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Every species known today is linked through time to life that originated some 3.8 billion years ago. Since then, millions, if not billions, of times, populations have diverged into new species. The most commonly accepted mechanism for explaining that divergence is allopatric speciation.
Allopatric speciation is a three-step process that includes:
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| • | A geographic (physical) separation of a population by a natural barrier, such as a river or a canyon, or by a man-made barrier, such as a road that isolates a terrestrial population or a dam that isolates an aquatic population. |
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| • | Separation of the population for a long enough period that the two populations diverge in traits such as habitat use or mating habits. This divergence is a product of mutation, variation, natural selection, and genetic drift. |
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| • | Finally, the separate populations become so different they become reproductively isolated. Since they can no longer produce fertile offspring, they are considered to be different species. |
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Evolution above the species level (genera, orders, etc.) is simply speciation expanded over a much longer period of time, with increasing differences accumulating to very high levels.
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