The theory of natural selection is based on four basic facts:
 

• In each generation more offspring are being born than can survive until the maturity and reproduction stage.
• Because of that, a continual "struggle for life" takes place determining which ones survive and have their own children.
• Members of a given population are different. The ones that are better adapted to the environment have a better chance to win this struggle.
• The individuals that survived and can reproduce, transmit their adaptive traits – the ones that allowed them to survive until the maturity and reproduction – to their progeny.

They, in turn, transmit to their children, which transmit to theirs and so on. Accumulation of such changes in a suitably long period of time can produce surprising results. Natural selection explains an astonishing diversity among living organisms and their perfect adaptation to the environment.

Although Gregor Mendel lived in the same period as Darwin, his work was almost unknown at that time. Darwin noticed the existence of evolution but could not explain its mechanism. He did not know why the organisms were different and how their adoptive traits could be transmitted to their progeny.