Endless Forms Most Beautiful (Sean Carroll)

Thảo luận trong 'Sách tiếng nước ngoài' bắt đầu bởi Despot, 6/10/13.

  1. Despot

    Despot Lớp 11

    Endless Forms Most Beautiful: The New Science of Evo Devo and the Making of the Animal Kingdom

    Review by Steven Rose, The Guardian UK, 2006:

    Identify the author and subject of this 19th-century quotation: "Endless forms most beautiful." Goethe on nature? Ruskin on art? Wrong. As any biologist should know (but too few do), the phrase comes from the majestic coda to Darwin's Origin, reflecting on the grandeur of the evolutionary view of life in all its multitudinous forms.

    How does this variety emerge? How to explain the astonishing unrolling processes of development, from the minuscule inchoate fertilised egg to the perfection of a newborn human baby, the dazzling display of a butterfly's wing or the stripes on a zebra's back? With Darwin came a second question: the origin not of the egg or the individual, but of the species of which these are but exemplars. And when, by the 1930s, Darwinian evolution and Mendelian genetics were unified into what became known as the Modern Synthesis, the time might have seemed ripe for a further synthesis, that of genetics and developmental biology.

    It was not to be. For most of the past 150 years, genetics has been the science of difference - what distinguishes a blue-eyed from a brown-eyed person, or both from a chimpanzee. By contrast, development has been the science of similarities: how is it that humans are so extraordinarily identical; nearly all of us growing up bilaterally symmetrical, with two arms and two legs, with exquisitely functioning though almost unimaginably complex brains? And while genetics became increasingly clever at accounting for such differences as blue versus brown eyes, it gulped and gave up the attempt to explain why, although humans are 98.8% genetically identical to chimpanzees, no one would mistake one for the other. What in this tiny 1.2% could account for the dramatic differences between two such closely related species? Meanwhile, generations of developmental biologists had studied in painstaking detail the seamless cellular cascade that leads, for instance, from the fertilised chick egg to the formation of its wings. But their work seemed to stall. It was time for the geneticists to come to the aid of the embryologists.

    Yet despite valiant attempts to bring together these crucial pieces of the biological jigsaw, progress was painfully slow until the last couple of decades, when new insights into the mechanisms and control of gene action have begun to pour out of the molecular biology labs. It is these new findings that are contributing to what Sean Carroll, a distinguished researcher in the field, calls the Third Synthesis, of evolution, genetics and development, or, in the argot its practitioners so enjoy, Evo Devo.

    The new starting point came from the recognition of the surprising continuities in genes between one species and another. We not only have nearly 99% of our genes in common with chimps, but some 35% in common with daffodils. Throughout much of the animal and even plant kingdoms, almost the same genes code for almost the same proteins. And further, to everyone's astonishment, the genes involved in making the complex eyes of fruitflies are close matches to those involved in making the very different eyes of octopuses and people. But it turns out that these protein-coding genes account for only a tiny fraction (about 2% in humans) of the cell's DNA. Embedded in the rest is a complex array of control mechanisms. Whether a particular gene is active at any time depends on other regulatory genes, which are in turn switched on and off by proteins coded for by yet other genes. What matters in development, and what in large measure helps determine whether you are a chimp or a human, depends on when and where these switches are thrown.

    Developmental biology, as Carroll describes it, is essentially a matter of topography and history. Zebras' stripes or butterfly wing patterns depend on the switching on and off at precise moments during development of genes that code for proteins - enzymes which are in turn responsible for synthesising the pigments that create the patterns. So too are our bilaterally symmetrical bodies, and Carroll tries hard in the first half of the book to make the complex molecular genetics of these processes clear without dumbing down.

    As for the larger history, that of evolution, since the 1930s evolutionary biologists have increasingly defined change in genetic terms - a change in gene frequency in a population. For such geneticists, unlike for Darwin, an organism is merely, in Richard Dawkins's phrase, a gene's way of making copies of itself. But given the unexpected similarity of protein-coding genes between different species, it becomes clear that what contributes novelty and change - and therefore the materials on which natural selection can act during evolution - is not an alteration in the protein-coding genes so much as subtle changes in the behaviour of the regulatory genes and their protein switches, which can subvert the entire pattern of development, providing new varieties available for selection.

    The second half of Carroll's book is devoted, therefore, to showing how Evo integrates with Devo, and to firmly relocating evolutionists' attention from genes to organisms - organisms, of course, in environmental context. Carroll fails to go the full distance, though. He hints, but never spells out, that what actually evolves, as Susan Oyama has argued, is not a set of genes or even of organisms, but of entire developmental systems. Which genes are expressed depends not just on development but, in the estate agent's sense, on location, location, location.

    So to return to the topography. A fertilised egg is not entirely symmetrical. It is marked by the point at which the sperm enters it. From this singularity one can define north and south poles (top and bottom), east and west (right and left). As the cell divides, these geographical markers define the axes of development of the organism. All its daughter cells contain the same genes and regulators, but it is the positional information that ensures which genetic switches are thrown when. Time depends on place, and the emerging structure depends on both. Carroll of course knows this, but doesn't, I think, quite appreciate the extent to which it undermines a merely gene-based model of development. It is, to put it simply, not the genes, but the developing organism that at each moment of its history determines the next phase of its life.

    But, and here again one must go beyond Carroll's account, such development is not unconstrained. There are limits to what forms the developing organism can generate, constraints given by physics and chemistry, what some holistically minded developmental biologists call "the laws of form". I would have liked to have seen Carroll address these arguments, if only to counter them. But in attempting to make Evo Devo accessible to a wider readership, there is only so much one book can do, and Carroll does it splendidly.

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    Người viết bài: contraucay
    Nguồn: TVE
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