Invasion Fitness

Invasion fitness is the per-capita growth rate of a rare mutant introduced into a population dominated by a resident phenotype. It is the central quantitative construct of adaptive dynamics: if invasion fitness is positive, the mutant can establish and potentially replace the resident; if negative, the mutant dies out before reaching appreciable frequency. The invasion fitness landscape — plotting mutant success as a function of both resident and mutant trait values — determines the long-term trajectory of evolution in trait space. The concept generalizes classical relative fitness from population genetics by making fitness explicitly a function of the resident-mutant competition rather than merely environmental parameters.