Natural selection is a process that is also known as "survival of the fittest." Every time an animal or plant reproduces, tiny changes in the genes make the offspring slightly different from its parent. Sometimes these changes can make offspring more successful than its parents. For example, a young giraffe with a longer neck than normal can graze higher in the trees than other giraffes, and so has access to more food. As a result it will become healthier and stronger, and will be more likely to breed successfully, passing its new genes for longer necks to its offspring.
Other genes could be damaging, so the animals carrying them will be less successful and may eventually die out. The theory of natural selection is based on the great variation found among even closely related individuals. In most cases, no two members of a species are exactly alike. Each has a unique combination of such traits as size, appearance and ability to withstand cold or other harsh conditions.
Men Behind the Expectation
In 1858, Charles Darwin and another British naturalist, Alfred R. Wallace, presented similar theories of natural selection. Many biologists rejected the idea at first.