Most individuals that are born do not reach adulthood and as a result population size is generally stable. Survival to reproductive maturity is not random. Whether or not an individual survives to reproductive maturity depends, in large part, on the genetic makeup of the individual. The individuals that are best at leaving descendants are, on average, those whose inherited characteristics fit them best to their environment. This differential success is called natural selection.
Natural selection is the main mechanism of evolution.
The result of natural selection is adaptation of organisms to their environments. Adaptations evolve gradually over many generations by the greater reproductive success in each generation of individuals who were innately better than the average member of the population. Mutation creates genetic variation. Natural selection acts on this variation, producing adaptation. Speciation is the division of one species into two different species.