Natural selection has been termed "the cornerstone" of evolution because it is the process of the adaptation of a species to its environment which can then lead to the evolution of this species. Evolution is a
...term [that] refers ... to a large collection of well-established laws and facts about the ways organisms change over time.
One example of how a species evolved from it original composition through adaptive change is the Galapagos marine iguana. This is the only iguana that can live and forage in the sea, thus making it uniquely a marine reptile. In fact, it lacks agility on the land, but can swim proficiently. Also, it is darker in color than other iguanas, a condition which allows it to absorb the heat of the sun more quickly. When iguanas are on the beach of the islands, they use the sun to warm their body temperature back to normal from the cool waters of the ocean in which they have been feeding on seaweed and such. Like other lizards, too, their digestive processes are dependent upon heat; and, while they are warming themselves and digesting, iguanas are very susceptible to predators. So, their darker color allows them to speed up natural processes.
This natural selection of iguanas from South America who were able to adapt to their new environment on the Galapagos Islands explains the changes that happened over time, thus evolving into a new species.
Another example is found in the horse which has developed from a smaller animal who moved through woodlands and thickets to the larger animal with longer legs who could move more quickly across open grasslands. This obvious adaptation came as a result of the horse's being a prey animal; those too small or slow were soon killed off. The horse's head has also evolved over time for similar reasons. As it grazed, the horse needed to be able to see all around him, so through natural selection, his head grew longer, a state which allowed his vision to not be blocked by his legs. Again, those without this improved visionary condition were more likely to be killed.