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Random with Respect to Fitness or External Selection? An Important but Often Overlooked Distinction

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Mutations are often described as being “random with respect to fitness.” Here we show that the experiments used to establish randomness with respect to fitness are only capable of showing that mutations are random with respect to current external selection. Current debates about whether or not mutations are directed may be at least partially resolved by making use of this distinction. Additionally, this distinction has important mathematical, experimental, and inferential implications.

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  1. Even viruses, which do lack DNA polymerase genes, can do so because they have other biophysics which allow for it Even for a virus, there are configurations which are not fit in any environment.

  2. By “reasonably-encounterable” I only mean to exclude conditions that one would only be able to achieve in a carefully controlled laboratory environment and not in the wild.

  3. There is utility in also considering edge cases as well, such as where a subsystem of an organism may have internal selective constraints, but that subsystem is not a critical subsystem of the organism, and, thus, losing the subsystem could be neutral or even advantageous in some environments. These are useful distinctions, but for the purpose of the present discussion, we will consider these to be under the broad category of being subject to external selection.

  4. As an example, even teleonomical processes such as predators hunting prey do not exclusively yield their goals, even by Merlin’s qualified definition. Vermeij (1982) noted that many predatory species have capture rates less than 25%, yet none would qualify these as undirected. While there is some amount of chance in the process, the mere existence of stochastic variations is not what the authors of the modern synthesis had in mind when they developed the concept of random mutations.


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JB wrote all parts of the article and performed all analyses.

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Correspondence to Jonathan Bartlett.

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Bartlett, J. Random with Respect to Fitness or External Selection? An Important but Often Overlooked Distinction. Acta Biotheor 71, 12 (2023).

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