Constraints Evolve: Context-Dependency of Gene Effects Allows Evolution of Pleiotropy

Evolutionary constraint due to pleiotropy refers to a situation in which mutations in genes shared among traits generate trait covariance; therefore, traits that are not directly exposed to selective challenge show a correlated response. When such a correlated response is deleterious, it may constrain the trait from evolving. Here, we argue that the idea of absolute constraints draws from the perception that gene effects are inherent to alleles and thus invariant across genetic and environmental backgrounds. However, evidence from studies involving genetic effects on multiple traits, observed across different genetic backgrounds and environments, supports the notion that genes’ effects on traits change. Consequently, pleiotropy also varies across backgrounds. We argue for a stronger emphasis on interaction effects when describing a trait’s genetic basis and its evolutionary potential. By discussing different cases of trait individuation, we demonstrate how this approach can lead to new insights.