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Particular genes or accidental changes in the genes of plants or animals may give them characteristics which enable them to survive better. Over time this may result in entirely new species. There are different theories of evolution but Charles Darwin’s theory is the most widely accepted. His theory is based on natural selection or 'survival of the fittest'.
Variation of individuals of a species occurs naturally - variation makes you different from everyone else. If variation gives an animal an advantage in the wild, it is likely to live longer and have more offspring. It is likely that the variation is passed on to the offspring, making them more successful. Bad variations will mean that the individual is less successful and could even die so the individuals who are best adapted to the environment will survive. So as this carries on over a period of millions of years, animals and plants will slowly evolve through the process of natural selection.
Jean-Baptiste Lamarck suggested that evolution happened because of use and disuse. A characteristic which is used more by an organism becomes bigger and stronger, and one that is not used eventually disappears. Any feature of an organism that is improved through use is passed to its offspring. An example of this is the neck of a giraffe. He thought that early giraffes had short necks and as they used them to reach as high as possible they stretched. He thought that this stretched neck would then be inherited by the next generation of giraffes and so on.
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