Desert dwellers offer evidence that genes carried by an individual store information that literally reaches back millions of years

Sitting in my campsite at Goblin Valley State Park, I saw an antelope ground squirrel standing erect on its back feet, which I found amusing. I soon found that this was a common posture evoked by vigilance. Antelope ground squirrels are in the genus Ammospermophilus, which has five species, all in North America. I was watching white-tailed antelope ground squirrels, A. leucurus, the only antelope ground squirrel in Colorado and Utah.

Antelope ground squirrels (AGS) occur primarily in deserts, including Great Basin, San Joaquin, Mojave, Peninsular, Sonoran and Chihuahuan. They also occur in dryland environments like sagebrush communities and some grasslands. Most species of ground squirrels hibernate, but living in relatively warm and dry environments allows AGS to be active year round.

AGS have several adaptations that allow them to live in the deserts of the western United States and Mexico. Later that day, in the heat of the afternoon, AGS were walking with their white tails coiled above their backs to shed their own portable shade. They would also linger in the shade of a piñon pine, dumping heat by stretching out their legs and pressing their bellies onto the soil. This posture is used frequently in their burrows, between bouts of foraging on the surface. Their body temperatures can rise to 108 to 110 degrees F without damage, much higher than most mammals. 

AGS are adapted to deserts or drylands and A. leucurus occupies the greatest distribution, including Oregon, Idaho, California, Nevada, Utah, Colorado, Arizona, New Mexico and the Baja California Peninsula. Background reading turned up a paper in a scientific journal that nicely demonstrated, with AGS, how biologists can utilize DNA sequences to infer an evolutionary tree of the genus, and to not only estimate the date that the genus first arose but also infer when and where each species arose. 

From 10 million years ago to the end of the Miocene, 5.33 million years ago, a single lineage sustained the ancestors of AGS, but approximately 4 million years ago, as deserts were spreading and developing in the Southwest, the lineage split into three clades. That is, from a solitary trunk the tree of AGS sprouted three branches.  A. interpres evolved east of the Sea of Cortez, A. leucurus south ranged from the southern tip of Baja to the middle of the peninsula and A. leucurus north ranged from the middle of Baja to Oregon and Idaho. 

Fewer than 1 million years ago, another three species evolved. Pioneers from the leucurus south clade colonized two small islands east of Baja in the Sea of Cortez and evolved into A. insularis. The leucurus north form spread into the San Joaquin Desert in California and evolved into A. nelsoni, and subsequently the AGS in Arizona and northern Mexico evolved into A. harrisii. A. leucurus still ranges from the southern tip of Baja to Oregon and Idaho, but within A. leucurus nine subspecies are recognized today.

Dates on the AGS phylogenetic tree were estimated with mutation rates in three genes and with fossil data. A. insularis, A. harrisii and A nelsonii evolved recently, with an average of 0.32 million years ago. On a different continent, modern humans evolved around 0.20 to 0.30 million years ago—approximately the same time.

At first, the differentiation of A. leucurus into northern and southern forms or clades seems curious, but similar vicariances or taxonomic boundaries have been noted in systematic and biogeographic studies of other mammals, birds, fish and insects. The barrier has been attributed to the Vizcaíno Seaway, which is now the Vizcaíno Desert. While systematists agree that there was a barrier to gene flow near the middle of the Baja Peninsula, estimates from different studies yield different estimates, which vary from 1 to 3 million years ago. One description of the modern desert mentions multiple marine terraces, but another states flatly that there is no convincing evidence of an open, freely flowing seaway. Perhaps the marine terraces were formed by recurrent, ephemeral lagoons or marshes that were sufficient to disrupt gene flow.

Studies like this one emphasize the point that the genes carried by an individual store information that literally reaches back millions of years. Historical biogeographers working with genetic data in animals or plants or microbes can peer through the roiling mists of time to infer relationships among species, to detect speciations and extinctions and to map the migrations of species driven by glacial cycles. Similar techniques to those used in this study of AGS were used to map the migration routes that brought humans from southern Africa to every continent, archipelago and island in the world. Furthermore, our genome carries the evidence that humans hybridized with Neanderthals in Europe and the Middle East and Denisovans in Siberia.

Jeff Mitton is a professor emeritus in the Department of Ecology and Evolutionary Biology at the University of Colorado Boulder. His column, "Natural Selections," is also printed in the Boulder Daily Camera.

Did you enjoy this article? Subscribe to our newsletter. Passionate about ecology and evolutionary biology? Show your support.