Darwin's finches are pecking their way through evolution
Charles Darwin observed finches in the Galápagos while developing his theory of evolution by natural selection. Those finches' beaks are still evolving today, and scientists have identified a gene associated with their size.
Courtesy of © Peter R. Grant
More than 150 years after Charles Darwin proposed his theory of evolution by natural selection, the birds that helped shape his thinking are still actively evolving. And scientists have the genes to prove it.
A team of researchers has identified a gene involved in shaping the beaks of Darwin's finches – small, seed-eating songbirds in the Galápagos islands, according to a paper published Thursday in the journal Science. The gene, HMGA2, is still actively molding the famous finches today.
Darwin's birds were in fierce competition over scarce seeds after a severe drought struck Daphne Major Island a decade ago – a tiny scrap of land about five hours, by plane, off the coast of Ecuador. Two species in particular, the large ground finch and the medium ground finch, faced off.
But the competition wasn't as simple as species against species.
Medium ground finch beaks range in size, explains Leif Andersson, an animal geneticist at Uppsala University in Sweden, who led the new study. So the medium sized birds with larger beaks were competing against the large ground finches for larger seeds while the individuals with smaller beaks had the smaller seeds to themselves. The larger medium ground finches were often out-competed by the large ground finches, so the smaller-beaked birds stood a better chance in the drought.
"Those with larger beaks tended to die during this drought," Dr. Andersson tells The Christian Science Monitor in a phone interview. And because the birds with smaller beaks survived that drought best, "there's a tendency toward smaller beaks now."
Now, in their most recent paper, the team has identified a gene that contributes to that process, which helps the birds adapt to their drier environment.
After digging into the birds' DNA, the research team found that variation in the HMGA2 gene affects beak size. The genomes of many of the birds that survived the drought contained a variation of that gene associated with smaller beaks.
But that doesn't mean that the HMGA2 variation for larger beaks in the medium ground finches has gone extinct, Andersson says. There are still individuals with that version of the gene and larger beaks.
And, as natural selection dictates, different ecological pressures may lead to a resurgence of that variant, too. "As long as both variants are present in the medium ground finch, they may to respond to what is available, so to speak," Andersson says. It will depend on which birds can forage for food and reproduce most effectively.
The HMGA2 gene isn't the only gene involved in beak size, Andersson says, but it is remarkably influential. And this same part of the genome is associated with stature in humans, too.
"This research shows the power of genome-wide analysis to find genes of importance in fitness-related traits across the entire genome, the equivalent to finding a needle in the haystack," Jaime Chaves, an evolutionary biologist at the Universidad San Francisco de Quito in Equador who was not part of the study, tells the Monitor in an email. "Identifying these genes allows us to understand how these building blocks of beak dimensions are responsible for such dramatic changes (from a bird perspective, as we are talking differences of millimeters)."
This team of researchers also identified a gene involved in the beak shape of Darwin's finches last year. The variations of the gene they identified then, ALX1, relates to either blunt or pointed beaks.
Beak shape and size are two of the ways that Darwin's finches are distinct from each other. As different lineages diverged in order to exploit different resources on the Galápagos islands, differently sized and shaped beaks would have allowed birds to pick up and snack on different seeds.
Darwin's finches are "a model for evolutionary biodiversity on Earth," Andersson says.
The first finches that colonized the Galápagos were just one species, but today scientists recognize 18 different species across the islands.
The birds underwent a process that is a cornerstone of evolutionary biology: adaptive radiation. They began with one common ancestral species, but as they adapted to the different ecological niches on the islands, they began to diversify.
As the birds diversified, their morphology changed to best exploit whichever environment they were in. Ultimately new, distinct species emerged in a process called speciation.
"Many of us wonder and appreciate the amazing diversity of life on earth, both in the present day and in the fossil record. In order to understand how that bio-diversity arose we need to understand the process of speciation. When inferring how evolution occurred in the past, we need examples of how it occurs in contemporary time," study co-author Rosemary Grant, an evolutionary biologist who studied Darwin's finches in the Galápagos in a research team with her husband, Peter, for 40 years, says in an email to the Monitor. "Our research on Daphne Island in the Galapagos measured and demonstrated an unusually strong natural selection event during a two and a half year drought. This study extends our knowledge by showing the actual genes that were selected upon."
"We know that bacteria can evolve quickly in the laboratory but it is quite exceptional to find such a strong evolutionary change in a vertebrate animal," she says. "We are unable to study such events in humans, but we can in long-term studies of natural populations of birds and some other organisms."