Galapagos Finch Activity
Goal: Learn how fitness is affected by environmental conditions and how frequency of features of a population can change.
Overview: Read a summary of Galápagos finches and answer questions about the population.
In the Galápagos, Daphne Island is home to the medium ground finch (Geospiza fortis). The medium ground finch eats seeds of the puncture vine plant (Tribulus cistoides), using its beak to crack them. On the island, large changes in rainfall affects the food supply.
Instructions:
Smaller finches with small beaks can only eat smaller and softer seeds, while larger finches with larger beaks can only eat larger and harder seeds.
Assume each finch needs to eat one seed in order to survive and reproduce.
Assume the finches live one year to reproduce three offspring with another same-sized finch.
Year 1: The year is going well on Daphne Island. The rainfall has been an average 10 inches, allowing the caltrop plants to produce small and large seeds.
Population:
10 small finches
40 large finches
Food supply:
10 small seeds
40 large seeds
Surviving population:
10 small finches
40 large finches
Offspring:
15 small finches
60 large finches
Population ratio of small to large finches:
1:4
Year 2: Drought has hit Daphne Island, bringing only 6 inches of rain.
Population:
___ small finches
___ large finches
Food supply:
6 small seeds
44 large seeds
Surviving population:
____ small finches
____ large finches
Offspring:
____ small finches
____ large finches
Population ratio of small to large finches:
______
Year 3: The draught has continued on Daphne Island, bringing only 4 inches of rain.
Population:
___ small finches
___ large finches
Food supply:
4 small seeds
46 large seeds
Surviving population:
____ small finches
____ large finches
Offspring:
____ small finches
____ large finches
Population ratio of small to large finches:
______
Year 4: Rain has returned with a force on Daphne Island, bringing 30 inches of rain.
Population:
___ small finches
___ large finches
Food supply:
30 small seeds
20 large seeds
Surviving population:
____ small finches
___ large finches
Offspring:
____ small finches
____ large finches
Population ratio of small to large finches:
______
Year 5: Rain has continued to fall with a force, on Daphne Island, totaling 40 inches.
Population:
___ small finches
___ large finches
Food supply:
40 small seeds
10 large seeds
Surviving population:
____ small finches
___ large finches
Offspring:
____ small finches
____ large finches
Population ratio of small to large finches:
______
Year 6: A normal amount of rain happens on Daphne Island, totaling 10 inches.
Population:
___ small finches
___ large finches
Food supply:
10 small seeds
40 large seeds
Surviving population:
____ small finches
___ large finches
Offspring:
____ small finches
____ large finches
Population ratio of small to large finches:
______
Year 7: Another normal amount of rain happens on Daphne Island, totaling 10 inches.
Population:
_15__ small finches
_21__ large finches
Food supply:
10 small seeds
40 large seeds
Surviving population:
____ small finches
___ large finches
Offspring:
___ small finches
____ large finches
Population ratio of small to large finches:
______
Questions:
1. Over the 7 years, describe what happened to the small beaked finches. Include fitness.
2. Over the 7 years, describe what happened to the large beaked finches. Include fitness.
3. Over the 7 years, describe what happened to the population as a whole. Include fitness and what it shows about any patterns of evolution.
4. What evolutionary pressures caused what you described?
5. What could happen to the population if either draught or wet weather continued on Daphne Island for centuries? How would the population compare to another population on another island?