Natural selection can sometimes work one gene at time, a new study of Darwin’s finches suggests.
Variants of one gene had a major effect on rapid changes in beak size after a drought, researchers report in the April 22 Science. The finding may help explain how Darwin’s finches evolved into 18 species in an evolutionarily speedy 1 million to 2 million years.
A drought that struck the Galápagos island of Daphne Major in 2004 and 2005 put adaptation of some of Darwin’s iconic finches on fast-forward. Competition for scarce seeds pitted medium-sized ground finches (Geospiza fortis) with big beaks against large ground finches (G. magnirostris) with big beaks. Big-beaked medium ground finches lost that contest. They died, but medium ground finches with small beaks survived by eating small seeds. As a result, medium ground finches on the island tended to have smaller beaks after the drought than before.
Genetic variants of the HMGA2 gene controls beak size in the birds, evolutionary geneticist Leif Andersson and colleagues now report. Large-beak varieties of the gene were at a strong disadvantage during the drought, the researchers found.
The study takes “arguably the most elegant story in evolutionary biology and then fills in the nitty-gritty molecular details,” says Harvard University evolutionary biologist Hopi Hoekstra. While the researchers don’t know precisely how the gene influences beak size, the work may help scientists better understand the genetic underpinnings of evolution, she says. “This is a beautiful and big first step.”
Andersson, of Uppsala University in Sweden, teamed up with Princeton University’s Peter Grant and B. Rosemary Grant, who have been documenting changes in Darwin’s finches populations for decades. The researchers previously discovered that a gene called ALX1 controls whether beaks are blunt or pointy (SN: 3/7/15, p. 7). But beak shape didn’t seem to play a role in surviving the drought, the team found. Instead, beak size changed.
Andersson’s group narrowed the search for the gene controlling beak size to a stretch of DNA that contains HMGA2 and three other genes. The researchers can’t rule out that the other genes also affect beak size, but say HMGA2 is the gene most strongly associated with the trait. It accounts for almost 30 percent of the shift in beak size during the drought, Andersson says. HMGA2 protein is known to help regulate how other genes are turned on or off. In humans and mice, the gene is associated with height, face development and other traits.
Birds that have two copies of the large-beak variant of the gene have big beaks, while two copies of the small-beak variant produce little beaks. Birds with one of each have intermediate-sized beaks. The small-beak variant was found 61 percent of the time in finches that survived the drought, but only 37 percent of the time in birds that died.
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It’s unusual for a single gene to have such a strong effect on survival, says plant ecologist Thomas Givnish of the University of Wisconsin–Madison. Researchers have documented many cases in which multiple genes each affect a trait a little bit. Genes with big effects may explain rapid evolutionary shifts.
Small beak variants of HMGA2 aren’t new changes brought on by the drought, Andersson says. The variant has been around for a long time, perhaps 1 million years or more, and may have come from interbreeding with tree finches, the researchers discovered.
Interbreeding between species is proving to be a powerful evolutionary force, says evolutionary biologist Daniela Palmer of the University of Chicago. Researchers are documenting more and more examples of interbreeding influencing evolution of organisms as diverse as butterflies and humans (SN: 3/5/16, p.18), she says. “Hybridization contributes major genetic variation to shape adaptation.”