Defining Fitness in Natural and Domesticated Populations

The term ‘fitness’ has been applied differently and with different definitions throughout the history of population genetics. Five concepts and definitions are presented~– distinguishing (phenotypic) fitness, (genotypic) fitness, adaptedness, adaptability and durability. While the heritabilities of fitness components are low, substantial genetic change is achievable, and breeding programs should include in the breeding objectives genotypic fitness (for known QTLs) and fitness traits such as fertility and longevity, as well as production traits.