WASHINGTON - Finches on the Galapagos Islands that inspired Charles Darwin to develop the concept of evolution are now helping confirm it �?by evolving.

A medium sized species of Darwin's finch has evolved a smaller beak to take advantage of different seeds just two decades after the arrival of a larger rival for its original food source.

The altered beak size shows that species competing for food can undergo evolutionary change, said Peter Grant of Princeton University, lead author of the report appearing in Friday's issue of the journal Science.

Grant has been studying Darwin's finches for decades and previously recorded changes responding to a drought that altered what foods were available.

It's rare for scientists to be able to document changes in the appearance of an animal in response to competition. More often it is seen when something moves into a new habitat or the climate changes and it has to find new food or resources, explained Robert C. Fleischer, a geneticist at the Smithsonian's National Museum of Natural History and National Zoo.

This was certainly a documented case of microevolution, added Fleischer, who was not part of Grant's research.

Grant studied the finches on the Galapagos island Daphne, where the medium ground finch, Geospiza fortis, faced no competition for food, eating both small and large seeds.

In 1982 a breeding population of large ground finches, Geospiza magnirostris, arrived on the island and began competing for the large seeds of the Tribulus plants. G. magnirostris was able to break open and eat these seeds three times faster than G. fortis, depleting the supply of these seeds.

In 2003 and 2004 little rain fell, further reducing the food supply. The result was high mortality among G. fortis with larger beaks, leaving a breeding population of small-beaked G. fortis that could eat the seeds from smaller plants and didn't have to compete with the larger G. magnirostris for large seeds.

That's a form of evolution known as character displacement, where natural selection produces an evolutionary change in the next generation, Grant explained in a recorded statement made available by Science.

The research was supported by the National Science Foundation.