When thinking about evolution, fitness is not a measure of how many hours you log at the gym that week. Rather, fitness describes how effective an organism is at leaving its genes in the next generation through its offspring.
In order for an organism to be considered the fittest, they don’t have to be the flashiest, the strongest, or the biggest; they simply have to survive, find a mate, and produce offspring, which leaves its genes in the next generation (Orr, 2010).
Biologists have offered many definitions of fitness over the years. They all agree on the main idea of fitness, but differ in their ideas of how fitness is analyzed. One way involves the actual components that yield the differences in fitness, while the other way to analyze involves a more mathematical approach (Orr, 2010).
Orr, A. H. (2010). Fitness and its role in evolutionary genetics. Nat Rev Genet, 10(8), 531-539.