Pronghorn antelope evolved roaming across large tracts of land undivided by roads and fences. But the highways that now crisscross their grassland habitats in the Southwest form barriers to mingling and interbreeding. They’re like giant swim-lane dividers in the gene pool.

Over time, limiting pronghorn genes to swimming in just one lane lowers genetic variability. That reduces fertility and limits the ability of isolated populations to cope with drought and deadly diseases like Bluetongue virus.

Biologists have long known that pronghorn don’t like to jump fences. In collaboration with landowners such as the Babbitt Ranches, they’ve been raising the bottom wire of livestock fences so that pronghorns can slip underneath. Moving fences back from roads also makes the antelope more likely to wriggle under.

But what wildlife managers haven’t understood so well is what other aspects of roads affect pronghorn behavior. Game and Fish biologist Scott Sprague has been tracking the movements of radio-collared pronghorn in northern Arizona. By tracking locations, he can monitor how roads form barriers—and by comparing the genetic profiles of pronghorn populations, he can assess how split up those populations are.

He’s found that busy Highway 89 forms the largest barrier to gene flow. Route 64 south of Grand Canyon is a lesser barrier. And pronghorn populations on either side of the lightly traveled Highway 180 show few genetic differences.

For busy highways, then, some biologists advocate installing crossing structures that allow pronghorns to move across the entire landscape – keeping their genes swimming healthily throughout the pool.