The bigger the bird's beak, the easier it is to crack open the seeds' coatings.
The already smaller-beaked medium ground finch couldn't keep up with the newly arrived large ground finch, which is about twice as big and dominates feeding grounds.
Apparently in response, the medium ground finch evolved to have an even smaller beak, making the species more adept at eating small seeds that didn't interest the larger finch.
"This is a phenomenon known as character displacement," Peter Grant said.
"It is a very important one in studies of evolution, because it shows that species interact for food and undergo evolutionary change which minimizes further competition."
The researchers say they have seen other types of evolution in action in Galápagos finches before.
But this was the strongest shift they've seen in their 33 years of study, the scientists say.
Nutcrackers, Woodpeckers, Vampires, Oh My!
The Galápagos Islands' 14 species of finches all evolved from one ancestral species, which arrived from the South American mainland about two to three million years ago.
That original species branched out into many others, with each one specialized for different roles.
The woodpecker finch, for example, has evolved to the point where it can drill holes in trees, while the vampire finch drinks other birds' blood (watch video of vampire finches).
Ironically, naturalist Charles Darwin missed signs of evolution among these finches during his 1831 visit to the Galápagos.
Only later, with the help of other collectors and scientists, was he able to see how evolution was responsible for the variety of finches. (Read "Was Darwin Wrong?" in National Geographic magazine [November 2004].)
Since then, the 1982 arrival of the large ground finch on Daphne is the first known instance of a new finch arriving in the Galápagos.
"The event we observed is the only one that we know about, the only establishment of a new breeding population anywhere in the archipelago," Peter Grant says.
"Once this happened before our eyes, we realized we had a very unusual and potentially very important event to follow."
The two bird species immediately began competing for larger seeds.
The situation reached a tipping point when a severe drought hit the island in 2003 and 2004.
Both finches suffered, since there were far fewer seeds overall. The dominant large ground finch ate most of the available large seeds.
"With the near removal of the supply of large seeds, the large-beaked birds [among] the medium ground finches did not have enough food to survive," Peter Grant said.
"They died at a faster rate than the small-beaked members of the population."
The effects of competition are apparent when this event is compared to a drought in 1977, before the large ground finch arrived on the island, the researchers argue.
During the earlier drought the medium ground finches' average beak size actually increased.
Jonathan Losos is an evolutionary ecologist at Harvard University in Boston, Massachusetts, who was not involved with the Grants' work.
"This study will be an instant textbook classic," he said.
"The most intriguing aspect of the study is its nuanced understanding of how and when character displacement occurs," Losos added.
"It supports suggestions by the Grants and others that [natural] selection will be most intense during crunch times."
David Pfennig at the University of North Carolina at Chapel Hill agrees that the study has important implications.
For Pfennig, the study's greatest surprise was "the apparent speed with which the character displacement occurswithin a single year!"
Usually we think of evolution as being a slow grind, he says.
But, Pfennig added, the study suggests that evolution due to competition between closely related species "paradoxically may often occur so rapidly that we may actually miss the process taking place."
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