Re: [asa] Two questions

From: George Murphy <>
Date: Wed Apr 15 2009 - 12:18:42 EDT

Mike -

Just a guess about your 1st question. The idea of continuity, what Huxley describes with the classic phrase natura non facit saltum (nature does not make jumps) seems to have been pretty well established in the physical sciences by the time the Origin was published. Note, e.g., how much of a revolution was started in physics 41 years later when Planck introduced the idea of discontinuous changes. & the fact that Mendel's particulate view of genetics was unknown at the time meant that the old "paint mixing" idea model of heredity, in which changes can be arbitrarily small, could still be maintained. (I know something about the history of physics but little about the history of genetics prior to Mendel, so my 2d comment carries little weight.)


  ----- Original Message -----
  From: Nucacids
  Sent: Wednesday, April 15, 2009 12:00 AM
  Subject: [asa] Two questions

  Let me quote from Eugene Koonin's recent paper, "Darwinian evolution in the light of genomics" (Nucleic Acids Research, 2009, 1-24). These are twp excerpts from where he is outline the principal concepts of the Modern Synthesis.

  "Evolution proceeds by fixation of the rare beneficial variations and elimination of deleterious variations: this is the process of natural selection that, along with random variation, is the principal driving force of evolution according to Darwin and the Modern Synthesis. Natural selection which is, obviously, akin to and inspired by the 'invisible hand' (of the market) that ruled economy according to Adam Smith, was the first mechanism of evolution ever proposed that was simple, plausible, and did not require any mysterious innate trends. As such, this was Darwin's second key insight. The founders of population genetics, in particular, Sewall Wright, emphasized that chance could play a substantial role in the fixation of changes during evolution not only in their emergence, via the phenomenon of genetic drift that entails random fixation of neutral or even deleterious changes. Population-genetic theory indicates that drift is particularly important in small populations that go through bottlenecks (6,16). However, the Modern Synthesis, in its 'hardened' form (13), effectively, rejected drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution (17)."




  "The beneficial changes that are fixed by natural selection are 'infinitesimally' small, so that evolution proceeds via the gradual accumulation of these tiny modifications. Darwin insisted on strict gradualism as an essential staple of his theory: 'Natural selection can act only by the preservation and accumulation of infinitesimally small inherited modifications, each profitable to the preserved being . . . If it could be demonstrated that any complex organ existed, which could not possibly have been formed by numerous, successive, slight modifications, my theory would absolutely break down.' [(1), chapter 6]. Even some contemporaries of Darwin believed that was an unnecessary stricture on the theory. In particular, the early objections of Thomas Huxley are well known: even before the publication of the Origin Huxley wrote to Darwin ''You have loaded yourself with an unnecessary difficulty in adopting Natura non facit saltum so unreservedly' (18)."


  Here are my two questions:


  Why did Darwin insist on strict gradualism as an essential staple of his theory?


  Why did most proponents of the Modern synthesis reject drift as an important evolutionary force, and adhered to a purely adaptationist model of evolution?


  - Mike

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Received on Wed Apr 15 12:19:32 2009

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