April 2012

If you’d visited Tuolumne Meadows in Yosemite National Park 100 years ago, you probably would have encountered the alpine chipmunk, Tamias alpinus. Today, however, park visitors will have to hike up a nearby mountain to see one of these critters. That’s because this species is sensitive to temperature — and over the last hundred years of global climate change, Yosemite has warmed by about 5.5 degrees Fahrenheit. As the temperature increased, the chipmunks retreated to higher and higher elevations where it was cooler. Today, they occupy a fraction of their original range. If climate change continues, they could be squeezed right off the tops of their mountains and out of existence. Now, new research has revealed that the chipmunks (and probably other temperature-sensitive species) are losing not just territory and individuals, but genetic variation as well — a hidden threat that adds to the risk of extinction.

Where's the evolution?

Genetic variation refers to the idea that different individuals in a population are likely to carry different genetic sequences for corresponding regions of the genome. For example, in humans, one individual may carry genetic sequences that code for type A blood, and someone else may carry sequences that code for type O blood. Similarly, people may carry different sequences in regions of the genome that don’t code for anything at all. Both are examples of genetic variation. Populations that have fewer genetic differences from one individual to the next have lower levels of genetic variation.

In the case of the chipmunks, scientists were able to study DNA from museum specimens collected in the early 1900s and compare those to samples from chipmunks collected in the last 10 years. The modern chipmunks were much more genetically homogenous — i.e., had fewer different genetic sequences at the seven DNA regions studied — than their historic counterparts. Though the study only looked at a few non-coding regions, it is very likely that all parts of the chipmunks’ genome — those that help produce important traits, as well as those that don’t code for anything — have lost genetic variation.

Climate change is the obvious culprit. As shown on the maps below, the range of the alpine chipmunk (and almost certainly, its population size) has decreased significantly over the last 90 years as the climate has warmed. And of course, as the population lost individuals, it also lost the genetic variants they carried, reducing its level of genetic variation. For comparison, the lodgepole chipmunk, T. speciosus, is not as finicky and has maintained its historic range despite the heat. When scientists studied the lodgepole chipmunk, they found similar levels of genetic variation in historic and modern samples. Since the main difference between the two species seems to be their response to rising temperatures, climate change is the most likely cause of reduced genetic variation in the alpine chipmunk.

What does reduced genetic variation mean for the future of the alpine chipmunk? There are several implications:

• First, it corroborates the idea that the population is shrinking and becoming more fragmented — i.e., small subpopulations are being cut off from one another. Both of these factors increase the species’ chance of extinction.
• Second, since genetic variation is the raw material of evolution, it reduces the ability of the species to evolve in response to future changes in its environment. Natural selection acts on genetic variation, increasing the frequency of those gene versions that are advantageous in a species’ current situation. If the right gene versions aren’t present in a population, adaptation will not occur. So for example, if the chipmunks were to face a new, deadly pathogen introduced by an invasive species, there may not be any individual in the population carrying gene versions that allow it to resist the pathogen, and hence, the population could be wiped out. In a more genetically variable population, this sort of thing is less likely to happen.
• Third, low levels of genetic variation can lead to decreased survival and reproductive rates through inbreeding depression. When individuals in a population are very similar to one another genetically, it can mimic a situation in which close relatives mate and can lead to reduced fitness of offspring (see boxed text). If this occurs, the chipmunks would suffer poor health and diminished reproductive rates as a result of their low levels of genetic variation.

All three of these factors have the potential to increase extinction risk for the chipmunks.

Of course, at the moment, the chipmunks seem to be doing fine and are not listed as a threatened species. But the new research highlights the risks they are likely to face in the near future, as well as the risks faced by many other species whose ranges are contracting due to the warming climate. Alpine chipmunks are one of the few species for which we can easily examine changes in levels of genetic variation over the last century because we happen to have the right museum specimens. Many other high-altitude species have been moving uphill as well and are likely facing the same double-whammy that the chipmunks are — decreased population size and the hidden side effects of reduced genetic variation.