Fitness

Fitness is the central concept of natural selection. The crudest definition of fitness is the capacity of organisms to survive and reproduce in their environment. But in order for fitness to be measured or analyzed, a quantitative definition is required. Several theoretical definitions of fitness have been proposed. We describe three alternative formulations of fitness: the Price equation, evolutionarily stable strategy (ESS) theory, and information theory. Each theory has a specific mathematical definition of fitness that is not necessarily the same as the conceptual definition of fitness, reflecting relative fitness in the Price equation, invasive fitness in the ESS theory, and the long-term growth rate in information theory. To use these theories to answer empirical questions, we also describe the concept of fitness that biologists measure in the field, and discuss its potential link to the mathematical definition of fitness in the theories. Lastly, we discuss the meaning of fitness beyond the individual level, covering inclusive fitness, heritable influences on the environment, and the possibility of considering combined fitness effects in different organisms.