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Explaining Complex Traits: The Intersection of Mutation, Selection, and Intelligent Design

This text explores how natural selection and mutation can account for complex traits like eyes and blood clotting cascades, which some argue exhibit "irreducible complexity." It critiques the intelligent design perspective by presenting probability arguments about evolutionary processes, such as simulating mutations through metaphorical examples (e.g., a chimp typing "Methinks it is like a weasel"). Additionally, it delves into the evolutionary history and mechanisms of blood clotting, discussing how gradual modifications can lead to complex biological systems and countering claims of irreducibility.

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Explaining Complex Traits: The Intersection of Mutation, Selection, and Intelligent Design

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Presentation Transcript

  1. Complexity: questions How can mutation and selection explain complex traits such as: eyes? blood clotting cascade? Intelligent design claim: natural selection and mutation cannot explain such traits

  2. Complexity: a metaphor • How likely is it that a chimp would type “Methinks it is like a weasel”?

  3. Complexity: metaphor II • How likely is it that a chimp would type “Methinks it is like a weasel”? • Probability: 28 sites. • 27 possibilities (26 letters, plus space). • 27 * 27 * . . . . 27 = 2728 = 1.12 x 1040

  4. Complexity metaphor III; A faster way • Odds of one correct letter: • How long to get it right? A simple simulation:

  5. Complexity metaphor IV: simulation Gen. 01 WDLTMNLT DTJBKWIRZREZLMQCO P Gen. 02 WDLTMNLT DTJBSWIRZREZIMQLO P Gen. 10 MDLDMNLS ITJISWHRZREZ MECS P Gen. 20 MELDINLS IT ISWPRKE Z WECSEL Gen. 30 METHINGS IT ISWLIKE B WECSEL Gen. 40 METHINKS IT IS LIKE I WEASEL Gen. 43 METHINKS IT IS LIKE A WEASEL

  6. Complexity: What good is half an eye? Fig. 3.15

  7. Complexity – eyes: How can lens proteins evolve?

  8. Origins of lens proteins

  9. Complexity: Blood clotting (“What good is half a scab?”) “Irreducible complexity?”

  10. Complexity: blood clottingserine proteases serine protease

  11. Complexity: blood clottingEvolutionary hypothesis? • 600 million years ago, an invertebrate ancestor . . . • Low pressure circulatory system • White blood cells present, can form plugs • Injury releases cAMP, leading to smooth muscle contraction • and proteases that cut circulatory proteins into fragments (which can clot)

  12. Complexity: blood clottingEvolutionary hypothesis • Duplicate serine protease (trypsin: digestive enzyme) • Express duplicate gene; product goes to circulatory system • Proteases are inactive until cleaved • Cellular proteases at wound activate protease • More fragments, better clot. • 1st clotting – specific gene.

  13. Next step: mutation • Recombine blood protease with an EGF domain (epidermal growth factor) • Would bind to cell surfaces when active.

  14. Complexity: blood clottingPrediction from evolutionary hypothesis • Clotting proteins had earlier functions • Therefore, expect similar proteins found in organisms without clotting system

  15. Complexity and Natural selection • If variation, heritability, and selection occur (=differential survival and reproduction), populations will evolve due to natural selection • Natural selection retains useful variation • Natural selection can lead to complex traits that could not evolve by mutation alone

  16. Additional readings and questions Miller, K. 2003. Finding Darwin's God. (Contains some excellent examples of the evolution of complex traits, including the clotting system. A very good account of intelligent design and its flaws, by a Christian evolutionist) Xu, X and R. F. Doolittle. 1990. Presence of a vertebrate-like fibrinogen in an echinoderm. Proceedings of the National Academy of Sciences 87: 2097-2101. Question: In Darwin's Black Box (1996), Behe argues that irreducibly complex systems, such as blood clotting, the immune system, and the eukaryotic flagellum, could not have evolved by natural selection. “By irreducibly complex I mean a single system which is composed of several well-matched, interacting parts that contribute to the basic function, wherein the removal of any one of the parts causes the system to effectively cease functioning. An irreducibly complex system cannot be produced directly by slight, successive modifications of a precursor system, because any precursor to an irreducibly complex system that is missing a part is by definition nonfunctional . . . Since natural selection can only choose systems that are already working, then if a biological system cannot be produced gradually it would have to arise as an integrated unit, in one fell swoop, for natural selection to have anything to act on.” Critique Behe's thinking using the evolution of blood clotting systems. What are the key pieces of evidence that would be necessary to test whether blood clotting is "irreducibly complex"?

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