About three million years ago, the isthmus of Central America arose separating the Pacific from the Atlantic oceans. Even at only 50 kilometers across at the narrowest point, you might think this an insurmountable barrier for marine snails. You’d be correct for snails traveling under their own power, but not for snails employing the services (and intestinal tracts) of shorebirds, from which they emerge unscathed a surprising percentage of the time. How often did this interocean transfer occur? According to researchers from the Smithsonian Tropical Research Institute, UCLA and UC Santa Barbara, only twice.The researchers compared the mitochondrial DNA of horn snails on both the Pacific side (from California to Panama) and on the Atlantic side (Texas to Panama) of central America. In conjunction with molecular dating techniques, the data suggest that gene flow between the two sides occurred on just two occasions, first from the Pacific to the Atlantic 750,000 years ago, and again in the opposite direction about 72,000 years ago.
1940, the paleontologist George Gaylord Simpson dubbed these kinds of unlikely events ‘sweepstakes dispersals’. In some cases, especially on remote islands, a single fluke transfer of new species (by bird or storm) can completely alter an ecosystem forever.
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