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Evolution Comes in Bursts, Gene Study Finds

Brian Handwerk
for National Geographic News
October 5, 2006
 
Evolution is not always a gradual process but often a series of genetic fits and starts, a new report suggests.

Looking back over evolutionary "family trees," it appears that adaptations—such as changes in skin color or beak shape—occur in spurts, the study says. And those spurts seem to occur when new species arise.

This suggests that genetic mutations are more likely to succeed—to be passed on to future generations and gradually adopted across a species—when a species is relatively young and trying to find its place in the world.

These times of supercharged evolution are separated by long stretches with relatively few adaptations, according to the study, which looked at genetic histories of plants, animals, and fungi.

(See our quick overview of human genetics.)

"It seems to be the case that speciation can accelerate evolution," said Mark Page, a professor of biological sciences at the University of Reading in the United Kingdom.

"There's something special about it."

(Related: "'Instant' Evolution Seen in Darwin's Finches, Study Says' [July 14, 2006].)

Punctuated Equilibrium

The controversial concept of "punctuated equilibrium"—the idea of golden ages of evolution separated by "calmer" times—arose from studies of the fossil record more than three decades ago.

Instead of studying fossils for their new report, Page and his colleagues examined the evolutionary family trees of 122 species—charting their genetic changes over millions of years.

Their report, to be published in the journal Science tomorrow, compares the genetic changes in species (changes not drastic enough to qualify is indicators of entirely new species) to the number of "speciation events"—the debuts of new species—in those family trees.

The team found that speciation and genetic divergence are related in about a third of the 122 plants, animals, and fungi studied.

"One-third is a big number," Pagel said. "Detractors might have said that number was zero. We've shown, I think, that this is at least common. It doesn't have to happen every time, but it happens—in plants, animals, and fungi."

New Species May Boost Evolution

Pagel suggests that the link between speciation and accelerated evolution may have roots in traditional Darwinian theory.

The correlation appears more common in plants, perhaps because so many plants trade pollen, hybridize, and readily form new species.

"When [new species arise] they start out in really small groups of a few individuals, and that's where we think the mechanisms that cause the acceleration of evolution are most likely to occur," Pagel said.

"Genetic drift" is one such mechanism.

If a population splits into two groups, and one group is very small, a single random genetic change in the small group can quickly make that entire group diverge rapidly.

A change in skin color, for example, can quickly become a common trait among a small population of a dozen or so individuals.

Larger groups would require many such chance events to produce the same kind of species-wide change.

Another possible evolution-boosting mechanism is the role of new species within existing ecosystems.

"When a new species forms, there's a very good chance that it will occupy a different niche from the ancestral species," Pagel said.

"If it didn't, it would be competing directly with the ancestral species, so new species often have to find new ways of making a living."

New niches may require new traits or skills. Therefore, a greater number of genetic mutations may prove successful, resulting in more adaptations living on in the species and getting passed on to later generations—natural selection in overdrive.

"We think that at speciation there's a rapid burst of adaptive evolution, and once [the new species] has occupied a niche, it can settle down and there's no longer as much pressure for rapid adaptive change."

Genetic results suggest that some 22 percent of all genetic evolution could be associated with speciation—even though such events are rare and short-lived.

"We found that to be a huge number," Pagel said.

Underscoring how rarely successful new species appear, he added, "Consider that, along the lineage leading from our common ancestor with chimpanzees to modern humans, there may be something like ten events of speciation or fewer but perhaps 5 million or more years of gradual evolution."

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