Fitness in evolutionary terms is not physical fitness, like muscle tone and health. Fitness is the number of offspring an individual produces which is the equivalent of its reproductive success.
Some variations are more advantageous than others. Organisms with advantageous traits will survive and have more offspring who will carry those advantageous traits and pass them on to their offspring. Differential reproduction occurs when individuals with advantageous traits reproduce more than those without the traits. The increased survival and reproductive rates of an individual's offspring contribute to that individual's fitness being greater than another's. This phenomenon occurs because a trait is being "selected for" or because it is being "favored" by natural selection.
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