Does mountain living slow rate of molecular evolution?

A study of hummingbirds living high in the Andes Mountains suggests that life at the top slows the pace of genetic evolution.

Findings from the study, which compares the DNA of 26 hummingbird species, is some of the first evidence to link an animal’s physical environment to the rate at which genes evolve, something traditionally attributed mostly to the size, mass or other feature of the animal itself.

“This adds elevation to the mix of factors that one might consider as contributors to rates of molecular evolution,” says Robert Bleiweiss, professor of zoology and the author of the recent study published in the Proceedings of the National Academy of Sciences.

In all animals, change is governed at the molecular level as genes change or mutate. Over long periods of time and many generations, these genetic changes accumulate and can manifest themselves in altered body plans or behaviors. But what initially sparks those changes at the genetic level is largely unknown.

The evidence that life at higher elevations slows the “molecular clock,” says Bleiweiss, was a surprise since “the more extensive subdivision and speciation of bird populations living at high elevations” predicts the opposite of what was found. Bleiweiss compared the DNA of hummingbirds living at a variety of elevations, from sea level to the equatorial snow line at more than 5,000 meters. Using a technique that combines the DNA of different species, Bleiweiss was able to compare and chart “genetic distances” between the amino acid base pairs that make up hummingbirds’ genetic material. Those distances, says Bleiweiss, have a direct correlation to the altitude at which a species lives, suggesting that the higher you go, the slower the molecular clock ticks.

While adding elevation to what is likely to be a stew of factors that govern molecular evolution, the study also confirmed that mass is “the overriding factor” for such change: “The larger you are or the higher you live, the slower the clock ticks. All of this makes a more interesting soup than DNA simply evolving in its own black box.”

The finding that elevation may influence the rate of genetic change in an animal hints, too, that the rigors of life at high altitudes may play a role in slowing the molecular clock. Colder temperatures in general, and the fact that some high-living hummingbirds enter a state of torpor when things get too cold, may exert an influence on physiology that, over time, is expressed genetically.

Moreover, available oxygen also decreases with altitude and — given the extraordinary physiology of hummingbirds — metabolic factors such as these could have an important influence on genetic mutation rates.

But whatever the cause of the slower molecular clock, the new findings provide evidence that the pace at which evolution marches may be dependent, to one degree or another, on environment. And those influences, Bleiweiss argues, may prove to be widespread and not limited to mountain peaks.