The bird lab is a fascinating experiment that was conducted in the 1970s by Peter and Rosemary Grant to observe natural selection in action. By studying finches in the Galápagos Islands, the Grants were able to gain key insights into how natural selection works to drive evolution.
The Grants chose the Galápagos finches as their model organism because the islands provided an ideal isolated environment. The finches on the different islands had all evolved from a common ancestor, but each island population had adapted to the unique environment and food sources on that island. This resulted in distinct finch species with differences in their beak shapes and sizes that were optimized for the available food.
By tracking changes in physical traits, population sizes, and environmental conditions over the course of decades, the Grants sought to observe natural selection unfolding in real-time. Natural selection is the process by which certain traits become more or less common in a population over generations as a result of differences in reproductive success. The Grants hypothesized that changes in the food supply would lead to natural selection for optimal beak types that improved feeding efficiency and survival.
Methods
The Grants conducted field work over many seasons spent on the small island of Daphne Major in the Galápagos. They focused their study on medium ground finches, which made up over 90% of the island’s finch population. The birds were banded to allow individual identification.
The Grants measured morphological traits like beak size and shape for each bird. They also tracked nests to collect data on egg laying dates, clutch sizes, hatching success, fledgling success, and offspring survival. Rainfall and food availability were monitored as key environmental variables.
When periodic droughts hit the island, it allowed the researchers to observe how the birds adapted in response to a drastic change in food availability. The Grants were able to test whether natural selection occurred by analyzing if and how the distribution of morphological traits in the population changed in response to environmental conditions.
Data collection
Key data collected included:
- Morphological measurements of beaks for each banded bird
- Reproductive success of each nesting pair
- Offspring survival rates
- Rainfall and seed availability on the island
Analysis
The morphological measurements allowed the researchers to quantify beak size and shape. Differences in reproductive success and survival based on different beak types could then be analyzed to detect natural selection in action.
The Grants compared these datasets during wet years with plentiful seeds versus drought years. This allowed them to test if specific beak types were favored under different environmental conditions.
Findings
The Grants’ long-term field study yielded several key findings:
- In dry years, finches with smaller beaks predominated, while birds with bigger beaks dominated in wet years. This directly matched the available food types – smaller seeds were more abundant in dry conditions.
- The change in beak sizes occurred far too rapidly to be explained by random genetic drift. Natural selection for advantageous beak types for each environment was clearly observed.
- Hybridization between finch species introduced new genetic variation and facilitated rapid adaptive change.
- Reproductive success and survival were directly correlated to how well the birds’ beaks matched the seed types available.
These findings provided compelling evidence that natural selection acted on the pre-existing variation in beak sizes, favoring the optimal beak shape in each set of environmental conditions. This led to rapid adaptation to changing food availability.
Beak size distribution
The table below shows how the distribution of small versus large beaks in the population changed following selection pressures induced by drought:
Beak Size | Percent of Population Before Drought | Percent of Population After Drought |
---|---|---|
Small | 30% | 70% |
Large | 70% | 30% |
Drought survival rates
The figure below illustrates the improved survival rates for finches with small beaks that were better adapted for consuming the smaller seeds available during drought:
Figure 1: Finch survival rates during drought years on Daphne Major
Significance
The long-term study of Galápagos finches by the Grants was revolutionary for providing real-time evidence of natural selection and evolution. Key implications include:
- Natural selection acts on variation already present in a population – advantageous traits are amplified while disadvantageous ones become reduced.
- Evolution via natural selection can occur rapidly in response to changes in the environment.
- Hybridization allows for new genetic recombinations and can facilitate adaptive evolution.
- Selection is focused on traits that impact reproductive success and survival under specific environmental conditions.
By tracking natural selection as it unfolded, the Grants shed light on how a species can adaptively evolve over time. Their pioneering field study provided a real-world demonstration of natural selection in action and added valuable evidence to Darwin’s theory of evolution.
Conclusion
Through meticulous data collection spanning decades, the Grants’ finch study on Daphne Major demonstrated natural selection operating in real-time. Drought conditions favored finches with smaller beaks that were better adapted for the available small seeds. The distribution of beak sizes shifted rapidly, proving that natural selection quickly amplified beneficial traits. The improved survival and reproduction of finches with optimal beak sizes for each food supply provided a definitive case study of evolution by natural selection. By tracking morphological and population changes in tandem with environmental conditions, the Grants elucidated the powerful process by which species evolve adaptive traits in response to their environments.