Original article: Bosse, Mirte, et al. “Recent natural selection causes adaptive evolution of an avian polygenic trait.” Science 358.6361 (2017): 365-368. DOI: 10.1126/science.aal3298
Echoing Charles Darwin’s study of Galapagos finches, biologists in Great Britain have found that the size of birds’ beaks is adapted to help them eat certain types of food. But unlike Darwin’s finches, the British food sources influencing bird evolution aren’t natural features of the environment. They’re backyard bird feeders.
Researchers reported in Science that great tits, common songbirds throughout Europe, are rapidly evolving larger beaks in Great Britain. Long-beaked birds appear to be better at plucking food from the ubiquitous feeders Britons keep in their gardens, allowing these birds to raise more offspring.
The scientists analyzed the genetic code of great tits from Great Britain and the Netherlands, where bird feeders are less common. They found that DNA of the British birds contained genes for bigger beaks. One of these genes, called COL4A5, affects beak size in Darwin’s finches and is even responsible for facial abnormalities in humans. The variant of the COL4A5 gene causing larger beaks has grown more prevalent in recent decades among great tits in feeder-friendly Great Britain.
After scrutinizing the DNA, the scientists measured beak length of hundreds of great tits to confirm that their genetic findings actually translated into longer bird beaks. The researchers also used museum specimens to show that birds within Great Britain have increased their beak size by one eighth of a millimeter over the last 25 years, a remarkably fast change by evolutionary standards.
The culprit could be hiding in the garden
The authors claim that the rapid rise in beak size is an adaptation to “supplemental feeding.” Birds with longer beaks may have easier access to feeders in peoples’ gardens. In fact, larger beaks are such an advantage that birds with the big-beaked gene variant were able to raise more chicks than birds without the variant.
These findings surprised the study’s own lead author, Dr. Mirte Bosse from Wageningen University in the Netherlands.
“I did not expect to find evolution at work as a possible unknown side-effect of human generosity towards birds,” she said. However, Bosse doesn’t believe her findings will change the practice of bird feeding in Great Britain.
“I think supplementary feeding will stay in an urban environment,” she predicted, “so for birds to adapt to that environment, a longer beak seems to be beneficial.”
Combining multiple lines of evidence
Prior research has shown that other human activities alter bird behavior too. For example, birds in cities sing at a higher pitch than their rural counterparts to cut through the din of traffic and construction. However, Bosse believes this study is unique because it combines beak measurements with genetic data, proving that the observed size differences are evolutionary adaptations written into birds’ DNA code.
“The fact that we are now able to find a trait under selection in natural populations, starting with a screening of their genes, is very promising for future studies,” says Bosse.
It’s too early to predict what these evolutionary changes may mean for bird populations as a whole. But more research that combines DNA sequencing with observations of anatomic or behavioral change could go a long way toward understanding which environmental pressures are impacting the organisms around us.