Darwin Theory of natural selection was proposed by Charles Darwin in 1858. Darwin believed all plants and animals had evolved from a few common ancestors by means of natural selection. Plants and animals produce many offspring, but some of the young die before they can become parents. According to Darwin's theory, natural selection determines which members of a species die prematurely and which ones survive and reproduce. All living things must compete for a limited supply of food, water, space, and other necessities. The individual plants and animals whose variations are best adapted to conditions have an advantage in this struggle. These organisms, on average, tend to leave a larger number of offspring than other members of their group. As a result, the proportion of the group sharing the traits of the best-adapted organisms increases from generation to generation. Scientists use the term fitness to refer to the ability of an organism to reproduce. For this reason, natural selection is often called the "survival of the fittest."