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Separate species can arise by allopatric evolution after one population is forced into two by a natural event such as glaciation. This has been the classic explanation of the existence of numerous sister species of birds in the eastern and western regions of North America. In this issue, Klicka and Zink show that molecular data derived from mitochondria are not compatible with this scenario. Rosenzweig tells us in his Perspective how many of our previous beliefs about mechanisms of evolution are called into question by these results and how many are still valid.
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Michael L. Rosenzweig (1997) studied this question.
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