Back in 1835, Charles Darwin was captivated by the incredible diversity in beak shapes among finches living in the Galápagos archipelago off Ecuador. Now, after sequencing the genomes of all of Darwin’s finches, researchers have pinpointed the genetic basis of their beak variation. The findings are published in Nature this week, which happens to be right around Darwin’s 206th birthday.
Galápagos finches are some of the most famous animals when it comes to studying natural selection and the evolution of new species. As Darwin wrote: “Seeing the gradation and diversity of structure in one small, intimately related group of birds, one might really fancy that from an original paucity of birds in this archipelago, one species had been taken and modified for different ends.”
With beaks that are remarkably suited to diverse habitats and food resources, finches are (literally) textbook examples of adaptive radiation. Different species from the genus Geospiza have different bill depths for cracking seeds of varying sizes and hardness. Platyspiza feed on buds and fruits, Camarhynchus snack on insects hiding in wood, Cactospiza use cactus spines to extract bugs from crevices, and Certhidea have thin, sharp bills for sipping on nectar and gleaning insects off twigs.
Fourteen of the currently recognized species evolved from a common ancestor in the Galápagos in the past 1.5 million years based on mitochondrial DNA dating, and a fifteenth species lives on Cocos Island off Costa Rica. Their relatively rapid radiation likely happened as a result of strong isolation, new islands thanks to volcanic activity, climate oscillations like El Niño, and varying sea level changes.
Now, a large international team led by Leif Andersson from Uppsala University sequenced the genomes of 120 birds representing all 15 of Darwin’s finches, as well as two closely related tanagers. The team found DNA regions that looked different in species with blunt beaks versus species with pointed beaks. Within that stretch of DNA, they identified a gene called ALX1, and its variants are strongly related to how sharp or dull the beak becomes. "This is an interesting example where mild mutations in a gene that is critical for normal development leads to phenotypic [observable] evolution," Andersson says in a news release. (This same gene has been linked to human facial and cranial development.)
What’s more, the team found evidence of gene flow—that’s when genetic material moves between species, resulting in the development of new lineages. Hybrids are often sterile, but it turns out, offspring of different finches can mate with birds from either of the two parental species. The resulting chicks might sound and look like one or the other parent species, but they’ll carry genes from both. Interspecies hybridization has helped them maintain their genetic diversity.
Furthermore, the gene varies among birds of the same species. Some medium ground finches (Geospiza fortis, right) on Daphne Major have blunter or pointier bills than others. Hybridization mixed the different ALX1 variants, allowing the species to undergo rapid evolution in response to drought during the mid-1980s. With sudden changes in food supply, a bird who exploits multiple sources (like seeds, insects, and nectar from cactus flowers) will survive to pass on their genes. "This is the genetic variation upon which natural selection can work," Princeton’s Peter Grant says in a university statement. He and his wife, Rosemary Grant, have studied Galápagos finches for 40 years.
And one last thing! The sharp-beaked ground finch (Geospiza difficilis) is actually three species, and the large cactus finch (Geospiza conirostris) may actually be two.
Images: B. R. Grant