The Definition of Individual Biological Fitness

“Fitness” is one of the central concepts in biology. Despite this, the concept is still not clearly defined. Previous attempts at definition refer to what should be called an individual’s “potential fitness,” or, when mathematized, the relative fitness of genetic alleles. In contrast, the present work defines the actual fitness of an individual, exactly what is referred to in the expression “survival of the fittest.” This represents a new conceptualization, a mathematical definition that will extend across the entire set of ideas related to evolution.