The theory of natural selection was proposed by Charles Darwin, a British naturalist, in the mid-19th century. His groundbreaking ideas were presented in his seminal work, "On the Origin of Species," published in 1859. Darwin posited that species evolve over time through a process where individuals possessing advantageous traits are more likely to survive and reproduce in their environments. This process leads to the gradual adaptation of species to their surroundings. Natural selection is a fundamental mechanism of evolution, explaining the diversity of life on Earth and how species adapt to changing environments over generations.