Study: Darwin's Finches Evolving Fast

July 13, 2006 — A Galapagos finch that helped reveal the origins of species to Charles Darwin has now undergone a spurt of rapid climate-driven evolution, biologists have reported.

The "medium ground finch," a.k.a. G. fortis, of Daphne Island was nudged, and then shoved, to evolve a smaller beak by the combination of competition from another finch that arrived on the island more than 20 years ago and more recent drought conditions.

"It happened very fast," said biologist Peter Grant of Princeton University. He and Rosemary Grant have published their discovery in the July 14 issue of the journal Science.

In fact, it happened in a single bird generation, Grant explained.

The evolutionary nudging began when some larger finches settled on Daphne during an exceptionally wet El Nino in 1982.

In the years since, the larger G. magnirostris finches have been eating most of the larger, thorny seeds of the island’s puncture vine plants and steadily pushing the smaller finches to rely on smaller seeds from other plants.

As a result, G. fortis birds with smaller beaks that did not compete with the larger birds did better, and were more likely to leave offspring behind. That essentially enriched the gene pool with small beak genes and led to more G. fortis with smaller beaks.

But the matter really came to a head in 2003 and 2004, when little rain fell on the island and seeds of any kind were scarce.

"Most of the birds that had large beaks before the drought disappeared," said Grant. That included almost all of the recently arrived G. magnirostris and any remaining G. fortis with especially large beaks.

The only birds that survived enough to mate and produce offspring in 2005 were the G. fortis with smaller beaks and an ability to exploit small seeds like those of the drought-tolerant Optunia cactus.

In Darwinian jargon, the small-beaked birds were naturally "selected" for perpetuating the species, just as a dog breeder might select for speed in a greyhound.

"For years, this has been the classic textbook example of rapid evolution," said David Skelly, an ecologist and rapid evolution researcher at Yale University, referring to the competitive pressure on G. fortis by the larger beaked G. magnirostris.

"When I was a student, (Grant’s) work was sometimes taught as the exception to the rule," said Skelly.

That is, normally evolution is thought of as slow and gradual in large animals like fish, birds, reptiles and mammals. Beak sizes changing measurably in just decades seemed awfully fast. The Galapagos finches were considered an extreme case of quick evolution caused by an extreme environment.

"Now it appears that the Grants' work shows a pattern that is likely to be widespread," said Skelly, who has studied rapid evolution of amphibians in response to global warming. "Environmental changes severe enough to cause sharp population declines, as seen with the finches, are also selection events."

As more and more species undergo the stresses of climate change, more cases of rapid evolution can be expected, Skelly said. It’s not likely to save most species facing the climatic bottleneck, of course, but it does give a few a fighting chance, he explained.

"I don't think there is any reason to suspect this is an unsual occurrance," said biologist Richard Shine of the University of Sydney in Australia.

Shine has charted the rapid evolution of longer legs in invasive cane toads in Australia, as well as adaptive changes in native snakes where the toads have invaded over the past few decades.

We'd see more evidence of rapid evolution if there was more support for long-term field studies like that of the Grants' 30-year work on the finches, he said. "It's incredibly difficult to maintain these long-term studies."