Natural selection is a key mechanism of evolution, which is the change in the heritable traits characteristic of a population over generations. It is the process through which populations of living organisms adapt and change. Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. Organisms that are more adapted to their environment are more likely to survive and pass on the genes that aided their success, causing species to change and diverge over time.

The concept of natural selection was first published by Charles Darwin and Alfred Russel Wallace in a joint presentation of papers in 1858, and was elaborated in Darwins influential 1859 book On the Origin of Species by Means of Natural Selection, or the Preservation of Favoured Races in the Struggle for Life. Darwin described natural selection as analogous to artificial selection, a process by which animals and plants with traits considered desirable by human breeders are systematically favored for reproduction.

Natural selection can act on any heritable phenotypic trait, and selective pressure can be produced by any aspect of the environment, including sexual selection and competition. However, selection for adaptation is not the only cause of evolution. Species change can also be caused by neutral mutations that have no detriment or benefit to an individual, genetic drift, or gene flow.

In summary, natural selection is a non-random difference in reproductive output among replicating entities, often due indirectly to differences in survival in a particular environment, leading to an increase in the proportion of beneficial, heritable characteristics within a population from one generation to the next.