In honor of Charles Darwin’s 206th birthday, scientists from Princeton University and Uppsala University in Sweden have published a timely article that sheds light on the evolutionary history of Darwin’s finches, as well as identified a gene that helps explain the variation in beak shape within the species.
Importantly, this study illustrates the underlying genetic basis of evolution, describing the nature of how genes stream between species; as well as how alternate versions of a gene within a single species can ultimately contribute to the formation of entirely new species.
Results from this study were published today in Nature under an article entitled “Evolution of Darwin’s finches and their beaks revealed by genome sequencing”.
Darwin’s finches, which inhabit the Galápagos archipelago and Cocos Island, are the quintessential example of speciation and adaptive radiation. Roughly two million years ago the finch’s common ancestor landed on the islands and has since evolved into 15 recognized species that vary in beak shape, body size, song, and feeding behavior.
Charles Darwin described the varying beak shape and discussed them at great length in passages about species divergence within his most famous text, On the Origin of Species. He noted that changes in size and form of the beaks have allowed different finch species to utilize food from various sources, such as seeds, insects and nectar, all of which are driven by natural selection.
“We have now sequenced 120 birds including all known species of Darwin's finches, as well as two closely related species in order to study their evolutionary history,” explained Sangeet Lamichhaney, Ph.D. student and shared first author on the paper.
An important question that the scientists were trying to address was how prominent was gene flow between finch species throughout evolution. The researchers were able to trace strong indications of hybridization between a warbler finch and the common ancestor of tree and ground finches, which they concluded was an event that must have occurred approximately one million years ago.
“During our field work on the Galapagos we have observed many examples of hybridization between species of Darwin's finches but the long-term evolutionary effects of these hybridizations have been unknown,” stated Peter Grant, Ph.D., and Rosemary Grant, Ph.D., Princeton University evolutionary biologists, who have been studying Darwin’s finches in the Galapagos for the past 40 years.
“Now we can safely conclude that interspecies hybridization has played a critical role in the evolution of the finches, and has contributed to maintaining their genetic diversity,” said Dr. Peter Grant.
The Princeton and Uppsala teams examined the genetic basis underlying the variations in beak shape through comparisons of two bird species with bunt beaks with two with pointed beaks. They found 15 regions within the genomes of their comparative species that struck them as very different. Moreover, six of those had been previously characterized to be associated with craniofacial and beak development.
“The most exciting and significant finding was that genetic variation in the ALX1 gene is associated with variation in beak shape not only between species of Darwin's finches but also among individuals of one of them, the medium ground finch,” explained Leif Andersson, Ph.D., professor of functional genomics at Uppsala University and head of the current study.
“This is a very exciting discovery for us since we have previously shown that beak shape in the medium ground finch has undergone a rapid evolution in response to environmental changes. Now we know that hybridization mixes the different variants of an important gene, ALX1,” stated Dr. Rosemary Grant
The ALX1 gene encodes for a transcription factor that has been observed to be critical for the development of normal craniofacial structures. Mutations within in this gene that lead to inactivation are known to cause severe birth defects, including frontonasal dysplasia in humans.
“This is an interesting example where mild mutations in a gene that is critical for normal development leads to phenotypic evolution. I would not be surprised if it turns out that mutations with minor or minute effects on ALX1 function or expression contribute to the bewildering facial diversity among humans,” concluded Dr. Andersson.