Genome Study Reveals 30 Years Of Darwin’s Finch Evolution
A landmark study on contemporary evolutionary change in natural populations released by an international team of researchers led by Texas A&M University professor Dr. Leif Andersson reveals that 45% of the variation in the highly heritable beak size of Darwin’s finches can be attributed to only six genomic loci (fixed positions on a chromosome). Among these is a supergene that comprises four genes, which was under strong natural selection as a result of a two-year drought.
The study, published on Sept. 29 in Science, examines one of the largest genomic datasets ever produced for a wild animal — comprising nearly 4,000 Darwin’s finches — to reveal the genetic basis of adaptation in this iconic group.
The results of this international effort paint a dynamic picture of how species adapt to changing environments. As the global environment continues to change, the finches of the Galápagos Islands provide a valuable window into understanding how birds, their genetic constitution, and their environment interact to shape the future of wild populations.
“One of the remarkable things we found is that only a few genetic loci explain a great deal of the variation in the beak of the finch,” said Andersson, the study’s senior author who holds a dual professorship at Sweden’s Uppsala University and within Texas A&M’s School of Veterinary Medicine and Biomedical Sciences. “It seems that one of the ways these genetic changes evolve is by bundling together multiple genes into supergenes, which are then subject to natural selection as the environment changes.”
Andersson said these results may surprise human geneticists, because most gene variants associated with multifactorial traits like height and disease susceptibility only explain a tiny fraction of the heritable variation.
Ever since Charles Darwin wrote about the finches of the Galápagos Islands, biologists have studied these small songbirds to understand the mechanisms of evolution.
“The strength of Darwin’s finches as a model for evolution lies in what they can show about the early stages of speciation,” said Erik Enbody, the lead author of the study, a University of California Santa Cruz postdoctoral scholar, and former post-doctoral fellow at Uppsala University. “On the Galápagos, one ancestral species has evolved into 18 different species in the last million years. One of the things that makes the finches such an amazing system is that we can observe evolution in real-time by measuring finch beaks and following individuals through their life.”
The research team’s discoveries were made possible in part by Peter and Rosemary Grant, from Princeton University, who tracked nearly every individual finch on Daphne Major, a volcanic island in the Galápagos, starting in the 1970s. The Grants’ work demonstrates that the finches of Daphne Major evolved in response to changes in the environment and interactions among species.
“This study highlights the value of long-term studies to understand the mechanism of evolutionary change,” Peter said.
“By collecting samples throughout the study, we had them available for genomic study when the technology became available,” Rosemary added.
The international team’s findings began with sequencing the genomes of nearly every finch the Grants studied on Daphne. By studying the entire community of four species of finches present on the island, the researchers revealed the genetic architecture of adaptive change.
The Common Cactus-Finch, for example, experienced a gradual change toward blunter beaks as conditions on the island changed and hybridization with the Medium Ground-Finch increased.
Over the three decades studied, the beak of the Medium Ground-Finch also has become smaller. Using the genomes of all the finches on Daphne, the researchers show that this partially results from gene variants transferring from the Small Ground-Finch through hybridization and because periods of drought between late 2003 and early 2005, where individuals with smaller beaks survived better, explains Rosemary Grant.
This episode of drought led to dramatic shifts in the frequency of the gene variants associated with beak size, showing that the gene variants described in this study have a major effect on fitness in these birds, according to Andersson.
“I think it’s a really exciting opportunity to tie together our understanding of evolutionary change in the deep past with observations in current time,” Enbody said. “Genomic data is a powerful tool to take our observations of birds in the field and learn about the factors that have shaped their evolution.”