If I have one population of monkeys, then some monkeys travel over to the other side of a mountain range where they are effectively isolated from the original population of moneys, and the environment is different in that new location like types of foods or predators or weather, then the new population of monkeys will have different survival pressures than the original population and their offspring will begin to exhibit different physical characteristics that reflect those different pressures. Over a long enough time frame they can become very unique and distinct from the other population back in the original location.
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