The scientist who proposed the theory of evolution by natural selection is Charles Darwin. His seminal work, "On the Origin of Species," published in 1859, outlines his observations and conclusions on how species evolve over time through the process of natural selection. This theory posits that individuals with advantageous traits are more likely to survive and reproduce, passing those traits to the next generation. Darwin’s contributions fundamentally changed the understanding of biology and laid the groundwork for modern evolutionary biology.