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Coming to Terms with Chance in Evolution

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Discussions of `chance' and other related concepts (such as 'stochasticity', 'randomness', 'indeterminism', etc.) are found throughout philosophical work on evolutionary theory. By focusing on three commonly recognized distinctions, I identify four 'chance'-like concepts: randomness, subjective unpredictability, collapsible objective chance, and non-collapsible objective chance. These are not, however, merely semantic distinctions: it is demonstrated that conflation of these clearly separate notions undermines widely-cited arguments in the philosophy of biology -- particularly, in the debate over the interpretation of fitness, natural selection, and genetic drift.

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Coming to Terms with Chance in Evolution

  1. 1. . . COMING TO TERMS WITH CHANCE IN EVOLUTION Louisiana State University, 2/3/2014 Charles H. Pence Program in History and Philosophy of Science Department of Philosophy
  2. 2. color by Mads Madsden
  3. 3. . . NATURAL SELECTION
  4. 4. t=0
  5. 5. t=0 t=1
  6. 6. t=0 t=1 t=2
  7. 7. . . FITNESS
  8. 8. .
  9. 9. .
  10. 10. . >
  11. 11. Orange organisms . leave more offspring than teal organisms.
  12. 12. Orange organisms . leave more offspring than teal organisms. A circle: the tautology problem
  13. 13. Orange organisms will probably (are disposed to) leave more offspring than teal organisms.
  14. 14. . . GENETIC DRIFT
  15. 15. t=0
  16. 16. t=0 t=1
  17. 17. t=0 t=1 t=2
  18. 18. . . THE GOALS
  19. 19. . . Distinguish four notions of chance in evolution Show that conflation of all four leads to problems — then fix the problems
  20. 20. . . FOUR CONCEPTS OF CHANCE
  21. 21. . ‘chance’-like concepts
  22. 22. . ‘chance’-like concepts process concepts outcome concepts
  23. 23. . ‘chance’-like concepts process concepts outcome concepts randomness
  24. 24. . ‘chance’-like concepts process concepts subjective objective outcome concepts randomness
  25. 25. . ‘chance’-like concepts process concepts subjective unpredictability objective outcome concepts randomness
  26. 26. . ‘chance’-like concepts process concepts subjective unpredictability outcome concepts objective collapsible randomness non-collapsible
  27. 27. Pr( heads ∣ coin flipped ) = 0.5
  28. 28. Pr( heads ∣ coin flipped ) = 0.5 Pr( heads ∣ Ω) = 0 or 1 Pr( heads ∣ Ω) = x
  29. 29. Pr( heads ∣ coin flipped ) = 0.5 Pr( heads ∣ Ω) = 0 or 1 COLLAPSIBLE Pr( heads ∣ Ω) = x NON-COLLAPSIBLE
  30. 30. from Diaconis (1998)
  31. 31. . . randomness unpredictability collapsible objective chance non-collapsible objective chance
  32. 32. . . SOLVING PROBLEMS
  33. 33. . . BC: Natural selection is chancy because genetic drift is chancy.
  34. 34. “drift clearly is a stochastic or probabilistic or indeterministic phenomenon” (BC, 324)
  35. 35. “drift clearly is a stochastic or probabilistic or indeterministic phenomenon” (BC, 324)
  36. 36. “drift clearly is a stochastic or probabilistic or indeterministic phenomenon” (BC, 324) “the inferences we can make” about drift (BC, 322); what drift “can predict” or “cannot predict” (BC, 323)
  37. 37. “if one is a realist...then one should conclude that [evolutionary theory] is fundamentally indeterministic” (BC, 336)
  38. 38. (P1) Drift is unpredictable. (P2) Drift is an autonomous statistical law. (C1) Drift is chancy (a fortiori from P1 and P2). (P3) Natural selection and drift are “inextricably connected” (BC, 324). (C2) Natural selection is objectively chancy (from C1 and P3).
  39. 39. . . Why fix this?
  40. 40. (P1*) Drift exhibits collapsible objective chance. (P2) Drift is an autonomous statistical law. (C1*) Drift is chancy (a fortiori from P1 and P2). (P3) Natural selection and drift are “inextricably connected” (BC, 324). (C2) Natural selection is objectively chancy (from C1* and P3).
  41. 41. Why think this is right?
  42. 42. Why think this is right? • The “hidden variables” argument • Brandon’s other work on drift
  43. 43. “Are the probabilities employed in the theory [subjective] or not?” (GHR, 146) GOOD: unpredictability
  44. 44. “Ungrounded probabilistic propensities are not mechanisms; they are admissions that there is no mechanism operating....” “[P]ure probabilistic propensities are viewed as an uncomfortable but unavoidable conclusion in quantum mechanics.” (GHR, 154) BAD: non-collapsible objective chance
  45. 45. . . THE TAKE-HOME
  46. 46. . . BC conflate unpredictability, non-collapsible, and collapsible objective chance GHR conflate unpredictability and non-collapsible objective chance
  47. 47. . . Most importantly: we can fix BC’s argument, if we resolve this conflation
  48. 48. . .
  49. 49. . .
  50. 50. Pearson, Biometrika 5:1 (1906)
  51. 51. If we want to make a statement about the stature of Englishmen, we must find a way of describing our whole experience … so that we can easily remember and communicate to others how many men of any given height we find among a thousand Englishmen. We must give up the attempt to replace our experiences by a simple average value and try to describe the whole series of results our observation has yielded. Weldon 1906, p. 94
  52. 52. . . QUESTIONS? charles@charlespence.net http://charlespence.net @pencechp

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