Natural selection is one of Darwin's central ideas that leads to the evolution of organisms.
Natural selection claims that populations overproduce. This leads to competition within the species. There are variations of traits amongst individuals of the same species. The individuals that have the most advantageous trait for the environment of interest will be more likely to survive. Therefore, these individuals are more likely to find a mate and pass on their genetic material to future generations (this is the biological definition of "fitness"). Organisms with less advantageous traits will pass and are less likely to find a mate. Thus, the genetic information responsible for their traits will eventually be lost from the species' genetic pool. In this way, nature (the environment) selects (chooses) which genes are maintained in the species' genetic pool. If there is no genetic variation within a population, then there will be no selection - all individuals will have equal chances at survival and reproduction.