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It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
It’s Okay to Call Genetic Drift a ‘Force’
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It’s Okay to Call Genetic Drift a ‘Force’

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One hotly debated philosophical question in the analysis of evolutionary theory concerns whether or not evolution and the various factors which constitute it (selection, drift, mutation, and so on) …

One hotly debated philosophical question in the analysis of evolutionary theory concerns whether or not evolution and the various factors which constitute it (selection, drift, mutation, and so on) may profitably be considered to be “forces” in the traditional, Newtonian sense. Several compelling arguments assert that the force picture is incoherent, due to the peculiar nature of genetic drift. I consider two of those arguments here – that drift lacks a predictable direction, and that drift is constitutive of evolutionary systems – and show that they both fail to demonstrate that a view of genetic drift as a force is untenable.

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  • 1. University of Notre DameProgram in the History and Philosophy of ScienceDepartment of PhilosophyIt’s Okay to CallGenetic Drift a “Force”PSA 2012, San Diego, CaliforniaCharles H. Pencecharles@charlespence.net
  • 2. The Plan• What is genetic drift?• What is the force interpretation?• First problem: The direction of drift• Second problem: Inertial states and deviations
  • 3. The Plan• What is genetic drift?• What is the force interpretation?• First problem: The direction of drift• Second problem: Inertial states and deviationsThe goal: Both the problems are solvable; it really is okay tocall genetic drift a “force.”
  • 4. What is Genetic Drift?“What most of the phenomena so designated have in commonis one or another biological form of random or indiscriminatesampling, and consequent sampling error.” (Beatty, 1992)1. Random Mendelian segregation2. Random fluctuations in equally fit types3. Causes that fail to differentiate based on fitness(“lightning strikes”)4. The “founder effect”: random geographic splitting of agroup (possibly)5. The cause of any non-adaptive variation (possibly)
  • 5. What is Genetic Drift?“What most of the phenomena so designated have in commonis one or another biological form of random or indiscriminatesampling, and consequent sampling error.” (Beatty, 1992)1. Random Mendelian segregation2. Random fluctuations in equally fit types3. Causes that fail to differentiate based on fitness(“lightning strikes”)4. The “founder effect”: random geographic splitting of agroup (possibly)5. The cause of any non-adaptive variation (possibly)
  • 6. Random Mendelian Segregation00.10.20.30.40.50.60.70.80.91100 200 300 400 500 600 700 800 900 1000fixation at AAfixation at aano fixation (yet)Five simulations of a heterozygous population (N 100) undergoing only genetic drift (no selection or mutation)
  • 7. The Force Interpretation“[E]volutionary biology has also developed a theory of forces.This describes the possible causes of evolution. The variousmodels provided by the theory of forces describe how apopulation will evolve if it begins in a certain initial state and issubject to certain causal influences along the way.” (Sober,1984)
  • 8. The Force Interpretation
  • 9. The Direction of Drift“In any case, drift is not the sort of thing that can play the roleof a force – it does not have predictable and constantdirection.” (Matthen and Ariew, 2002)“Consider first the idea that a force has both a magnitude anda direction. Drift has a magnitude that can be probabilisticallypredicted prior to the fact, and can be quantitatively accessedafter the fact. But drift definitely does not have a direction.”(Brandon, 2006)
  • 10. A First Response: Stephens“Drift does have a direction – it serves to eliminateheterozygosity.” (Stephens, 2004)
  • 11. A First Response: Stephens“Drift does have a direction – it serves to eliminateheterozygosity.” (Stephens, 2004)But:• Is “homozygosity-space” sufficiently well-defined tosupport defining directions for forces? (Filler, 2009)• Is this “direction” really what genetic drift is intended todescribe?
  • 12. A Second Response: FillerTake two criteria for forcehood from the literature on forces(Filler, 2009):• Forces must have a “mathematically specific magnitude”• Forces can “unify a wide array of seemingly disparatephenomena”
  • 13. A Second Response: FillerTake two criteria for forcehood from the literature on forces(Filler, 2009):• Forces must have a “mathematically specific magnitude”• Forces can “unify a wide array of seemingly disparatephenomena”Can we find a way to salvage the force metaphor that runs lessrisk of being ad hoc?
  • 14. Brownian Motion-0.5-0.4-0.3-0.2-0.100.10.2-0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8 1Simulation of a particle released at 0 0 undergoing Brownian movement (after Perrin, 1909)
  • 15. Brownian Motionmd2xdt2= −6πμadxdt+ X−6πμa = damping coefficientX = random variable
  • 16. Possible Objections• Reject both Brownian motion and genetic drift as forces?
  • 17. Possible Objections• Reject both Brownian motion and genetic drift as forces?• Not “giving up” complete predictability
  • 18. Possible Objections• Reject both Brownian motion and genetic drift as forces?• Not “giving up” complete predictability• Fully able to model these systems
  • 19. Possible Objections• Reject both Brownian motion and genetic drift as forces?• Not “giving up” complete predictability• Fully able to model these systems• Stochastic forces often already countenanced (by Brandon,Matthen and Ariew)
  • 20. On Inertial States and DeviationsMaudlin (2004) on “quasi-Newtonian” theories: when is“nothing happening,” and when is a system “deviating” fromthat inertial trajectory?Brandon (2006; McShea and Brandon, 2010): Drift is “part andparcel of a constitutive process of any evolutionary system,” a“default-causal” explanation, while mutation, selection, andmigration are “special” forces. Thus drift is not a force.
  • 21. Two Force InterpretationsSober BrandonInertial state Hardy-WeinbergequilibriumGenetic driftSpecial forces Drift, selection,mutation, etc.Selection,mutation, etc.
  • 22. Why is Drift Inertial?• Drift is necessarily going to act in any circumstances inwhich evolution itself is possible.
  • 23. Why is Drift Inertial?• Drift is necessarily going to act in any circumstances inwhich evolution itself is possible.• But what about Newtonian gravitation? Is it equally“constitutive?”
  • 24. Why is Drift Inertial?• Drift is necessarily going to act in any circumstances inwhich evolution itself is possible.• But what about Newtonian gravitation? Is it equally“constitutive?”• Response: we can build test cases where we eliminategravity.
  • 25. Why is Drift Inertial?• Drift is necessarily going to act in any circumstances inwhich evolution itself is possible.• But what about Newtonian gravitation? Is it equally“constitutive?”• Response: we can build test cases where we eliminategravity.• But the same holds for drift:• A test mass “at infinity,” undergoing no gravitationalacceleration, versus• A “drift-free” evolving population – infinite, but with bothselection and mutation
  • 26. Conclusions• Problem: Drift cannot be a force, because forces musthave specifiable directions• Solution: We already countenance stochastic forces, suchas Brownian motion• Problem: Drift is a “first-law” inertial condition, not a“second-law” special force• Solution: Drift is no more “constitutive” of evolvingsystems than gravity is of Newtonian systems• Overall: The force metaphor lives to fight another day
  • 27. ..Questions?charles@charlespence.net
  • 28. A Drift-Free Population• Four types: A, B, C, and D• Population: Infinite, equally distributed (initially)• Reproduction: Each individual produces exactly 1offspring and then dies, if it survives to reproductive age• Heredity: Clonal, with a slight mutation rate• Selection: Types C and D have a 10% chance of dyingbefore reproductive ageResults: C and D die off (thanks to selection); no bottlenecks orfinite population size; all probabilistic or stochastic influencesare selection or mutation, not drift. In the limit: roughly half Aand half B (modulo the mutation rate).

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