of variation and selection with developmental mechanisms that translate genetic information into body plan". Yes, indeed. Raff reminds us that when plants occupied the land about 400 million years ago they ramified into new phyla such as mosses, horsetails, ferns and seed plants. These phenomena point to the differences of intrinsic organisational properties that characterise different types of organism, rather than the consequences of random variation and natural selection acting on plastic bodies. Molecular sequencing techniques have made it possible to plot the evolutionary histories of organisms by comparing nucleotide base sequences in highly conserved parts of genomes. These include the sequences which code for basic cell machinery such as ribosomes and mitochondria. We can measure the historical distance between species by observing how the genes coding for these components have changed. Then we can address question about the evolutionary relationships between groups such as segmented worms and arthropods. Because these two groups share a basic body plan of segments, taxonomists classified them together on morphological grounds as the Articulate. Gene sequencing does not support this view. This leaves us without a consensus on how to organise the evolution of segmented animals coherently, which in Darwinian Mmis means identifying their common ancestor. But however the lineages are arranged, some degree of convergent evolution must have Occurred: the same character appears to have arisen independently in different lineages. To account for this in Darwinian terms, we would have to show that adaptation to particular habitats has forced this convergence. This is highly @Plausible for characteristics as fundamental as those that are shared by annelids and arthropods. It implies that there are properties intrinsic to 41,imal development that act as "attractors" for @r'icular patterns such as segmentation, just as the laws of physics and chemistry act to limit the range of patterns in the non-living world. This is where developmental biology begins to emerge as a "third force" in evolution, alongside genes and natural selection. Raff still sees convergent evolution as a problem rather than accepting that it requires a significant shift of emphasis in explanations. The problem becomes acute in a phenomenon which Raff has himself studied extensively. Within any major group of animals, such as insects or vertebrates, there is a stage of embryonic development at which the diverse species bear a striking resemblance to one another. Among vertebrates, salamander, chick and human embryos are remarkably alike at the stage when all the major organs have formed. Looking rather like fish, they share characteristics such as a prominent head and spinal cord, gill pouches and a tail-to mention only "external structures. It now appears that the phenomenon is essentially developmental. Different members of a group can diverge from one another in early development, but if they share a common body plan as adults this requires that they converge on a basic form characteristic of the group during their development. Again we face a problem of convergence that relates to how organisms are made, not to genes or to natural selection. Raff proposes a general conceptual scheme to explain this. Early embryos are organised according to general spatial features such as head-to-tail and inner-outer axes, while size of egg, yolk content and extraembryonic structures can all vary greatly. Embryos are modularised at later stages: their bodies consist of modules (developing limbs, eyes and so on), which can vary significantly. But in between, as the modules are emerging within the body axes, organisation demands convergence. Why, we do not yet know. There is plenty about genes in this book, how they act in development and their pattems of evolution, which have turned out to be much more constrained than expected. Again, where do these constraints come from? They seem to be closely connected with constraints on development, but how the causal pattems flow remains to be elucidated. So what emerges from this lucid, thoughtful and intelligent report is that development holds promising and intriguing clues to solutions of evolutionary paradoxes. We just don't yet know how to formulate a new biology in these terms. What we have is still life with paradoxes.