Whose theory was natural selection?

The theory of evolution by natural selection is credited chiefly to the English naturalist Charles Darwin, who developed it after his voyage on HMS Beagle and published it in his 1859 book On the Origin of Species. Natural selection explains how species change over time because individuals with traits better suited to their environment tend to survive and reproduce more successfully, passing those traits on. Remarkably, the naturalist Alfred Russel Wallace independently arrived at essentially the same idea, and their work was first presented jointly in 1858. Darwin had delayed publishing for nearly two decades, aware of how controversial the idea would be.