In Chapters 11 and 12, fitness phenotypes were always regarded as properties of individual organisms. However, even in those chapters, we saw that the fitness values of specific individuals were important only through their contribution to genotypic values; that is, the average fitness phenotype for a group of individuals sharing a common genotype. Consequently, to study natural selection, all we need is a group that shares a common genetic state. We can then assign a "genotypic value" or average fitness deviation to that group. There is nothing about the theory of natural selection or the concept of average excess that requires that the biological units that are pooled together and assigned an average fitness value have to be individual organisms. As long as the group we define has some genetic identity that displays a fitness phenotype and there is variation in the population for those genetic identities, natural selection can occur. The genetic identities do not have to correspond to individual-level genotypes, although the response to selection will still be modulated by what fitness variation can be passed through the gametes produced by individuals. In this section, we will discuss targets of selection that are at levels of biological organization below the level of individuals (that is, nested within individuals).
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