Natural selection, also known as the “survival of the fittest,” is naturalist Charles Darwin’s term (1869) for the process of adaptation by living organisms to change in their environments. Variants within a species–that is, physical differences among members of the same species–are evidence of genetic changes from one generation to the next. The genetic adaptations that help a species survive are passed down to offspring. Members of a species group who do not possess those genetic adaptations are less likely to thrive and reproduce. In that way, a variation, over time, becomes a dominant characteristic in the species.