SCIENCE / EVOLUTION : Sunken Isle Discovery Bolsters Darwin Theory
Scientists have discovered evidence that could resolve a long-simmering feud that has grown out of Charles Darwin’s historic study of the evolution of species on the Galapagos Islands.
The islands are only about 3 million years old, and some biologists have argued that that is not enough time for the wide diversity of wildlife to have evolved there, as Darwin’s work suggested.
But geologists aboard the Thomas Washington, a Scripps Institution of Oceanography research vessel, have found something that Darwin could not possibly have known: There were other, much older islands in the region that have long since slipped beneath the ocean, and some of the creatures on Galapagos that needed more time to evolve may have begun their ancestral journey on islands that are no longer there.
The finding does not prove Darwin’s theory of evolution, but it does make the theory compatible with the current understanding of genetics and how long it would have taken for some species to diverge into two separate groups.
“The controversy may disappear,” Hampton L. Carson of the department of genetics and molecular biology at the University of Hawaii wrote in an analysis of the research, published in today’s issue of the journal Nature.
Darwin was only 22 when he began an expedition aboard the HMS Beagle in 1831 that was to revolutionize human thinking. Biologists thought then that species either had continued unchanged since their creation, or acquired characteristics that could simply be inherited by their offspring. But the young naturalist found evidence during the voyage that made him doubt both theories, and he concluded that all plant and animal species change over time to meet environmental demands.
On the Galapagos Islands, off the coast of South America, Darwin studied birds that were quite distinct from those found on the mainland. Some were also different from others found on the 10 widely separated islands. Darwin concluded that the birds had evolved through natural selection, or “survival of the fittest,” and the evidence from the islands formed a key part of his seminal research published a quarter of a century later.
As other experts studied the research many years later, they found no proof that the birds that Darwin studied could not have evolved during the 3-million-year history of the present islands. But there were other creatures on the islands, including iguanas, whose “genetic time clocks” would have required much longer for them to evolve, Carson said.
How could that be if the islands are only 3 million years old?
Creationists and other critics of the theory of evolution have cited such problems as evidence that Darwin was simply wrong.
But David M. Christie, a geologist at Oregon State University in Corvallis, and several colleagues returned to the Galapagos aboard the Scripps vessel in the summer of 1990. They found evidence that could explain how the critters found time to evolve.
“We really went there to study volcanism,” Christie said in a telephone interview.
The Galapagos Islands were created in much the same way as the Hawaiian Islands. In both cases, a “hot spot” deep inside the Earth sends plumes of hot mantle boiling toward the surface, where it burns through the crust and forms volcanoes. Each “hot spot” remains in a fixed position relative to the Earth, but the huge tectonic plates that make up the crust are constantly moving.
As a result, the hot spot constantly produces new volcanoes as the crust passes over. The youngest volcanoes are directly over the hot spot, and older islands are left behind as the plate moves on.
Scientists have long understood that process, largely because of extensive research on the Hawaiian Islands and a series of subsurface hills--called seamounts--that stretch northwestward across the Pacific. Similar evidence has been found near the Galapagos, but it was not clear whether the small seamounts found there had ever been tall enough to reach above the surface of the Pacific and become islands.
Christie said images created with sonar equipment aboard the Scripps vessel reveal that some of the old seamounts have terraces that look as though they were cut by waves, suggesting that the mounts once existed as islands. But that was not the most conclusive evidence. The vessel also has dredging equipment that allowed the scientists to retrieve rocks from the slopes of the seamounts.
“The real clincher is we found lots of beautiful rounded pebbles like you would find on an Oregon beach,” he said. “You can’t make those in the deep ocean.”
Instead, he said, they must have been formed by waves washing against the seamounts when they pierced through the Pacific.
The seamounts are part of an extremely complex mosaic of tectonic activity, with some plates pushed in various directions since the hot spot first formed about 80 million years ago, Christie said.
Today, the hot spot is beneath the Galapagos’ youngest island, Fernandina. The small Nazca plate is moving east, so in time other islands will be formed to the west of Fernandina, which could gradually erode and subside into the Pacific, much as other islands apparently have done in the past.
But now, because of the recent work, scientists may have finally learned why some species on the Galapagos seem to be so old, and some so young.
“There is no reason why the birds and the lizards should show identical patterns of divergence and descent,” Carson wrote in a companion piece to the research. “One lineage may be indeed new, and the other old.”
An Island Find
Scientists have found evidence near the present Galapagos Islands that there were many older islands that have subsided beneath the ocean.
A “hot spot” deep in the Earth created the volcanic islands as the crust, made up of tectonic plates, passed over. The hot spot, which remains fixed, is responsible for forming the ancient islands now below the ocean’s surface. That hot spot is currently beneath the youngest island, Fernandina and the Nazca tectonic plate is moving east.
