The four steps or conditions necessary for natural selection to occur are that more organisms are born than can survive, characteristics vary within a species, variations are inherited, and differences in reproduction and survival are due to variations. All four of these conditions must occur for natural selection to happen.
The basic mechanisms of evolution include natural selection, genetic drift and mutation. Natural selection is a process in which the population of organisms that have adapted to their environment increases more than the population of less well-adapted forms. This concept was first explained in Charles Darwin's theory of evolution. Natural selection is sometimes referred to as "survival of the fittest."