Natural selection can change a species in small ways, causing a population to change color or size over the course of several generations. This is called "microevolution."
But natural selection is also capable of much more. Given enough time and enough accumulated changes, natural selection can create entirely new species, known as "macroevolution." It can turn dinosaurs into birds, amphibious mammals into whales and the ancestors of apes into humans.
Take the example of whales — using evolution as their guide and knowing how natural selection works, biologists knew that the transition of early whales from land to water occurred in a series of predictable steps. The evolution of the blowhole, for example, might have happened in the following way:
- Random genetic changes resulted in at least one whale having its nostrils placed farther back on its head.
- Those animals with this adaptation would have been better suited to a marine lifestyle, since they would not have had to completely surface to breathe.
- Such animals would have been more successful and had more offspring.
- In later generations, more genetic changes occurred, moving the nose farther back on the head.
- Other body parts of early whales also changed.
- Front legs became flippers.
- Back legs disappeared.
- Their bodies became more streamlined and they developed tail flukes to better propel themselves through water.
Darwin also described a form of natural selection that depends on an organism's success at attracting a mate, a process known as sexual selection. The colorful plumage of peacocks and the antlers of male deer are both examples of traits that evolved under this type of selection.