Natural variations, also known as genetic variations, are mutations that randomly occur during sex cell division. These variations lead to different phenotypes that change an offspring's ability to survive and reach sexual maturity.
When a genetic variation gives offspring a competitive advantage over its counterparts, it is genetically more fit. A classic example is Darwinian finches. Depending on the type of food available, different beak shapes lead to greater fitness and reproductive ability. As the food sources change, the relative fitness of each beak shape also changes, leading individuals with a different beak shape to reach sexual maturity. These individuals pass their genes on, leading to an overall change of beak shape in the species.