After visiting the Galapagos, Charles Darwin proposed that species who compete for the same, limited food resources tend to diverge from each other to reduce competition. Seed-eating finches have hardy bills, for example, while those who drink nectar from cactus flowers have thin, sharp bills. This was called the principle of character divergence, now known as ecological character displacement. And nearly two centuries later, researchers have finally identified a gene that can explain differences in beak size. The findings are published in Science this week.
The common ancestor of all of Darwin’s finches arrived in the archipelago around 2 million years ago. Nowadays, there are 18 recognized species displaying a dazzling range of body sizes, beak shapes, songs, and feeding strategies. A study published last year revealed that a gene called ALX1 helps determine beak shape – specifically how sharp or blunt the beak becomes. Now, the same team, led by Uppsala University’s Leif Andersson, has focused on the rapid evolution of beak size as model for the evolutionary process.
The gray warbler finch has a small, pointed beak for eating insects. The common cactus finch has a large, pointed beak for feeding on medium-sized seeds and cactus pollen. The large ground finch has a large, blunt beak for feeding on large seeds. B. Rosemary Grant
After sequencing a total of 60 birds from six different species – the small, medium, and large ground finches, and the small, medium, and large tree finches – the team identified a gene of interest: HMGA2. It’s highly linked to beak and body size in medium ground finches (Geospiza fortis).
The researchers then analyzed the genomes of 71 medium ground finches who either survived or succumbed to a severe drought on Daphne Major in the Galapagos from 2004 to 2005. HMGA2 has two variants: one that’s common in finches with small beaks, another that’s common in finches with large beaks. Turns out, medium ground finches with two sets (one from each parent) of the small-beak HMGA2 variation survived more than those with the variation associated with larger beaks. The frequency of the small-beak variation among survivors was 61 percent, compared to the 37 percent of those who died.
When environmental conditions changed, the gene contributed to a rapid shift in beak size – in this case, it took less than a year for the gene to respond to the drought-induced food shortage and bring about a permanent physical change – allowing the medium ground finch to diverge from its competitor, the large ground finch (Geospiza magnirostris, pictured to the right). It’s a clear example of ecological character displacement.
"It was an exceptionally strong natural-selection event," study co-author Peter Grant of Princeton said in a statement. "We have demonstrated that the HMGA2 locus played a critical role in this evolutionary shift and that natural selection acting on this gene during the drought is one of the highest yet recorded in nature."
HMGA2 has previously been linked to variations in the sizes of dogs and horses and stature in humans. It’s also associated with cancer. "It is very fascinating that this gene pops up in many different species as a gene affecting growth and, in humans, as a gene affecting dysregulated cell growth in cancer," Andersson said in a statement. Though exactly how it controls human stature and beak size in Darwin's finches is still a mystery.
Image in the text: Large ground finch (Geospiza magnirostris). K. Thalia Grant