Natural barriers keep populations of organisms partly or entirely isolated from each other. They can be rivers, mountain ranges, or open grasslands; anything that, for a specific kind of organism, is unsuitable. Without barriers, genetic material—genes—tend to be passed back and forth between different populations of the same species. With shared genetic material, populations usually remain quite similar. With a barrier in place, though, the populations can evolve independently and may eventually become separate species.
The Rio Grande in New Mexico and Trans-Pecos Texas apparently is a good
barrier for packrats. The White-throated Packrat occurs west of the river while the
Eastern White-throated Packrat occurs on the other side. There is one instructive
exception, though. East of the River near El Paso, both species occur. How come? The
Rio Grande, before it took its present route, flowed east of the Franklin Mountains,
which, of course, supported the western species. Later, the river switched to its
present route west of the mountains, suddenly marooning a population of White-throated
Packrats east of the river. Talk about suddenly getting new neighbors!
Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.
Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.