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Natural Selection, in biology, the non-random survival of randomly varying hereditary units, resulting in the evolution and maintenance of adaptive improvements. The importance of natural selection was first recognized, independently, by Charles Darwin and Alfred Russel Wallace. They compared it to the breeding of domestic strains by artificial selection and saw it as the tendency for individual organisms of differing heredity to vary in survival and reproductive success. It is often called by Herbert Spencer's name, "Survival of the Fittest". Later "neo-Darwinians" see evolution by natural selection in genetic terms, as the tendency for genes to become, as a result of their beneficial or adverse effects on organisms, systematically more frequent, or systematically less frequent, in gene pools. The hereditary variation upon which natural selection acts results ultimately from mutation, more proximally from sexual recombination. Mutation is random in the sense that it is not biased towards improvement. It is controversial whether all, or only some, of evolutionary change is influenced by natural selection, the rest being neutral, but it is widely accepted that natural selection is the only force leading to adaptive evolution.

It has also been debated whether natural selection works only at the level of the genetic success of individual organisms (the orthodox view among professional evolutionists), or whether it sometimes also works among higher units in the hierarchy of life, such as group or species.