Charles Darwin was one of the founders of evolutionary biology. In his book On the Origin of Species he articulated two major points: descent with modification and natural selection.
Descent with Modification
All the contemporary species arose from the succession of ancestors that differed from them. That shows unity and diversity of the life. The unity is in the fact that species share same ancestor and diversity is in great variety of different species of the same origin.
Darwin started with the following three observations from the nature. First, individuals in populations vary in their traits, many of which seem to be heritable (passed on from parents to offspring). Second, a population can produce far more offspring than can survive to produce offspring on their own. With more individuals, than the environment is able to support, the competition is inevitable. Third, species generally suit to their environments, which means they are adapted to their environments.
Making inferences from these three observations, Darwin arrived at his theory of evolution. Individuals with inherited traits that are more suited to local environment are more likely to survive and reproduce than less-suited individuals. Over many generations a higher and higher proportion of individuals will have advantageous traits. Evolution occurs as an unequal reproductive success of individuals ultimately leads to adaptation to their environments, as long as the environment remains the same.
Darwin called this mechanism of evolutionary adaptation natural selection because the natural environment selects individuals with certain traits among naturally occurring variant traits in the population.