What was once a continuous population is divided into two or more smaller populations. This can occur when rivers change course, mountains rise, continents drift, or organisms migrate. The geographic barrier isn't necessarily a physical barrier that separates two or more groups of organisms — it might just be unfavorable habitat between the two populations that keeps them from mating with one another.
University of California Museum of Paleontology's Understanding EvolutionPhoto credit: http://www.goldiesroom.org/
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