Like other living things, animals evolve by adapting to and exploiting their surroundings. In the billion-year history of animal life, this process has created vast numbers of new species, each capable of using resources in a slightly different way. Some of these species are alive today, but these are a minority; an even greater number are extinct, having lost the struggle for survival.
Speciation, the birth of new species, usually occurs when a group of living things becomes isolated from others of their kind (see Species and Speciation). Once this has occurred, the members of the group follow their own evolutionary path and adapt in ways that make them increasingly distinct. After a long period—typically thousands of years—their unique features mean that they can no longer breed with their former relatives. At this point, a new species comes into being.
In animals, this isolation can come about in several different ways. The simplest form, geographical isolation, occurs when members of an original species become separated by a physical barrier. One example of such a barrier is the open sea, which isolates animals that have been accidentally stranded on remote islands. As the new arrivals adapt to their adopted home, they become more and more distinct from their mainland relatives. Sometimes the result is a burst of adaptive radiation, which produces a number of different species. In the Hawaiian Islands, for example, 22 species of honeycreepers have evolved from a single pioneering species of finch-like bird.
Another type of isolation is thought to occur where there is no physical separation. In this case, differences in behavior, such as mate selection, may sometimes help to split a single species into distinct groups. If the differences persist for a long enough time, new species are created.
The fate of a new species depends very much on the environment in which it evolved. If the environment is stable and no new competitors appear on the scene, an animal species may change very little in hundreds of thousands of years. But if the environment changes rapidly and competitors arrive from outside, the struggle for survival is much more intense. In these conditions, either a species changes, or it eventually becomes extinct.
During the history of animal life, on at least five occasions, sudden environmental change has triggered simultaneous extinction on a massive scale. One of these mass extinctions occurred at the end of the Cretaceous Period, about 65 million years ago, killing all dinosaurs and perhaps two-thirds of marine species. An even greater mass extinction took place at the end of the Permian Period, about 200 million years ago. Many biologists believe that we are at present living in a sixth period of mass extinction, this time triggered by human beings.