Boreal mammals in the Great Basin have long been viewed as island-bound Pleistocene relicts because they occupy island-like patches of montane habitat separated by desert lowlands that presumably are impermeable to dispersal. Recent work, however, raised the possibility that dispersal among mountain ranges is an important process in the biogeography of boreal mammals in the Great Basin. We test this proposition using genetic variation in a representative species, the yellow-bellied marmot (Marmota flaviventris). A total of 332 marmots was sampled from 10 ranges and genotyped at six microsatellite loci. If the intervening desert lowlands are impermeable barriers to dispersal, then there should be no relationship between genetic distance and geographic distance among mountaintop populations, and genetic diversity should be diminished because gene flow would not be available to replace alleles lost over thousands of generations of isolation. Our results did not support these predictions. There was a strong correlation between genetic and geographic distance, demonstrating an isolation-by-distance pattern, and genetic diversity was high. Our results suggest that marmot populations in the Great Basin may be linked by dispersal, providing a mechanism to replenish genetic variation lost by drift. However, global climate change over the next several decades could make the desert lowlands more difficult to traverse, eventually transforming the boreal faunas of Great Basin mountaintops into the isolated relicts they were originally portrayed to be.
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