When offspring fitness is determined by the joint influences of offspring and parental traits, selection may favor particular combinations that generate high offspring fitness. Parents often have important influences on their offspring's traits and/or fitness (i.e., maternal or paternal effects).
The available empirical evidence regarding the evolution of behavior expressed in interactions, although limited, supports the predictions of our model. Further, unlike most models of phenotypic evolution, a lack of variation in direct genetic effects does not preclude evolution if there is genetic variance in the indirect genetic contributions. Phenotypic evolution is greatly enhanced or inhibited depending on the nature of the direct and indirect genetic effects. The relative rate of evolution in interacting phenotypes can be quite different from that predicted by a standard quantitative genetic analysis. We then explore the ramifications of this model of inheritance on the evolution of interacting phenotypes. Therefore, to consider the evolution of interacting phenotypes we simultaneously consider changes in the direct genetic contributions to a trait (as a standard quantitative genetic approach would evaluate) as well as changes in the environmental (indirect genetic) contribution to the phenotype. However, because it has a genetic basis, this environmental component can evolve. When the trait having the effect is heritable, the environmental influence arising from the interaction has a genetic basis and can be incorporated as an indirect genetic effect. We partition genetic and environmental effects so that traits in conspecifics that influence the expression of interacting phenotypes are a component of the environment. We adopt a quantitative genetic approach to assess genetic influences on interacting phenotypes. More extreme examples of interacting phenotypes-traits that exist exclusively as a product of interactions-include social dominance, intraspecific competitive ability, and mating systems. Commonly-studied interacting phenotypes include aggression, courtship, and communication. Interacting phenotypes are traits whose expression is affected by interactions with conspecifics.
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