In biology, "fitness" refers to a biological condition in which a competing variant is increasing in frequency relative to other competing variants in a population. It is also referred to, in particular, as Darwinian fitness. Darwinian fitness differs from the "physical fitness", which is more associated with health, muscle tissues, oxygen, and physical effort. Darwinian fitness is more concerned about reproductive success. Darwinian fitness describes how successful an organism has been at passing on its genes. The more likely that an individual is able to survive and live longer to reproduce, the higher is the fitness of that individual. Thus, it may indicate the relative measure of reproductive success of an organism in passing its genes to the next generation. It denotes the relative ability of an individual (or population) to survive, reproduce and propagate genes in an environment. Thus, when one says an organism is biologically fit, it means that the organism is adapted and suitable to its environment based on its relative reproductive success with respect to its population. There are two ways through which fitness can be measured: absolute fitness and relative fitness.