Galapagos Finch Activity
Teacher Version
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 (Geospizafortis). The medium ground finch eats seeds of the puncture vineplant (Tribuluscistoides), 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 yearis 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
40large finches
Offspring:
15 small finches
60large finches
Population ratio of small to large finches:
1:4
Year 2:Drought has hit Daphne Island, bringing only6 inches of rain.
Population:
_15__ small finches
_60__ large finches
Food supply:
6 small seeds
44 large seeds
Surviving population:
_6___ small finches
__44__ large finches
Offspring:
_9___ small finches
_66___ large finches
Population ratio of small to large finches:
__1:7.3____
Year 3: The draught has continued on Daphne Island, bringing only 4 inches of rain.
Population:
_9__ small finches
_66__ large finches
Food supply:
4 small seeds
46 large seeds
Surviving population:
_4___ small finches
__46__ large finches
Offspring:
_6___ small finches
_69___ large finches
Population ratio of small to large finches:
__1:11.5____
Year 4: Rain has returned with a force on Daphne Island, bringing 30 inches of rain.
Population:
_6__ small finches
_69__ large finches
Food supply:
30 small seeds
20 large seeds
Surviving population:
_6___ small finches
_20__ large finches
Offspring:
_9___ small finches
_30___ large finches
Population ratio of small to large finches:
__1:3.3____
Year 5: Rain has continued to fall with a force, on Daphne Island, totaling 40 inches.
Population:
_9__ small finches
_30__ large finches
Food supply:
40 small seeds
10 large seeds
Surviving population:
_9___ small finches
_10__ large finches
Offspring:
_12___ small finches
_15___ large finches
Population ratio of small to large finches:
__1:1.25____
Year 6: A normal amount of rain happens on Daphne Island, totaling 10 inches.
Population:
_12__ small finches
_15__ large finches
Food supply:
10 small seeds
40 large seeds
Surviving population:
_10___ small finches
_15__ large finches
Offspring:
_15___ small finches
_21___ large finches
Population ratio of small to large finches:
__1:1.4____
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:
_10___ small finches
_21__ large finches
Offspring:
_15___ small finches
_30___ large finches
Population ratio of small to large finches:
__1:2____
Questions:
1. Over the 7 years, describe what happened to the small beaked finches. Include fitness.
All of the small finches survived in the years when there was enough rain to produce enough small seeds.
In other years, less rain caused there to be less small seeds, so the numbers of the small finches reduced.
Often, reproduction led to population growth that was not sustainable the following year.
In the consecutive wet years, the smaller finches reproduced, so they had good fitness.
2. Over the 7 years, describe what happened to the large beaked finches. Include fitness.
All of the large finches survived in the years when there was low rain so the plants produced enough large seeds.
Often, reproduction led to population growth that was not sustainable the following year.
In other years, more rain caused there to be less large seeds, so the numbers of the large finches reduced.
In the consecutive draught years, the larger finches reproduced, so they had good fitness.
3. Over the 7 years, describe what happened to the population as a whole. Include fitness and patterns.
The ratio of small to large finches in the population was altered each year with the changing food supply. When there was enough small seeds, the small finches were more fit than the large finches. When there was enough large seeds, the large finches were more fit than the small finches.
It was a back and forth pattern.
4. What evolutionary pressures caused what you described?
The environmental pressures of rainfall, food supply, and competition caused the above.
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?
If draught continued, more of the population would be large finches.
If wet weather continues, more of the population would be small finches.
In either of these situations, if enough changes accumulated, the population could become a distinct species from the other island’s population.