Flexi Says: Darwin coined the term fitness to refer to an organism’s relative ability to survive and produce fertile offspring. Offspring which, by chance, had variations which “fit” or adapted them to their environment would have a greater chance to survive to maturity and a greater chance to reproduce. Offspring without such beneficial adaptations were more likely to die and either not reproduce, or not reproduce as much as others. Thus, well-adapted individuals produce more offspring. "Differential survival and reproduction" is a cornerstone of natural selection. The phrase "survival of the fittest" summarizes this idea.
