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Earth and evolution
The earth itself has a part in evolution. Its
surface changes slowly over time, making mountains higher, oceans
deeper, and canyons wider. When the earth changes, groups of plants and
animals become separated. Over time, they may become completely
different species.
 A population is a group of animals that lives
in a specific area. Members of a population belong to the same species,
so they can mate and produce young together. When geographical barriers
like mountains and canyons form, a population often gets split into two
groups, one on each side of the barrier.
The two groups begin to change through natural
selection.
5.3
After many generations, the groups are so different that even if
they come in contact with each other again–if the barrier disappears, or
they find a way around it–they can't produce young together. They have
become different species.
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