Resident-Mutant Competition

Resident-mutant competition is the pairwise interaction that determines whether a rare mutant can spread in a population dominated by a resident phenotype. It is the microscopic engine of adaptive dynamics: the outcome of each competition, measured by invasion fitness, drives the population's trajectory through trait space. The competition is typically asymmetric — the mutant's success against the resident may differ from its success when common — and this asymmetry explains why evolution does not always converge to optimal traits. The structure of resident-mutant competition underlies classical concepts like the ESS (where the resident wins against all mutants) and extends them to continuous trait spaces where the space of possible mutants is infinite. When fitness depends on the composition of the population rather than merely the environment, resident-mutant competition becomes a case of frequency-dependent selection.