Herbert Spencer first used the phrase “survival of the fittest” in his 1864 book Principles of Biology.
Spencer coined the expression while discussing Charles Darwin’s theory of natural selection. Darwin later adopted it in the fifth edition of On the Origin of Species, published in 1869. The phrase became widely associated with evolution, although Darwin generally preferred the more precise term “natural selection.”
“Fittest” does not necessarily mean strongest, largest, or healthiest. In evolutionary biology, fitness refers to reproductive success relative to other individuals in a particular environment. A smaller or less aggressive organism can be fitter if it leaves more surviving offspring.
The phrase has also been misused to justify social and political claims unrelated to biological evidence. Biological evolution describes changes in inherited traits across populations; it does not provide a moral rule saying that whatever happens in society is desirable or inevitable.