Runaway evolution from male-male competition

Understanding why and how elaborated traits evolve remains a fascination and a challenge. Darwin proposed both male-male competition and female mate choice as explanations for elaboration because such traits are often mediators of social interactions that govern access to mates. Although we have robust evolutionary quantitative genetic models for how mate choice can lead to runaway evolution, we lack an equivalent framework for understanding how male-male competition can drive extreme elaboration of traits. Here, we integrate the logic of optimality models into the quantitative genetic framework of interacting phenotypes to fill this gap. We assume that males modulate their aggression based on the relative size of a trait that signals willingness and ability to fight and identify conditions where the signal undergoes rapid and exponential evolution. Males receive fitness benefits from winning contests, but they may accrue fitness costs due to threats imposed by their opponent. This cost leads to a force of social selection that accelerates as the signaling trait is elaborated, which may cause runaway evolution of the signal. Even when a runaway is checked by natural selection, we find that signaling traits evolving by male-male competition can be elaborated well beyond their naturally selected optimum. Our model identifies simple conditions generating feedback between the behavioral and morphological traits mediating male-male competition, providing clear testable predictions. We conclude that, like the well-characterized case of female mate choice, male-male competition can provide a coevolving source of selection that can drive a runaway process resulting in evolution of elaborate traits.