The Grand Canyon as Evolutionary Force*

Certain spots on the globe are well known for their geographic isolation. The concept is easy to visualize simply by imagining an island. If an area is physically isolated from other spots very often there will be a drastic effect on the evolution of life in the region.

Two famous examples of geographic isolation are Australia and the Galapagos Islands. Australia is a continent-sized isle sequestered in its own area of Earth. Because of this extreme remoteness Australia is packed with bizarre flora and fauna, species which have diverged from common ancestors to exhibit unique qualities. For example, 70% of the world’s marsupial species are found only in Australia. Similarly, the Galapagos are known for their high number of endemic species, many of which were studied by Charles Darwin on his way to formulating the theory of evolution.

This type of evolutionary effect is known as allopatric speciation. It is defined as a “mode of speciation that occurs when biological populations of the same species become isolated from each other to an extent that prevents or interferes with gene flow.” Take 1000 kitties from a large population and dump them onto an island by themselves. Fast forward a large period. Come back and inspect the descendants and you are likely to find new species of kitties, new subspecies of kitties, or new traits in kitties not seen elsewhere.

But geographic isolation need not be confined to islands. All sorts of things can cause allopatric speciation, including mountain ranges, glaciers, bodies of water, human activity, or even enormous canyons. And thus we finally arrive at today’s topic: the Grand Canyon.

The Grand Canyon is many things; it is ineffable grandiosity; it is sublime beauty; it leaves all sane people who see it with a sense of immense wonder. In addition to these attributes, the Grand Canyon, in at least one situation, may have “caused” a new subspecies due to geographic isolation*. The behemoth is so large that a cute set of squirrels on the north rim is isolated from its ancestors on the south rim.

Meet the Kaibab Squirrel:

These adorable critters live nowhere else on our planet other than the Kaibab Plateau, which is bounded to the south by the north rim of the Grand Canyon. Most people rightly envision the area of the Canyon as arid desert, but the Kaibab Plateau (which shares a name with the top layer of rocks that from the Grand Canyon, Kaibab Limestone) is heavily forested. The plateau is cool and receives more precipitation, including snowfall, than the surrounding desert landscapes. Within the plateau is an area of approximately 20 by 40 miles where ponderosa pines thrive. Squirrels, as it turns out, love ponderosa pines. Every Kaibab squirrel in the world lives within this rectangle.

But Kaibab squirrels weren’t always Kaibab squirrels.

Soar across the great expanse of the Grand Canyon to the south rim. Here you will find many examples of Abert’s squirrel, which has a vast range from the southern Rocky Mountains all the way into Mexico. The Kaibab squirrel is a subspecies of Abert’s squirrels, a distinction which arose thanks to the geographic isolation of the Kaibab Plateau. Over 10,000 years ago the populations were the same. Somehow a group was isolated in the plateau and started to drift genetically from the main herd on the other side of the canyon.

You may have noticed the usage of an asterisk in the title of today’s article and in an above paragraph. As is often the case with science, in a never-ending pursuit of the truth, some disagreeing theories have arisen in this tale of cuddly critters. How did squirrels on opposite sides of the Grand Canyon become different from each other if they used to be the same? The answer might seem obvious, but sometimes obvious answers are wrong!

For a time scientists believed the Kaibab Squirrel was a distinctly new species, separate from Abert’s squirrel. Currently it is classified as a subspecies. The exact nature of the Grand Canyon’s role in this speciation is debated. Some scientists believed the canyon is the cause of the geographic isolation which caused genetic drift. It is easy to imagine this scenario; it’s a huge chasm. But others posit the geographic isolation is not due to the canyon, but instead to the Kaibab Plateau itself.

How could it not be the enormous rift in the ground that causes the isolation?

In this theory, after the last ice age, as temperatures rose, ponderosa pine stands were “forced” into higher elevations, such as the Kaibab Plateau. Where the pines remained, squirrels could live. The surrounding areas, including the other side of the Grand Canyon, lost all pockets of Abert’s squirrels, as the ponderosa pines could not adapt to the rising temperatures. Those Abert’s squirrels who survived in the Kaibab Plateau became the Kaibab squirrels of today, geographically isolated to the plateau. The rest of the wider zone’s Abert’s squirrels peaced out, moving elsewhere. Eventually the climate cooled again and ponderosa pines were able to spread to lower elevations, including the southern side of the canyon, at which time Abert’s squirrels arrived again.

In this scenario, it’s not that the canyon is so big that it keeps the populations from hanging out that mattered, but that for a time the only place an Abert’s squirrel could survive in this region was on the Kaibab Plateau. Those squirrels were isolated for long enough to become a new subspecies before their brethren were able to return to the general vicinity across the way.

If this theory is true, the popular notion that the Grand Canyon caused the geographic isolation isn’t exactly correct (other than its role in the exact limits of the plateau), but it can still be considered a force of evolution, as the gap between rims is so big that Kaibab Squirrels simply do not travel south of the canyon, where Abert and his squirrels wait for a reunion.