The classic finches that were integral in Darwin's development of natural selection are now being looked at under new light. What does their genome tell us?
Charles Darwin – his name is often heard tied to that of his greatest work, The Origin of Species, a pioneering work in the field of evolution. When Darwin embarked on the HMS Beagle as a naturalist (a position similar to today’s field biologist), he could not have predicted the impact his work would have on modern science. During the five-year journey, Darwin collected copious amounts of data on all the things he observed: rocks, fossils, different types of plants and animals. He filled notebooks with sketches and details notes about his surroundings. These notes would fuel the writing of the Origin of Species.
There is one particular group of organisms that is commonly associated with Charles Darwin: The finch species of the Galapagos Islands, more often referred to as Darwin’s Finches. What makes these finches so special? When Darwin brought his specimens back to England, he thought that they were just different varieties of the same species. After more work, he found that all twelve were unique species, each from different islands!
The finches differed from each other in physical characteristics, mainly beak size and shape. Scientists call these physical appearances “phenotypes”. The finches on some islands had phenotypes with long, thin beaks, while others had short, wide beaks. Some had pointed beaks, and others had blunt beaks. All of these different changes arose through the process of natural selection, which drives evolution. All of these finch species arose from one single species that originally lived on the mainland. After coming to the islands, different beak traits were better for survival depending on which island the finch lived on and what food was available. Over a very long time, the original group of finches evolved into the twelve individual species that Darwin found.
For years, scientists have grouped these finches into phylogenetic trees based on their phenotypic relationship to each other. The more closely related birds would have the most similar phenotypes. Phylogenetic trees help scientists understand the relationship between different species, as well as how different species evolved from their common ancestor. Darwin’s finches are especially important to scientists because none of the species have gone extinct, making them a good tool for studying evolution.
In a paper published this month in Nature, scientists reported that they have sequenced the genomes of all of the Darwin’s Finch species and two other closely related birds. By sequencing the complete genome (finding out how the DNA is arranged), the scientists were able to locate regions of the DNA that might be directly responsible for the differences in beak shapes among the finches. This team of scientists proposes that a gene called ALX Homeobox 1 (ALX1) is the gene changing the beak shape of the finches. This gene is found in many animals, even humans, and is used in the development of the head and cranial features. The finch species with blunt beaks showed similarities in the ALX1 gene, while finch species with pointed beaks had different ALX1 genes from their blunt beaked relatives.
Another surprising discovery appeared when the genomes of the different species were compared. There has been a history of hybridization between the finch species from different island. Hybridization occurs when organisms from different species interbreed. This interbreeding mixes genes from both species together. It has been long thought that the finches of the Galapagos Islands were too different for hybridization to occur. Now, this team of scientists has shown the some of the finch species have mixed lineage and share some genes.
Using their genetic data, the scientists arranged a new phylogenetic tree for Darwin’s Finches. Many of the species remained in their original places, but this new information moved others. The genetic sequences of some species were more similar than previously thought, meaning that they needed to be moved nearer to each other in the phylogenetic tree. One species in particular, Geospiza difficilis, may actually be three separate species. When the scientists compared Geospiza difficilis genomes from birds of different islands, they realized that there were genetic differences related to which islands the birds originated on. More research will be done before they are officially named as separate species, but without the genetic sequencing, we might never have known.
This paper is significant because it expanded our knowledge of how Darwin’s Finches evolved. These birds are commonly studied as an example of ongoing evolution, so any new information greatly develops our understanding of evolution as a whole. The techniques used in this research will be used in the future to explore the relatedness and evolution of other groups of species.