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Discontinuities of Darwinian Evolution and Revolution of Species

Tuesday 28 November 2017, by Robert Paris

Discontinuities of Darwinian Evolution and Revolution of Species

Stephen Jay Gould writes in "The Structure of the Theory of Evolution"

"Many evolutionists consider that a strict continuity between micro and macro-evolution is an essential ingredient of Darwinism and the necessary corollary of natural selection. (...) Thomas Henry Huxley had separated the natural selection from gradualism and warned Darwin that his frank and unsubstantiated adherence to gradualism could undermine his entire system. Fossils have too many abrupt transitions to show a gradual change and the principle of natural selection does not require it, because selection can act quickly. But this superfluous connection that Darwin invented became the central dogma of synthetic theory. Goldschmidt raised no objection to the classical theses of microevolution. He devoted the first half of his main work "The material foundations of evolution" to progressive and continuous change within species. However, he clearly distinguished himself from the synthetic theory by asserting that the new species suddenly appear by discontinuous variation, or macro-mutation. He admitted that the vast majority of macro-mutations could only be considered disastrous and he called them "monsters". But, Goldschmidt continued, a macro-mutation could, by the simple effect of luck, adapt an organism to a new mode of existence. We were dealing, in his terminology, with a "promising monster". Macro-evolution results from the infrequent success of these promising monsters, and not from the accumulation of minor changes within populations. (...) All paleontologists know that among fossils there are only a few intermediate forms; transitions between large groups are particularly brutal. Gradualists usually get out of this difficulty by invoking the extremely incomplete nature of the fossils that we possess; even if one step out of a thousand survived as a fossil, geology would not record continuous change. (...) Even in the absence of direct testimony in favor of these transitions smoothly can we invent a reasonable succession of intermediate forms, that is to say, viable organisms, between ascendants and descendants , in the main structural transitions? (...) What is the use of one half of the jaw and one half of the wing? (...) If we are to accept many cases of discontinuous transition in macroevolution, does Darwinism not collapse by surviving only as a theory concerning minor adaptive changes within species? The essence of Darwinism lies in one sentence: natural selection is the main creative force of evolutionary change. Nobody denies that natural selection plays a negative role in eliminating the unsuitable. Darwinian theories imply that it creates at the same time the adapted ones. Selection must accomplish this task by implementing adaptations in a series of steps, while preserving at each phase the beneficial role in a range of genetic variations due to chance. Selection must govern the process of creation and not be content with dismissing the unsuited after some other force has suddenly produced a new species completely completed in primitive perfection. One can very well imagine a non-Darwinian theory of discontinuous change, that is, a profound and brutal genetic modification that creates by chance (from time to time) and all at once a new species. Hugo de Vries, the famous Dutch botanist, was the defender of this theory. But these notions seem to face insurmountable difficulties. (...) The disruption of genetic systems as a whole does not produce creatures with benefits unknown to their descendants - and they are not even viable. But not all theories of discontinuous change are anti-Darwinian, as Huxley pointed out nearly a hundred and twenty years ago. Imagine that a discontinuous change in an adult form is born of a small genetic modification. Problems of incompatibility with other members of the species do not arise, this important and favorable mutation can then spread in the population in the Darwinian way. Imagine that this large-scale change does not produce a perfect form, but rather serves as a key adaptation allowing its owner to adopt a new model of existence. The pursuit of this new and successful life requires a large set of side-by-side changes in both morphology and behavior; these can occur on a more traditional, gradual route once key adaptation has led to a profound shift in selective pressures. Proponents of the present synthesis have given Goldschmidt the role of Goldstein by associating his imaginative expression - the promising monster - with non-Darwinian notions of immediate perfection resulting from profound genetic change. But this is not quite what Goldschmidt was arguing. In fact, one of its mechanisms leading to the discontinuity of adult forms was based on the notion of small underlying genetic change. Goldschmidt was a specialist in embryo development. He spent most of the early part of his career studying the geographical variations of the moth Lymantria dyspar. He discovered that large differences in the color distribution of the caterpillars resulted from small changes in the rate of development: the effects of a slight delay or enhancement of pigmentation at the beginning of growth increased through ontogeny and resulted in profound differences in fully grown caterpillars. Goldschmidt succeeded in identifying the genes responsible for these small changes in rhythm, and showed that the great differences observed at the end of development stem from the action of one or more genes controlling the rates of change acting at the beginning of the development. the growth. He codified the notion of "gene rate of change" (spleen genes) in 1918 and wrote twenty years later: "The mutant gene produces its effect (...) by changing the rates of partial processes of development. These can be growth or differentiation rates, production rates of the elements needed for differentiation, rates of reactions leading to specific physical or chemical situations at specific points of development, rates of these processes responsible for segregation of embryonic forces at given times. (...) In my opinion, very biased, the problem of reconciliation between the obvious discontinuity of macroevolution and Darwinism is largely solved if we observe that the small-scale changes occurring early in the embryo development accumulate during growth to produce profound differences in the adult. By prolonging the high rate of prenatal growth of the monkey brain in infancy, its size is approaching that of the human brain. (...) In reality, if we do not invoke discontinuous change by small changes in development rates, I do not see how most of the major transitions in evolution can be accomplished. Few systems have greater resistance to change than complex, highly differentiated, "higher" animals. How could one convert an adult rhinoceros or a mosquito into something fundamentally different? However, the transitions between the main groups have occurred well in the course of the life story. D’Arcy Wentworth Thomson (...) writes in "Growth and form": "(...) We can not transform an invertebrate into a vertebrate, nor a coelenterate into green, by any simple and legitimate distortion (...) nature goes from one type to another. (...) To look for steps to overcome the differences separating these types is to search in vain for ever. D’Arcy Wentworth Thomson’s solution was the same as Goldschmidt’s: transition can occur in embryos that are simpler and more similar to each other than the strongly divergent adults they form. No one would think of turning a starfish into a mouse, but the embryos of some echinoderms and some protovertebrates are almost identical. "

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Evolution in Modern Thought by William Bateson et al.

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How Stephen Jay Gould Explains Darwinism

Stephen Jay Gould and Revolution of Species

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