Alfred Russel Wallace, OM, FRS (8 January 1823 – 7 November 1913) was a British naturalist, explorer, geographer, anthropologist and biologist. He is best known for independently proposing a theory of natural selection which prompted Charles Darwin to publish his own theory.
Wallace did extensive fieldwork, first in the Amazon River basin and then in the Malay Archipelago, where he identified the Wallace Line that divides Indonesia into two distinct parts, one in which animals closely related to those of Australia are common, and one in which the species are largely of Asian origin. He was considered the 19th century's leading expert on the geographical distribution of animal species and is sometimes called the "father of biogeography". Wallace was one of the leading evolutionary thinkers of the 19th century and made a number of other contributions to the development of evolutionary theory besides being co-discoverer of natural selection. These included the concept of warning colouration in animals, and the Wallace effect, a hypothesis on how natural selection could contribute to speciation by encouraging the development of barriers against hybridization.
Wallace was strongly attracted to unconventional ideas. His advocacy of Spiritualism and his belief in a non-material origin for the higher mental faculties of humans strained his relationship with the scientific establishment, especially with other early proponents of evolution. In addition to his scientific work, he was a social activist who was critical of what he considered to be an unjust social and economic system in 19th-century Britain. His interest in biogeography resulted in his being one of the first prominent scientists to raise concerns over the environmental impact of human activity. Wallace was a prolific author who wrote on both scientific and social issues; his account of his adventures and observations during his explorations in Indonesia and Malaysia, The Malay Archipelago, was one of the most popular and influential journals of scientific exploration published during the 19th century.