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.
