Charles Darwin proposed the theory of evolution by natural selection, which states that organisms with beneficial characteristics are more likely to survive and reproduce so they pass them on to their offspring.
The theory of evolution was initially not accepted due to religious beliefs at the time, but is now widely accepted as true due to improvements in technology and the science behind it being understood.
Alfred Russel Wallace proposed the theory of evolution through population splits, where two groups become isolated for a long time and evolve differently, eventually forming new species.