The varying beak shape of the Galapagnos finches have been a textbook example to demonstrate evolution for decades.
By Lisa Carr
Their beaks were the quintessential models of evolution. Now Darwin’s Galapagos finches have become the subject of research endeavours again as scientists have identified the gene that caused the variation in their beak size and shape.
On his voyage to the Galapagos Islands in the 1800s, Charles Darwin observed thirteen different species of finches that lived in the isolated region. The finches each had a variety of bill shapes and sizes that suited their individual diets and lifestyles. By observing these differences in morphology in such a remote location, Darwin was able to conclude that all these finches was descendants from an original pair of finches that migrated to the Galapagos islands. It was natural selection over time that lead to differences in their appearance.
The common ancestor of all Darwin’s finches was thought to arrive on the Galapagos Islands around two million years ago. So by the time Darwin himself arrived on the shores of the islands, the birds had diversified into many different species, adapted to different ecological niches and different diets. The birds that cracked seeds for their diet had much larger beaks, whereas the birds that snatched insects had a smaller curved delicate beak. Unusually, some birds even had pointy sharp beaks for feeding on blood. In order to not compete with other finches over the same food source, each finch species evolved it’s own beak characteristics that best suited different dietary requirements so each species could thrive independently.
Natural selection favoured certain types of beak over others to suit the environment. The birds with the best traits for a certain diet thrived, reproduced and therefore passed their traits onto their offspring. However, until recently the genetic basis for this evolutionary action was unknown but now scientists have been able to identify the specific genes that played a role in the change.
The genomes of 60 birds representing six species of Darwin’s finches were examined, alongside 120 specimens from other species that allowed the scientists to identify the genes that contributed to physical characteristics. Closely related species had the most similar genomes, though this isn’t exactly news to science.
However, within the six finch species there was one region of the genome that seemed to account for size of the bird rather than relatedness. Smaller species only had one variation of this genomic region, larger species had more than one variation and medium-sized species had a mixture of the two. This region was the most likely location to hold the individual gene that accounted for size and therefore size of the beak. Further analysis confirmed this when it was found the HMGA2 gene was important for controlling size and beak size.
In addition, the researchers were able to look at the role of HMGA2 during the 2003 Galapagos drought that had severe evolutionary consequences. After drought struck the region, many of the medium-sized ground finches that fed on seeds with their large beaks starved to death. They were unable to compete with another species that fed on the same seed diet, a species that only fairly recently colonized the island. The newer species were better equipped morphologically to eat the seeds and therefore many members of the existing medium ground species died. However, the few that managed to survive from the original population had smaller beaks than those that perished, with the small-beak variant of HMGA2. They were better suited at eating smaller seeds, seeds that the newer population struggled to eat with their huge beaks.
DNA analysis on the medium ground finches that lived around the time of the drought showed that the large-beak HMGA2 variant was more common in the birds that starved to death and the HMGA2 variant that gave a smaller beak size survived and was therefore passed onto offspring. Genetic drift had occurred leading to an overall reduction in beak size.
Charles Darwin’s initial reflections paved the way for what we know today about evolution, natural selection and genetic drift. However, research is constantly ongoing to unravel the minute details that substantiate his observations. We are delving into the genetic level of evolution and selection for traits and we are always uncovering more about what shapes the world around us.