Charles Darwin proposed natural selection after years of studying living organisms, fossils, and geographical variation, and he published the theory in his 1859 book On the Origin of Species.
Darwin’s observations during the voyage of HMS Beagle helped him recognize that related organisms could differ between islands and environments. He later spent years gathering evidence, including a detailed study of barnacles, before presenting his evolutionary argument.
Darwin did not know about genes or DNA, so he could not explain the mechanism of heredity fully. Modern evolutionary biology combines natural selection with genetics, mutation, genetic drift, gene flow, and other processes.