Fitness may be defined as: (1) the number of copies of one’s genes that endure in later generations;. (2) the genetic contribution of an individual to the next generation's gene pool relative to the average for the population, usually measured by the number of that individual's offspring or close kin that survive to reproductive age;. (3) the patterns of activity or inactivity, endurance or frailty, and illness or health that influence the ability to manage tasks of independent daily living. Moreover, according to Darwin, Fitness is an organism's ability to survive and reproduce.