The theory of evolution by natural selection is the process through which organisms change over time as a result of changes in physical or behavioural traits. These changes will allow an organism to adapt better to its environment and thus help it survive and have more offspring. Natural selection can change a species in very small ways, causing a population to change colour or size over the course of several generations. This is called “microevolution." But natural selection is also capable of a lot more. Given enough time and enough accumulated changes, natural selection can create entirely new species, known as "macroevolution." It was able to turn dinosaurs into birds, amphibious mammals into whales and the ancestors of apes into humans.