Function in the Light of Frequency-Dependent Selection

Christie, Brusse, et al. claim that the ‘selected effect’ (SE) theory of function is premised on a simplistic view of evolution. In complex evolutionary scenarios, in particular those involving frequency-dependent selection (FDS), the SE theory fails, they argue, since citing a trait’s SE function does not serve to explain why the trait exists. I argue that where FDS leads to a stable equilibrium, at which all individuals’ trait values constitute a ‘best response’ to the rest of the population, the SE theory can be straightforwardly applied. However, matters are different if the evolutionary dynamics do not take the population to a stable equilibrium.