Natural selection
This is true even with children of the same parents. Some of these differences might make one organism better at surviving and reproducing than others in a particular habitat. When this organism reproduces, it passes along the genes, which gave it the advantage, to its children. Some adaptations are extremely long-lasting, useful in many habitats. The evolution of wings in birds is an example. Others are good only as long as the environment stays the same. If the environment changes enough, then another organism might do better.