Natural selection is a process whereby organisms best adapted for their environment will survive and reproduce.
Natural selection is based on several key observations proposed by Darwin:
Traits are inherited - This means that the best-adapted organisms will pass their characteristics down to their off-springs which over millions of years is effectively the process of evolution.
More off-springs are produced than can survive - This means that there is competition for food and resources which also means that only the fittest will survive (survival of the fittest)
Heritable traits will vary from organisms - Each off-spring will be slightly different to a different off-spring
After Darwin had made these observations, he devised a conclusion:
Organisms will have off-springs with different inherited traits that will have better traits suited for their environment. The individuals that have the most suitable traits will have more chances of survival and reproduction.
Since the most suited traits are heritable, and because the organisms with the most suit=able traits will eave ore off-springs, the most suitable traits will become more common.
Over many years, the population will become more adapted to the environment.