An example of this would be giraffes necks. To be able to eat food they would need long necks to reach the leaves on the trees. Giraffes with longer necks would be able to eat more and reproduce. Giraffes with shorter necks are not guaranteed to eat enough food and starve, no being able to pass their genes along. The natural selection favors giraffes with long necks.
What is an example of one of the conditions to natural selection: "Individuals within a population differ, variation is heritable if they are passed on from parent to offspring."