In previous years, fitness was defined as the capacity to carry out the day’s activities without undue fatigue. Physical fitness is now defined as the body’s ability to function efficiently and effectively in work and leisure activities, to be healthy, to resist hypokinetic diseases, and to meet emergency situations.
Many sources also cite mental and emotional health as an important part of overall fitness. This is often presented in textbooks as a triangle made up of three sub-sections, which represent physical, emotional, and mental fitness.
Physical fitness can also prevent or treat many chronic health conditions brought on by unhealthy lifestyle or aging, among other implications. An individual’s fitness is manifested through its phenotype. As phenotype is affected by both genes and environment, the fitness of different individuals with the same genotype are not necessarily equal, but depend on the environment in which the individuals live. However, since the fitness of the genotype is an averaged quantity, it will reflect the reproductive outcomes of all individuals with that genotype.
As fitness measures the quantity of the copies of the genes of an individual in the next generation, it doesn’t really matter how the genes arrive in the next generation. There are two commonly used measures of fitness; absolute fitness and relative fitness. Absolute fitness for a genotype can also be calculated as the product of the proportion survival times the average fecundity. Relative fitness is quantified as the average number of surviving progeny of a particular genotype compared with average number of surviving progeny of competing genotypes after a single generation.