The evolutionary process in which beneficial traits become more common over generations is called natural selection.
Natural selection occurs when individuals in a population vary, some of those differences are heritable, and individuals with advantageous traits tend to survive or reproduce more successfully. Over many generations, the traits linked to that advantage can increase in frequency. The process acts on individuals, but its evolutionary result appears in populations.
Charles Darwin and Alfred Russel Wallace independently developed the idea in the 19th century. Darwin presented natural selection in On the Origin of Species, published in 1859. Modern evolutionary biology combines the mechanism with genetics, explaining how inherited variation is passed through populations.
Natural selection is often confused with mutation or genetic drift. Mutations can introduce new genetic variation, but they are not automatically beneficial and are not themselves the process that increases an advantageous trait. Genetic drift changes trait frequencies through random chance, especially in small populations. Migration moves genes between populations. Natural selection is specifically the non-random increase of heritable traits that improve reproductive success in a particular environment.