The theory of evolution by natural selection, first proposed by Charles Darwin, explains how populations of living things can change over time. In this theory, Darwin proposed that individuals within a species vary in their traits, and that those variations can be inherited. He also proposed that individuals with certain traits are better adapted to their environment and are more likely to survive and reproduce, passing on their genes to the next generation. Over many generations, this can lead to significant changes in the population, as individuals with certain traits become more common while others become less common or disappear altogether.