Survival of the Fittest
Only the strongest survive. This is what usually comes to your head once you hear the term “survival of the fittest.” The term was intended to be used as a synonym for “natural selection” or the process in which traits that best fit the environment the species live in become more emphasized through repeated reproduction throughout the succeeding generations.
As a matter of fact, the term 'survival of the fittest' does not necessarily entail the meaning ‘in best physical shape’ as the general populace would believe it to be. On Charles Darwin's On the Origin of Species, Darwin meant to use the term as a metaphor for 'better adapted for the climate the species lives in.'
Depending on the environment one (or one animal, for that matter) lives in, fitness does not necessarily mean being a hulking muscular being that will probably out power anything it comes into a fight with. But no, fitness can mean the speed to outrun predators in its niche (or habitat), tree or height climbing skills, its resistance to heat or cold and many others. The premise here is that being ‘fit’ is associated as to how certain species survive depending on their habitat.
For example, in a cold, icy climate, an organism with say, relatively thin fur than most of its population would not be selected as a mate by the opposite gender because it has well, relatively less fur than the ‘competition.’ Instead the female (or the opposite gender for that matter) would rather mate with male with much thicker fur because it would ensure the survival of their offspring. The mere survival of the offspring is the main goal of animals. Survival would mean better adaptations for survival in a given climate. By genetics, traits are passed on from one generation to another, meaning, means of survival (physically speaking) better designed for the environment are much more preferred for reproduction.
But alas, it is not the case for humans. We have become so intelligent as to have means of survival that are out of the ‘survival of the fittest’ question. We are able to utilize materials to build shelters from harsh and extreme environments, and to find new methods of hunting and cultivating for our own food. Which may explain why our definition of ‘survival of the fittest’ has changed over the course of time since it was first coined.