One of the tenets of evolutionary biology is that it requires time, but some microevolutionary changes can occur with surprising rapidity. The Grants have studied Darwin's finches in the Galapagos for over 30 years and have observed many changes. Recently they reported a change in the beak size of the medium ground finch on Daphne Major. In 1977 a drought reduced the number of small seeds available for the birds, forcing them to rely on larger seeds requiring considerable force to open. Within a couple generations, beak size, and hence ability to open large seeds, had increased. In 2003 a second drought, combined with the presence of large ground finches with substantially larger beaks, drastically decreased the number of large seeds available. Now ability to eat small seeds became an asset. The attrition rate for large beaked birds was horrendous, leaving very few individuals with large beaks to reproduce. The beak of the medium ground finch was quickly reduced to pre-1977 size. This paper is important because it shows how fast change can occur, and it demonstrates an interplay between environmental effects (the droughts) and population numbers (presence or absence of the large ground finch) can drive evolution in different directions.
Links
Darwin’s Finches Evolve Before Scientists’ Eyes
Goudarzi, S., LiveScience, July 13, 2006
Competition Drives Big Beaks Out of Business
Pennisi, E., Science 311, 156 (2006)
Evolution of Character Displacement in Darwin's Finches
Grant, P. R. and Grant, B. R., Science 313 , 224(2006)
Darwin’s Finches – Wikipedia
AIBS video presentation by Peter and Rosemary Grant
�What Darwin�s Finches Can Tell Us About Evolution�
Jonathan Weiner. 1994. The Beak of the Finch: A story of Evolution in Our Time. Alfred A. Knopf.
Questions
1. What trait was tracked in this study? How does this trait influence survival?
2. How long did it take for measurable changes to occur in the trait?
3. What environmental factors influenced changes in the trait?
4. Predict what would happen to the beak size of the population if there was another drought.
Curriculum
BIRDD from BioQUEST
Natural Selection and Variation in Birds
Level: Advanced middle school, high school, introductory college
Time: Two 50 minute class periods
Overview: A single lesson to introduce students to the concept of variation within a population. Students measure a particular trait (bill size) in a collection of specimens and analyze the distribution within a population. Excellent materials and teacher guide.
Darwin thought that evolution took place over hundreds or thousands of years and was impossible to witness in a human lifetime. Peter and Rosemary Grant have seen evolution happen over the course of just two years.
The Grants study the evolution of Darwin's finches on the Galapagos Islands. The birds have been named for Darwin, in part, because he later theorized that the 13 distinct species were all descendants of a common ancestor. Each species eats a different type of food and has unique characteristics developed through evolution. For example, the cactus finch has a long beak that reaches into blossoms, the ground finch has a short beak adapted for eating seeds buried under the soil, and the tree finch has a parrot-shaped beak suited for stripping bark to find insects.
The Grants have focused their research on the medium ground finch, Geospiza fortis, on the small island of Daphne Major. Daphne Major serves as an ideal site for research because the finches have few predators or competitors. (The only other finch on the island is the cactus finch.) The major factor influencing survival of the medium ground finch is the weather, and thus the availability of food. The medium ground finch has a stubby beak and eats mostly seeds. Medium ground finches are variable in size and shape, which makes them a good subject for a study of evolution.
The first event that the Grants saw affect the food supply was a drought that occurred in 1977. For 551 days the islands received no rain. Plants withered and finches grew hungry. The tiny seeds the medium ground finches were accustomed to eating grew scarce. Medium ground finches with larger beaks could take advantage of alternate food sources because they could crack open larger seeds. The smaller-beaked birds couldn't do this, so they died of starvation.
In 1978 the Grants returned to Daphne Major to document the effect of the drought on the next generation of medium ground finches. They measured the offspring and compared their beak size to that of the previous (pre-drought) generations. They found the offsprings' beaks to be 3 to 4% larger than their grandparents'. The Grants had documented natural selection in action.
While beak size is clearly related to feeding strategies, it is also related to reproduction. Female finches tend to mate with males that have the same size beaks. These factors together can add to the development of new species.
The Grants return each year to Daphne Major to observe and measure finches. They have been collecting data on the finches for over 25 years and have witnessed natural selection operating in different ways under different circumstances.