The arrival of the large ground finch species on a Galápagos island in 1982 prompted its relative, the medium ground finch (inset), to evolve. Over just 20 years, the smaller species evolved a smaller beak to avoid competition for food, the first time this effect has been observed in action in the wild.
Photographs courtesy B. Rosemary Grant/Science
Evolution may sometimes happen so fast that it's hard to catch in action, a new study of Galápagos finches suggests. Researchers from New Jersey's Princeton University have observed a species of finch in Ecuador's Galápagos Islands that evolved a smaller beak within a mere two decades. Surprisingly, most of the shift happened within just one generation, the scientists say.
In 1982 the large ground finch arrived on the tiny Galápagos island of Daphne, just east of the island of San Salvador. Since then the medium ground finch, a long-time Daphne resident, has evolved a smaller beak—apparently as a result of direct competition with the larger bird for food. Evolutionary theory had previously suggested that competition between two similar species can drive the animals to evolve in different directions. But until now the effect had never been observed in action in the wild. In the new study, Princeton's Peter and Rosemary Grant closely tracked the two related species for decades and published their results the journal Science.
Changing Beaks, Changing Diet
For both finch species, the researchers note, feeding is a trade-off between effort and payoff. Both species preferentially eat larger seeds, which are harder for their nutcracker-like beaks to break open but hold a bigger reward inside. 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 successfully compete with the newly arrived large ground finch, which is about twice as big and dominates feeding grounds. Individuals among the medium ground finch that were able to specialize on smaller seeds because they had smaller beaks managed to obtain enough food to breed successfully. The competitive advantage of the larger beaked large ground finch resulted in greater success of the medium ground finches with the smallest beaks. A consequence of strong competition for the shared seed resource led to a greater divergence of beak size between the two species of finches.
Offspring of the medium ground finch inherited their success parents' smaller beaks, shifting the average beak size in the smaller direction. The large ground finch dominated the larger seeds and appeared not to take interest in the smaller seeds. A competitive situation that leads to the divergence of a characteristic of one or both species from its initial condition that results in the species specializing on different parts of the resource base is 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. 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.
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 occurs—within 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."