Charles Darwin is known for his theory of evolution by natural selection. Published in his seminal work, "On the Origin of Species" in 1859, Darwin's theory posits that species evolve over time through the process of natural selection, where individuals with advantageous traits are more likely to survive and reproduce. This leads to the gradual adaptation of species to their environments. Darwin's ideas challenged the prevailing views of his time and laid the groundwork for modern evolutionary biology. His research and findings have significantly influenced numerous fields, including genetics, ecology, and anthropology.