On the Unit of Selection in Sexual Populations

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Description/Abstract

Evolution by natural selection is a process of variation and selection acting on replicating units. These units are often assumed to be individuals, but in a sexual population, the largest reliably-replicated unit on which selection can act is a small section of chromosome – hence, the ‘selfish gene’ model. However, the scale of unit at which variation by spontaneous mutation occurs is different from the scale of unit at which variation by recombination occurs. I suggest that the action of recombinative variation and mutational variation together can enable local optimization to occur at two different scales simultaneously. I adapt a recent model illustrating a benefit of sexual recombination to illustrate conditions for two scales of optimization in natural populations, and show that the operation of natural selection in this scenario cannot be understood by considering either scale alone.