Final answer:
Alfred Russel Wallace and Thomas Malthus significantly influenced Charles Darwin's development of the theory of natural selection, through the observation of species and the understanding of population dynamics.
Step-by-step explanation:
Two individuals that greatly affected Charles Darwin's formulation of the theory of natural selection were Alfred Russell Wallace and Thomas Malthus. Wallace's observations of species in the Malay Archipelago and his conclusions about natural selection closely mirrored Darwin's findings from his voyage on the H.M.S. Beagle, specifically with regards to the finches of the Galápagos Islands. Additionally, both Darwin and Wallace were influenced by the essay of economist Thomas Malthus, which discussed the struggle for existence due to population outgrowing resource availability. This understanding of competition and survival among offspring with heritable variations formed the crux of the principle of natural selection.