Who co-proposed natural selection with Charles Darwin?

Alfred Russel Wallace co-proposed the theory of evolution by natural selection with Charles Darwin. Wallace reached the idea independently while conducting field research in the Malay Archipelago.

In 1858, while recovering from a fever on the island of Ternate—or possibly nearby Gilolo according to evidence from his journal—Wallace developed his explanation of how environmental pressures could cause species to diverge. He sent an essay outlining the idea to Darwin, whom he had previously met and corresponded with. Darwin had already been working on natural selection for about two decades but had not yet published the theory.

Darwin's colleagues Charles Lyell and Joseph Dalton Hooker arranged for Wallace's essay and extracts from Darwin's unpublished writings to be presented together at the Linnean Society of London on July 1, 1858. Darwin then rapidly prepared On the Origin of Species, published in 1859.

Wallace was also a major expert on biogeography. The Wallace Line, a boundary separating Asian and Australasian animal distributions, is named after him. A common mistake is to credit Wallace merely as Darwin's assistant or follower; he independently formulated natural selection and remains one of its principal co-discoverers.