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Did Darwin write the Origin backwards ?

Did Darwin write the Origin backwards ?. Elliott Sober Philosophy Department University of Wisconsin - Madison. Evolution by Natural Selection (of course). What is Darwin’s theory?. 2. Evolution by Natural Selection not far-sighted. What is Darwin’s theory?. 3.

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Did Darwin write the Origin backwards ?

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  1. Did Darwin write the Origin backwards? Elliott Sober Philosophy Department University of Wisconsin - Madison

  2. Evolution by Natural Selection (of course) What is Darwin’s theory? 2

  3. Evolution by Natural Selection not far-sighted What is Darwin’s theory? 3

  4. Evolution by Natural Selection not far-sighted environmentally contingent What is Darwin’s theory? 4

  5. Evolution by Natural Selection not far-sighted environmentally contingent acts on random variation What is Darwin’s theory? 5

  6. Evolution by Natural Selection not far-sighted environmentally contingent acts on random variation the “main but not exclusive cause” What is Darwin’s theory? 6

  7. Evolution by Natural Selection not far-sighted environmentally contingent acts on random variation the “main but not exclusive cause” usually individual, not group What is Darwin’s theory? 7

  8. What is Darwin’s theory? • Evolution by Natural Selection (with its importance, levels, etc. duly noted) …

  9. What is Darwin’s theory? • Evolution by Natural Selection (its importance, levels, etc. duly noted) … • This is true, but incomplete. Consider: S1 S2 S3 ……. Sn

  10. What is Darwin’s theory? S1 S2 S3 ……. Sn Here there is Evolution + Natural Selection + Separate Ancestry.

  11. What is Darwin’s theory? S1 S2 S3 ……. Sn Here there is Evolution + Natural Selection + SeparateAncestry. This is not D’s theory.

  12. Darwin’s theory = CommonAncestry + Natural Selection

  13. What does CA mean? In the concluding paragraph of the Origin on the “grandeur in this view of life,” Darwin says that, in the beginning, life was “breathed into a few forms or into one”

  14. What does CA mean? In the concluding paragraph of the Origin on the “grandeur in this view of life,” Darwin says that, in the beginning, life was “breathed into a few forms or into one” So is CA a claim about number of start-ups?

  15. No In the fifth edition, Darwin added: “No doubt it is possible …that at the first commencement of life many different forms were evolved; but if so, we may conclude that only a few have left modified descendants.”

  16. The point was already in the first edition. CA is about tracing back, not about start-ups.

  17. CA with multiple start-ups now: O1 O2 … On start-ups: S1 S2 … Sm

  18. Another example of CA with multiple start-ups now: O1 O2 … On bottleneck: B start-ups: S1 S2 … Sm

  19. Darwin’s theory = Common Ancestry + Natural Selection

  20. What sorts of observations would be evidence for CA+NS? Adaptive traits – ones that help organisms to survive and reproduce in their environments?

  21. What sorts of observations would be evidence for CA+NS? Adaptive traits – ones that help organisms to survive and reproduce in their environments? These support natural selection, but would they support common ancestry?

  22. Darwin’s answer: “… adaptive characters, although of the utmost importance to the welfare of the being, are almost valueless to the systematist. For animals belonging to two most distinct lines of descent, may readily become adapted to similar conditions, and thus assume a close external resemblance; but such resemblances will not reveal – will rather tend to conceal their blood-relationship to their proper lines of descent.”

  23. Two examples: The torpedo shape of dolphins and sharks. This does not provide strong evidence for common ancestry.

  24. Two examples: The torpedo shape of dolphins and sharks. This does not provide strong evidence for common ancestry. The gill slits of human foetuses and many fish. This does provide strong evidence for common ancestry.

  25. Darwin’s Principle • Useless or deleterious similarities provide strong evidence favoring CA over SA. • Useful similarities do not provide strong evidence favoring CA over SA.

  26. Darwin’s Principle • Useless or deleterious similarities provide strong evidence favoring CA over SA. • Useful similarities do not provide strong evidence favoring CA over SA. Does D’s principle have a deeper justification?

  27. the Law of Likelihood* Suppose O is a set of observations and H1 and H2 are competing hypotheses. Then: O favors H1 over H2 precisely when Pr(O│H1)> Pr(O│H2). *Ian Hacking, The Logic of Statistical Inference, 1965.

  28. A simple example H1: 80% of the balls in the urn are green. H2: 30% of the balls in the urn are green.

  29. A simple example H1: 80% of the balls in the urn are green. H2: 30% of the balls in the urn are green. O: you draw balls from an urn 100 times and observe that 81 of the draws are green.

  30. A simple example H1: 80% of the balls in the urn are green. H2: 30% of the balls in the urn are green. O: you draw balls from an urn 100 times and observe that 81 of the draws are green. O favors H1 over H2

  31. A simple example H1: 80% of the balls in the urn are green. H2: 30% of the balls in the urn are green. O: you draw balls from an urn 100 times and observe that 81 of the draws are green. O favors H1 over H2 Pr(O│H1) > Pr(O│H2)

  32. Darwin’s Principle and the Law of Likelihood Pr(both species have trait T │CA) Pr(both species have trait T │SA) How are these two probabilities related?

  33. Darwin’s Principle and the Law of Likelihood Pr(both species have trait T │CA) ≈ Pr(both species have trait T │SA) They are about equal if T is adaptive.

  34. Darwin’s Principle and the Law of Likelihood Pr(both species have trait T │CA) >> Pr(both species have trait T │SA) The first is much bigger than the second if T is neutral or deleterious.

  35. Darwin’s Principle applies outside of biology – for example, to plagiarism O = various observed similarities between two student essays

  36. Darwin’s Principle applies outside of biology – for example, to plagiarism O = various observed similarities between two student essays CA = the students copied from a common source (perhaps on the Internet). SA = the students wrote their essays separately and independently.

  37. Which similarities strongly favor CA over SA? • The fact that both essays use nouns?

  38. Which similarities strongly favor CA over SA? • The fact that both essays use nouns? • The fact that both essays misspell the same words?

  39. Useless or deleterious similarities provide strong evidence favoring CA over SA. Useful similarities do not provide strong evidence favoring CA over SA. Darwin’s Principle 39

  40. Useless or deleterious similarities provide strong evidence favoring CA over SA. Useful similarities do not provide strong evidence favoring CA over SA. There are some exceptions to both parts. Darwin’s Principle 40

  41. a case in which Darwin is right about adaptive similarities 2 populations that are each at 100% A A fitness B 0%A100

  42. a case in which Darwin is wrong about adaptive similarities 2 populations that are each at 100% A A fitness B 0%A100

  43. Multiple adaptive peaks. fitness trait value

  44. When can an adaptive similarity provide strong evidence for CA? When there are lots of adaptive arrangements, each stable once achieved.

  45. When can an adaptive similarity provide strong evidence for CA? When there are lots of adaptive arrangements, each stable once achieved. Example: the near universality of the genetic code.

  46. How are the two parts of Darwin’s theory(CA and NS) related to each other? • Darwin’s Principle says: to get strong evidence for CA, ignore traits that evolved via NS. • Other relationships of CA and NS?

  47. CA changes the question about whether new species can evolve from old • Darwin believed that NS could cause a population to evolve into a new species. His critics disagreed, arguing that species boundaries are walls.

  48. How CA changes the question about NS • Darwin believed that NS could cause a population to evolve into a new species. His critics disagreed, arguing that species boundaries are walls. • Given CA, species boundaries must have been crossed. The question of whether this is possible was solved by the evidence for CA (never mind NS).

  49. D’s order of presentation • TheOrigin begins by arguing for the power of NS. 2. Only later in the Origin does Darwin develop evidence for CA.

  50. D’s order of presentation • TheOrigin begins by arguing for the power of NS. 2. Only later in the Origin does Darwin develop evidence for CA. D’s order: NS first, then CA.

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