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According to Darwin's Theory of Natural Selection, living things in a population tend to over-reproduce. However, individuals within a population have variations(differences). Competition then occurs within the environment, which is the selecting agent and those individuals with the variations that allow them to survive may win the struggle for existence. If these variations are heritable, these individuals may mate and pass down those traits to their progeny. They would be deemed "fit", thus the term, survival of the fittest. Over time and many generations, if enough variations are passed down, a population may be significantly different than the ancestral stock. If they can no longer successfully mate with the original population, evolution by natural selection would have occurred resulting in a new species. Sometimes, when a population becomes separated by continental drift, a body of water, or some other barrier, over time, in the new environment, a new species may arise from the original one. This is theorized to have occurred in the Galapagos Islands, with the famous Darwin finches. It is believed that the ancestral finches migrated to these islands from Equador and due to different environmental conditions and pressures, evolved over time into a variety of different finch species with different bill sizes, behaviors and feeding habits.
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