Why there are so many definitions of fitness in models

“Fitness” quantifies the ability to survive and reproduce, but is operationalized in many different ways. Generally, short-term fitness (e.g., expected number of surviving offspring) is assigned to genotypes or phenotypes, and used to non-trivially derive longer-term operationalizations of fitness (e.g. fixation probability or sojourn time), providing insight as to which organismal strategies tend to evolve due to natural selection. Assigned fitness operationalizations vary, but all summarize currently expected organismal vital rates (i.e. births, deaths, organismal growth). Derived operationalizations depend also on assumptions regarding demographic stochasticity, environmental stochasticity, feedbacks whereby births, deaths, and organismal growth cause environmental change, and the impact of migration and niche construction on which environment is experienced. After reviewing existing derived fitness operationalizations, we propose a new one tailored to balancing selection. Population genetic models generally sidestep ultra-high-dimensional phenotype space and genotype spaces by instead deriving the long-term evolutionary fate of a lower-dimensional set of genetically encoded “strategies”. Strategies (e.g. costly developmental commitment to producing armaments) are causally upstream from realized phenotypes (e.g. armament size). While selection is best understood in terms of differences in organismal vital rates, its derived outcomes are most easily understood as properties of genetic lineages.