“Yes, fitness is the central concept of biology, but it is an elusive concept. Almost everyone who looks at it seriously comes out in a different place. There are literally dozens of genuinely different definitions, which I won’t review here. At least two people have called fitness indefinable, a biological primitive. (A primitive is an undefined initial term in logic.) I don’t think that helps. Stearns (1976) once described it as ‘something everyone understands but no one can define precisely.’ Or is it that we can’t define it because we don’t understand it?” – L. Van Valen, “Three Paradigms of Evolution,” Evolutionary Theory 9 (1989):1-17.
See also: Natural selection: Could it be the single greatest idea ever invented?
Can sex explain evolution?
Hat tip: Pos-Darwinista