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In biology, evolution is a change in the traits passed by a group from one generation to the next. These traits are found in the genes, and when they change it is called mutation. Evolution is said to have happened when these traits either become more or less commonplace in a group.
In a person's family, his or her grandparents are one generation. The parents, aunts, and uncles are another, and that person, and his or her brothers, sisters, and cousins, are a third. Even photographs of all these people at the same age would not show very many people who looked exactly the same. This is the fact of evolution: A family, like all groups of living things, is changing from generation to generation in small ways, and these changes can add up over many generations and lead to very big differences.
The theory of evolution tries to explain all the patterns found in the entire history of living things. One part of evolution that explains a lot of what we see is natural selection. Natural selection says that since children aren't exactly like their parents, some of them might have some slight advantage that makes them better able to live where they live. Since they have an advantage, these ones are more likely to survive. Children are more like their parents than other unrelated individuals. So the ones with a slight advantage are likely to have children with a slight advantage, or might even end up with some children with a bigger advantage. After many generations in the same place, the great-great-great-great-etc. grandchildren are likely to be really good at living where they live.
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