Many phenotypes emerge at the level of interactions between two or more individuals. If these interactions are recurrent across generations, they will have continuity over time and can be targets of selection. Targets emerging from interactions among individuals are not difficult to find. For example, of the three major fitness components outlined in Chapter 11, two in general are more appropriately assigned to an interacting pair of individuals: mating success and fertility/fecundity. An individual in a dioecious species does not truly have a phenotype of mating success or fertility; such phenotypes take on biological reality only in the context of an interaction with another individual. Another common target of selection is intraspecific competition, a phenotype emphasized by Darwin as being important in his theory of natural selection. But competition also takes on biological reality only in the context of interactions among individuals. It is always possible to assign an average phenotype to the individuals engaging in such interactions, but it is more accurate biologically to assign such phenotypes to the interacting individuals rather than to any single individual. Similarly, individuals who are relatives, particularly in species that have family structures, often interact in complex ways that can affect each other's fitnesses. As we shall see in this section, qualitatively new properties of natural selection emerge when we assign such interaction phenotypes directly to the set of interacting individuals rather than to each individual as a separate entity.
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