What was the theory Charles Darwin proposed?

Charles Darwin proposed the theory of evolution by natural selection, which suggests that species evolve over time through a process where individuals with favorable traits are more likely to survive and reproduce. This leads to the gradual adaptation of organisms to their environments. Darwin's theory emphasized the importance of variation within species and the role of environmental pressures in shaping these changes. His ideas, presented in "On the Origin of Species" in 1859, fundamentally changed the understanding of biological diversity and the interconnectedness of life.