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The word evolution means change and development. When we say "evolution", we normally mean biological evolution, changes in living things over generations. The theory of evolution is the scientific explanation for how and why the forms of living things change over time.
Evolution is what leads to living things changing over time. New species may appear, and others may disappear. Living things haven't always been here on the Earth. Life appeared at some point, and then started to evolve.
In nature, animals that are better adapted to their environment are more likely to survive. For example, a giraffe with a longer neck can reach more and fresher leaves on a tall tree, but one with a shorter neck can't reach those leaves and might starve to death. This means that the short-necked giraffe doesn't reproduce (have children), so it's genes are not carried on and only the long-necked giraffe gets to reproduce. The entire giraffe species changes over the generations because of this, as only long-necked giraffes are left over.
The theory of evolution was developed by the biologist Charles Darwin, who wrote the book "On the Origin of Species". When he visited the Galapagos Islands, he saw many bird species that were only a little different from each other. Darwin noticed that species that looked slightly different lived in slightly different environments. He theorized that it was because similar species evolved from a common ancestor but the environment forced that ancestor species to split into new species.
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