Final answer:
Charles Darwin referred to his explanation of the process by which species change over time as the theory of evolution by natural selection. This theory argues that favorable inherited traits become more common within a population due to better survival and reproductive success of individuals with these traits.
Step-by-step explanation:
Charles Darwin called his theory of how species change, the theory of evolution by natural selection. Natural selection is a mechanism for evolution, suggesting that species not only evolve but do so through the selection of individuals in a population that have favorable traits, which are then passed on to future generations. Recapitulating the process, it involves variation among individuals, a struggle for limited resources, and the idea that individuals with advantageous traits are more likely to survive and reproduce.
Darwin's observations of species like those on the Galápagos Islands, and the subsequent variation he noted, were foundational to developing this theory. He proposed that these variations would lead to differences in survival and reproduction rates, thereby altering species over generations in what he termed 'descent with modification'. Darwin and Alfred Russel Wallace independently arrived at the concept of natural selection, which they jointly presented before the scientific community.
Darwin's seminal work on this subject was published in a book called 'On the Origin of Species' in 1859, which detailed the evidence for his theory from both his observations and the fossil record.