Darwin's finches, with their varied beaks, are often cited as a perfect example of how new species arise by exploiting ecological niches. Now developmental biologists have added a new twist to this classic story. Two research teams have discovered that a protein normally associated with the development of the skull and other bones is one of the molecules that tailors the shapes of beaks.
While exploring the Galápagos Islands in 1835, Charles Darwin was intrigued by the varying shapes and sizes of the birds' beaks. Each beak appeared to be specialized for a task, such as cracking seeds or drinking nectar. Once Darwin formulated his ideas about evolution, he realized that these birds exemplified the principles he was proposing, and he described the finches in his famous Origin of Species.
Aided by Princeton University field biologists Rosemary and Peter Grant--renowned for their studies of these Galápagos birds-- developmental biologist Clifford Tabin of Harvard Medical School in Boston and colleagues collected eggs of six species. Three species, the ground finches, have stout bills for cracking seeds; the other three, the cactus finches, sport the slender, pointed bills needed for retrieving nectar.
The researchers looked at finch embryos at different points in development, documenting when and where the genes for 10 growth factors were expressed among the six species. Only one--a signaling molecule called bone morphogenic protein 4 (BMP4)--distinguished ground finch beaks from cactus finch beaks. The two groups of birds differed in both the amount of BMP4 and the timing of its activity. The ground finches, with larger beaks, make more BMP4 protein at an earlier stage, Tabin explains. And each ground finch species had its own distinct pattern of BMP4 expression, the team reports in the 3 September issue of Science.
In the same issue, a team led by Cheng-Ming Chuong, a evolutionary developmental biologist at the University of Southern California (USC) in Los Angeles, describes BMP4's role in building beaks in chickens and ducks. Different shapes arise depending on where and when BMP4 is turned on during development, they report.
The two groups' results provide a window into the molecular basis of diversity, says Dolph Schluter, an evolutionary biologist at the University of British Columbia in Vancouver, Canada. Jeff Podos, a behavioral ecologist at the University of Massachusetts, Amherst, agrees: "It translates genetic variation into something we can sink our teeth into," he says. "Maybe we are beginning to understand something about [morphological] plasticity.” Darwin would be pleased.