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Summary
Multicellular animals and plants may face geographic barriers to migration and dispersal, enabling isolated populations to diverge into different species. Because of their small size and enormous abundance, unicellular organisms are thought to be able to disperse unhindered around the world. In his Perspective, Fenchel discusses studies published here ( Whitaker et al.) and elsewhere, which suggest that populations of thermophilic microorganisms have diverged over evolutionary time as a result of geographic isolation.
