Evolution caught in the act

Competition between two species of finch in the Galápagos has caused the beak size of one species to shrink, and scientists have watched it happen. Detailed observations of the birds, which Darwin famously studied while formulating his theory of evolution, have provided one of the best descriptions of a characteristic trait evolving in the wild.

In a paper appearing in this week's issue of Science1, Peter and Rosemary Grant, both biologists at Princeton University, New Jersey, describe the struggle between the medium ground finch (Geospiza fortis) and the large ground finch (Geospiza magnirostris)....

The two species lived fairly happily together for many years, until two factors forced the birds into harsh competition.

The population of large finches grew, until there was enough of them to be battling with the medium finches, who were also going after Tribulus seeds. And then two years of drought, in 2003 and 2004, dramatically reduced the food supply, causing both populations to plummet as birds died of starvation.

In these mean conditions, the medium finches with smaller beaks had an advantage over those with big beaks, as they could more easily suck up smaller seeds that weren't being gobbled by the large finches.

"Small-beaked birds survived better than the large-beaked birds, to a strong extent, during the drought," says Peter Grant. And that trait was then passed down to the next generation.

In 2004 and 2005, the Grants observed a strong shift towards smaller beak size among the medium ground finch. The birds' feeding patterns changed too: they went for the large seeds only half as often as in previous years.

This kind of evolution, in which a characteristic of two similar species diverges due to competition over resources, is called character displacement. The idea has become widely accepted thanks to a number of well-detailed studies, says Jonathan Losos, a biologist at Washington University, St Louis.

•	Evolution caught in the act Smaller beaks in Galapagos finches make finding food easier Nature.com (UK), Thursday, July 13, 2006 Byline: Heidi Ledford