"Survival of the fittest" is a phrase originating in evolutionary theory, as an alternative description of natural selection. The phrase is today commonly used in contexts that are incompatible with the original meaning as intended by its first two proponents: British polymath philosopher Herbert Spencer (who coined the term) and Charles Darwin.

Herbert Spencer first used the phrase – after reading Charles Darwin's On the Origin of Species – in his Principles of Biology (1864), in which he drew parallels between his own economic theories and Darwin's biological ones, writing, "This survival of the fittest, which I have here sought to express in mechanical terms, is that which Mr. Darwin has called 'natural selection', or the preservation of favoured races in the struggle for life. "

Darwin first used Spencer's new phrase "survival of the fittest" as a synonym for natural selection in the fifth edition of On the Origin of Species, published in 1869. Darwin meant it as a metaphor for "better adapted for immediate, local environment", not the common inference of "in the best physical shape". Hence, it is not a scientific description.

The phrase "survival of the fittest" is not generally used by modern biologists as the term does not accurately convey the meaning of natural selection, the term biologists use and prefer. Natural selection refers to differential reproduction as a function of traits that have a genetic basis. "Survival of the fittest" is inaccurate for two important reasons. First, survival is merely a normal prerequisite to reproduction. Second, fitness has specialized meaning in biology different from how the word is used in popular culture. In population genetics, fitness refers to differential reproduction. "Fitness" does not refer to whether an individual is "physically fit" – bigger, faster or stronger – or "better" in any subjective sense. It refers to a difference in reproductive rate from one generation to the next.

An interpretation of the phrase "survival of the fittest" to mean "only the fittest organisms will prevail" (a view sometimes derided as "Social Darwinism") is not consistent with the actual theory of evolution. Any individual organism which succeeds in reproducing itself is "fit" and will contribute to survival of its species, not just the "physically fittest" ones, though some of the population will be better adapted to the circumstances than others. A more accurate characterization of evolution would be "survival of the fit enough".

"Survival of the fit enough" is also emphasized by the fact that while direct competition has been observed between individuals, populations and species, there is little evidence that competition has been the driving force in the evolution of large groups. For example, between amphibians, reptiles and mammals; rather these animals have evolved by expanding into empty ecological niches. In the punctuated equilibrium model of environmental and biological change, the factor determining survival is often not superiority over another in competition but ability to survive dramatic changes in environmental conditions, such as after a meteor impact energetic enough to greatly change the environment globally. The main land dwelling animals to survive the K-T impact 65 million years ago had the ability to live in underground tunnels, for example.

Moreover, to misunderstand or misapply the phrase to simply mean "survival of those who are better equipped for surviving" is rhetorical tautology. What Darwin meant was "better adapted for immediate, local environment" by differential preservation of organisms that are better adapted to live in changing environments. The concept is not tautological as it contains an independent criterion of fitness.