| 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. |
|
| • |
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. |
|
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. |
