Who coined the phrase “survival of the fittest” in evolutionary biology?

Herbert Spencer coined the phrase “survival of the fittest” in evolutionary biology. He used it in his 1864 book Principles of Biology after reading Darwin’s account of natural selection.

Spencer wrote that the phrase could summarize natural selection, although Darwin’s original wording emphasized the preservation of favorable variations. Darwin later included “survival of the fittest” in the fifth edition of On the Origin of Species, published in 1869.

The phrase is often treated as a direct synonym for natural selection, but it does not mean that the strongest individual always survives. In evolutionary biology, fitness refers to reproductive success: how effectively an organism passes its heritable traits to later generations. The phrase also does not imply conscious effort or moral superiority.