Differences Between Random Drift And Natural Selection

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nteractions between Natural Selection and Random Drift:
Some biologists such as Gillespie (1998) and Roughgarden (1979) often say that evolution is the outcome of the interaction of what they call evolutionary “forces” such as natural selection, migration, random drift, muatation, and mating preferences. Sober (1984b) argued for a realist interpretation of such talk, claiming that it describes factors which casually influence evolution. Just recently, some authors have attacked this realist interpretation of “force” talk those authors being Walsh (2000), Matthen and Ariew (2002). Aspects of Sober’s conception have been defended by Stephens (2005), Reisman and Forber (2005). However, advocates of a realist interpretation have not presented a
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This provides a way of responding to Plutynski’s (2005) argument that mere manipulability is not enough to show that drift is a cause of evolution. I will argue that selection is a population-level causal factor. What I add to Millstein’s claim is an account of how natural selection is nevertheless constituted by individual-level probabilities. My view is that natural selection as a force is constituted by fitness differences among individual organisms in a population, where fitnesses are constituted by certain probabilities.
If random drift is anything, it is not one thing. There seem to be variety of processes, effects, forces, anomalies, aspects of models, etc., all called “drift” by one biologist or another in different contexts. The term applies to many effects on populations or organisms which are said

to be due to “chance”, and to factors which are thought to help to produce such effects. However, many core senses of random drift make it something which varies inversely with population size.
The sense in which natural selection and drift causally interact is this: Both natural selection and drift are in part embodied in the same set of probability distributions over future frequencies in
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I also argued that natural selection and one sort of random drift can correspond to different aspects of such a population- level probability distribution. Natural selection is the aspect of the distribution which is under the control of fitness differences, and drift is the aspect of the distribution which is under the control of population size. If the probabilities involved are causal, then changes in the probability distribution are changes in causal probabilities of future frequencies in the population. Exactly how we view causal role of selection and drift in producing future frequencies will depend on what sort of causal character the probabilities have. For example, if they are dispositional, then the relation of selection and drift to actual frequencies will be the relation of dispositions to their manifestations. We therefore have a framework in which natural selection and drift can count as

causal factors, “forces” if you will. Moreover, it is now clear just what their causal character will
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