The bird beak experiment refers to a series of studies conducted in the 1970s by biologists Peter and Rosemary Grant on Darwin’s finches in the Galápagos Islands. The Grants were interested in studying evolution in action and focused their research on finch beak size and shape in response to environmental changes. Their findings provided important evidence for natural selection and evolution.
The Galápagos finches are well-suited for the study of evolution. There are 13 species descended from a common ancestor. They live on different islands with varying food sources, and their beaks have adapted to their specific diets. For example, finches with thick, strong beaks eat seeds and nuts whereas finches with long, slim beaks eat insects.
Background on Darwin’s Finches
Darwin’s finches are small songbirds found exclusively on the Galápagos Islands. There are 13 closely related species descended from a common ancestor that arrived on the islands from South America. The birds were first collected by Charles Darwin during his famous voyage on the HMS Beagle in 1835. At the time, Darwin did not realize they were all finches and failed to label which island each specimen came from.
Upon returning to England, ornithologist John Gould examined Darwin’s specimens and declared they were a related group of finches. This surprised Darwin because the finches on the different islands had unique beak shapes optimized for different food sources. The Galápagos finches played a key role in inspiring Darwin’s theory of natural selection as an explanation for how evolution occurs.
The different finch species fill ecological niches on the Galápagos Islands related to the types of food they consume. For example, the large ground finch has a thick, powerful beak for cracking seeds and nuts. The warbler finch has a slim, pointed beak for catching insects. This provided a perfect opportunity for the Grants to study natural selection and evolution of the finch beaks in response to environmental changes.
Beginning of the Study
In 1973, Peter and Rosemary Grant traveled from Princeton University to Daphne Major, a small volcanic island in the Galápagos. They decided to study the medium ground finch (Geospiza fortis) which feeds on seeds. Their goal was to observe natural selection and evolution in action by intensively studying the finches over a long-term period.
The Grants carefully captured, tagged, and measured medium ground finches on the island. They took a variety of measurements including beak size, body mass, and song characteristics. They also took blood samples and marked the birds with colored leg bands for identification. After releasing the finches, they methodically recaptured them several times per year to gather more data.
The Grants brought sophisticated equipment to record climatic conditions on Daphne Major including rainfall, temperature, and wind speed. They also collected detailed measurements of the availability of seeds each year. This allowed them to closely track how environmental conditions related to natural selection on beak size.
The 1977 Drought
In 1977, the Galápagos Islands experienced a severe drought. The shortage of rain negatively impacted plant growth and seed availability on Daphne Major. With limited food, many of the finches died including about 85% of the medium ground finches studied by the Grants.
The small, soft seeds preferred by the finches were scarce. However, there were still some large, tough seeds available. Finches with smaller beaks could not crack these hard seeds open. But finches with large, strong beaks could still eat these seeds and survive the drought.
This extreme natural selection event allowed the Grants to measure how the population of medium ground finches changed in response to the environment. The average beak size increased from one generation to the next as the finches with large beaks survived and passed on their genes.
Continued Study Showed Oscillating Beak Size
The Grants continued studying the finches on Daphne Major for decades after the 1977 drought. Their long-term dataset revealed that the average beak size oscillated back and forth depending on rainfall each year.
During wet years with an abundance of soft seeds, finches with smaller beaks once again dominated since they were more efficient at eating those seeds. In drought years when hard seeds became more prevalent, selection favored larger-beaked finches.
These types of oscillations in beak size occurred repeatedly over the course of the study in response to natural changes in seed availability. The Grants had remarkable evidence that evolution due to natural selection was occurring from one generation to the next in response to environmental conditions.
Genetic Basis of Beak Size
In addition to measurements of beak size, the Grants took blood samples from the finches to study the genetic basis of variation in beak size. In the 1980s, other researchers identified ALX1 as one gene involved in regulating beak morphology in the Galápagos finches.
Later genetic analysis including full genome sequencing revealed additional genes that control development of craniofacial structures and variation in beak shape. The availability of this genetic toolkit allows different beak shapes and sizes to arise and be selected for in various environmental conditions.
Parallels to Industrial Melanism in Peppered Moths
The rapid evolution of beak size in Galápagos finches has parallels to another well-known example of natural selection: industrial melanism in the peppered moth.
During the Industrial Revolution, soot and air pollution darkened the tree trunks that peppered moths rested on. Before this, the moths were predominately light colored and blended in with lichen-covered trees. But dark or melanistic variants began thriving in polluted areas because they had better camouflage against the sooty backgrounds.
Just as the frequency of melanistic peppered moths increased rapidly in response to changes in their environment, the frequency of large-beaked ground finches increased when hard seeds became more prevalent. Both cases illustrated evolution by natural selection occurring over short timescales.
Criticisms of the Bird Beak Experiment
The Grants’ groundbreaking long-term study of Galápagos finches has provided some of the best evidence of evolution in action through natural selection. However, some creationists have criticized certain aspects of the study.
One counterargument is that the finches show cyclical changes driven by environmental variation rather than a long-term directional trend. However, the Grants demonstrated significant beak size changes could accumulate over time and not just reverse back and forth.
Skeptics also argue that new finch species have not arisen. However, speciation would require far longer than the Grants’ few decades of study. Observing evolution on the timescale of speciation is not feasible for individual scientists. Nevertheless, the divergence of Darwin’s finches illustrates speciation over a longer time period.
Overall, most biologists consider the Grants’ work to be a remarkable demonstration of natural selection. Environmentally-driven changes in beak size distribution are well-documented and consistent with evolution by natural selection. Criticisms of the bird beak experiment have little merit in the face of the compelling data.
Conclusion
The bird beak experiment conducted by Peter and Rosemary Grant provides a compelling case study of natural selection and evolution. By intensively studying Galápagos finches for several decades, the Grants documented how the distribution of beak sizes changed in response to environmental pressures.
Severe drought led to the selection of large-beaked finches that could crack hard seeds. Wet years favored smaller beaks efficient for eating soft seeds. Repeated cycles of natural selection shaped the beak size distribution as the finches adapted to fluctuating food sources. These findings are a vivid demonstration of evolution in action unfolding through measurable natural selection on heritable traits.
The Grants shed light on the genetic basis of beak size differences and parallels with other well-known examples of evolution like industrial melanism. Despite some criticisms from creationists, the bird beak experiment remains one of the most definitive illustrations of real-time evolution driven by natural selection in the wild. It will continue inspiring biologists to study microevolution in action and understand the forces that drive biodiversity and adaptation.