Sources of evolutionary contingency: chance variation and genetic drift
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Sources of evolutionary contingency: chance variation and genetic drift T. Y. William Wong1 Received: 2 December 2019 / Accepted: 23 May 2020 © The Author(s) 2020
Abstract Contingency-theorists have gestured to a series of phenomena such as random mutations or rare Armageddon-like events as that which accounts for evolutionary contingency. These phenomena constitute a class, which may be aptly called the ‘sources of contingency’. In this paper, I offer a probabilistic conception of what it is to be a source of contingency and then examine two major candidates: chance variation and genetic drift, both of which have historically been taken to be ‘chancy’ in a number of different senses. However, contra the gesturing of contingency-theorists, chance variation and genetic drift are not always strong sources of contingency, as they can be non-chancy (and hence, directional) in at least one sense that opposes evolutionary contingency. The probabilistic conception offered herein allows for sources of contingency to appropriately vary in strength. To this end, I import Shannon’s information entropy as a statistical measure for systematically assessing the strength of a source of contingency, which is part and parcel of identifying sources of contingency. In brief, the higher the entropy, the greater the strength. This is also empirically significant because molecular, mutational, and replicative studies often contain sufficient frequency or probability data to allow for entropies to be calculated. In this way, contingency-theorists can evaluate the strength of a source of contingency in real-world cases. Moreover, the probabilistic conception also makes conceptual room for the converse of sources of contingency: ‘sources of directionality’, which ought to be recognised, as they can interact with genuine sources of contingency in undermining evolutionary contingency. Keywords Evolutionary contingency · Sources of contingency · Sources of directionality · Chance variation · Random mutations · Genetic drift · Information entropy
* T. Y. William Wong [email protected] 1
Department of History and Philosophy of Science, University of Cambridge, Free School Lane, Cambridge CB2 3RH, UK
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Introduction There is, presently, no consensus as to what evolutionary contingency amounts to. Minimally, the idea is that evolutionary outcomes could have been otherwise. Perhaps, had the evolutionary past been different, then present or future evolutionary outcomes would have been different. Or, perhaps, the biological natural laws allow for a multiplicity of possible outcomes—in other words, the biological natural laws underdetermine the evolutionary outcome. A number of contingency-theorists have attempted to characterise, or offer an account of, evolutionary contingency by satisfying a range of desiderata, such as (1) being faithful to Gould (1989), (2) concordance with macroevolutionary data, (3) emphasising the importance of history, and (4) avoiding tension with indeterminism. B
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