11. The term “exclusively” must not be interpreted literally. Moreover, it is not essential to the definition of the notion of “evolutionary chance mutation.” Millstein uses it to avoid some problems that arise in Sarkar’s “weak” definition of “directed mutation” (Sarkar 1991). Sarkar claims that a mutation is directed “if it occurs (or occurs more frequently) in the fitness-enhancing or ‘selective’ environment”, i.e., “in an environment where its associated phenotype has an enhanced fitness.” The inherent risk of this definition is that it would consider a mutation to be “directed” even if it turns out to be beneficial in a given environment but not clearly more probable than other deleterious or neutral mutations. Sarkar’s definition of “directed mutation” could also erroneously be applied to mutations which occur more frequently in a given environment when they are beneficial than in other environments where they are not, but not because of their adaptive value (in other terms, not because of the characteristics of the physico-chemical process causing them).

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