Carl Zimmer - Fast-Paced Evolution in the Andes

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Fast-Paced Evolution in the Andes

By CARL ZIMMER
Published: November 7, 2013

Páramos, mountainous grasslands that flourish thousands of feet above sea level in the Andes, are hot spots of evolutionary change. (Santiago Madriñán Restrepo)

In 1799 the great naturalist Alexander von Humboldt and his companions set out from Caracas, Venezuela, to climb the Andes. They struggled up a mountainside enveloped in mist so thick they had to clamber over rocks by hand. When the fog cleared, von Humboldt was left astonished by the view. Vast grasslands stretched all around him, home to an astonishing number of different trees, shrubs and flowers.

“Nowhere, perhaps, can be found collected together, in so small a space, productions so beautiful and so remarkable in regard to the geography of plants,” he later wrote.

Von Humboldt had stumbled into a remarkable ecosystem, known as a Páramo. Páramos blanket the Andes in Venezuela, Ecuador and Colombia, growing at altitudes 9,200 to 14,800 feet above sea level.

“They’re like islands in a sea of forest,” said Santiago Madriñán, an expert on Páramos at the University of the Andes in Colombia. All told, Páramos cover about 13,500 square miles — an area the size of Maryland. In that small space, Dr. Madriñán and other researchers have found 3,431 species of vascular plants, most of them found nowhere else on Earth. The Páramos are home to strange variations on familiar forms, such as a daisy known as Espeletia uribei that grows as tall as trees.

But according to a new study, the Páramos are even more remarkable than von Humboldt could have realized. They are the fastest evolving place on the planet.

Scientists have long known that in certain spots, evolution runs faster than normal. The Galápagos Islands, for example, are home to some 13 species of Darwin’s finches, which all evolved from a single group of birds that originally colonized them. The archipelago is just a few million years old, however, which means that all their diversity has evolved in a geologically short period of time.

In recent years, scientists have identified other regions where evolution is running fast. To measure its speed, researchers have looked at the DNA of species living in each place. The longer it has been since two species diverged from a common ancestor, the more time each lineage has had to accumulate mutations. Young species have relatively few mutations.

Dr. Madriñán has studied Páramos for over a decade, and he’s long suspected that evolution is running fast in them as well. “I don’t know if it was a hunch or what it was, but when you study the Páramos, it’s a marvelous place,” he said.

The geology also gave support to his hunch. The Andes started forming tens of millions of years ago, but it wasn’t until 2.5 million years ago that the northern Andes rose above the elevation where trees can survive. Only then could all the diversity of the Páramos emerge.

To calculate the speed of evolution in the Páramos, Dr. Madriñán and his colleagues surveyed 13 different lineages of plants that grow there. They estimated the rate at which species had split from each other in each lineage, and then combined those estimates into a single average. The scientists then looked at data on plants that grow in other fast-evolving places, such as Hawaii and the Mediterranean coast.

The results surpassed Dr. Madriñán’s suspicions. The Páramos weren’t just home to fast evolution, it turned out. Of the eight places he and his colleagues compared, the Páramos are evolving the fastest of all.

Other experts on evolutionary rates are intrigued by the new study, which was published last month in Frontiers in Genetics. “Of course these results are still very preliminary,” said Luis Valente of the University of Potsdam, noting that scientists have only sampled a few groups of plants from each evolutionary hot spot. But Dr. Valente thought the study persuasively demonstrates that the Páramos are a special place. “This may be a region where evolution is proceeding at a very fast pace, and where many new species may still be in the process of being formed,” he said.

Michael Sanderson of the University of Arizona thinks the contest won’t be settled definitively until biologists can draw large-scale evolutionary trees. “Then we’ll finally sort out the hottest hot spots,” he said.

Dr. Madriñán suspects that the peculiar climate of the Páramos is responsible for their fast evolution. Because the grasslands are at the equator, they are bathed in sunshine year-round. But to take advantage of that ample energy, the plants also have to contend with cold temperatures and harsh ultraviolet rays, not to mention weather that can turn on a dime. “You may be in total mist and then half an hour later you are in total sunshine,” Dr. Madriñán said.

When plants began to spread into the newly formed Páramos, Dr. Madriñán suspects, they evolved many solutions to surviving there. They specialized on different niches, from damp bogs to dry hillsides and stands of shrubs and trees. Páramo plants also evolved a wide range of defenses against the elements. Espeletia uribei, the daisy tree, grows white hairs on its flowers to protect them from damaging ultraviolet rays, while covering its stem with a thick layer of dead leaves to keep it warm.

Dr. Madriñán and his colleagues are now exploring the history of the plants to see if they can explain their remarkable speed. “Páramos are the new laboratory to study evolution happening at incredible rates,” he said.


~ A version of this article appears in print on November 12, 2013, on page D3 of the New York edition with the headline: Fast-Paced Evolution in the Andes.