Fitness is measured by complementarity. Two things are fit for each other when they complement each other in their function.
Fitness is a covenantal matter. Natural selection and the survival of the fittest is a process whereby competitors vie against each other for the most exact covenantal relationship. The more covenantally pure, the more fit; and, the more fit, the more likely to survive.
In Nature, competition is directed with respect to the context of the environment, not with respect to each other.