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Natural selection is the change that occurs in a species over time.
in other words, individuals in a species who are born with traits that give them advantage over other individuals of the same species will tend to survive better by adapting to their environment and hence have a greater likelihood of getting offspring...over time, these traits spread into the entire population hence the phrase natural selection. Thus it is not literally a new species but rather the species is now defined in new terms that was previously just a part of the species.
For natural selection to occur, there must be an element of reproductive isolation. This means hat two groups of species reproduce in separate habitat. Such as a different continent or in separate islands. As each group reproduces separately, natural selection occurs and thus over time the two groups differ genetically hence a new species is created.
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