Natural Selection in Galapagos Finches
Question: Did natural selection on ground finches occur when the environment changed?
Introduction
In 1973, biologists Peter and Rosemary Grant began a 30-year long study to find out if they could document
natural selection. On Daphne Major, a tiny island in the Galapagos, they caught, weighed, measured, and
recorded the diets of hundreds of ground finches, tiny birds about the size of sparrows. The diet of ground
finches consists mostly of seeds. Among ground finches, coloration, body size, wing length and the size and
length and depth of the beak are inherited traits that vary in the population. The Grants found that finches
with large beak depths also have big bodies. (Grant and Grant, 2002)
Finch beak depth is illustrated below:
Beak Depth
Ground Finch
In 1977 a severe drought occurred on Daphne Major; instead of receiving the usual 130mm of rain that
normally falls, they received only 24mm. Plants withered and seeds of all kinds were scarce. Small soft
seeds were quickly eaten by the finches, leaving mainly large, tough seeds that finches normally ignore.
Eighty-four percent of the finch population disappeared. Evidence of dead, emaciated birds supported a
hypothesis that many of the finches died of starvation. In addition, no individuals from Daphne Major were
found on adjacent islands, nor did they return when food supplies returned to normal. The drought ended
when a small amount of rain fell on the island in January of 1978. (Grant and Grant, 2002)
Hypothesis to be tested: Beak characteristics changed in response to changes in food
availability.
Modified and Adapted from Delaware Science Coalition Assessments
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Figures 1, 2, 3, and 4 illustrate data collected by the Grants on Daphne Major.
1976 Finch Population Beak Depth
(Total Number in Sample = 751)
80
70
Number of Finches
60
50
40
30
20
10
0
5.8
6.2
6.6
7
7.4
7.8
8.2
8.6
9
9.4
9.8
10.2
10.6
11
11.4
11.8
12.2
12.6
13
13.4
13.8
Beak Depth (mm)
Figure 1
= mean beak depth
1978 Finch Population Beak Depth
(Total Number in Sample = 89)
80
70
Number of Finches
60
50
40
30
20
10
0
5.8
6.2
6.6
7
7.4
7.8
8.2
8.6
9
9.4
9.8
10.2
10.6
11
11.4
11.8
12.2
12.6
13
13.4
13.8
Beak Depth (mm)
= mean beak depth
Figure 2
Modified and Adapted from Delaware Science Coalition Assessments
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Changes in Finch Population Before and After the
Drought of 1977
1600
Population Size
1400
1200
1000
800
600
400
200
0
Jan
0
1976
July
6
1976
Jan
12
1977
Jul
18
1977
Jan
24
1978
Jul
30
1978
Jan
36
1979
Figure 3
Changes in Seed Abundance Before and After the
Drought of 1977
Seed Abundance (g/m 2)
12
10
8
6
4
2
0
Jan
0
1976
Jul
6
1976
Jan
12
1977
Jul
18
1977
Jan
24
1978
Jul
30
1978
Jan
36
1979
Figure 4
Modified and Adapted from Delaware Science Coalition Assessments
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Use Information in the Introduction and Figures 1-4 in answering the following questions:
1. Using data in Figure 1, what is the range in beak depth in the finch population in 1976?
5.8 – 13.8 mm
Using data in Figure 2, what is the range in beak depth in 1978?
6.8 – 11.4 mm
Did variation in beak size increase or decrease after the drought?
Decrease
2. How does mean beak depth before the drought compare with mean beak depth after the drought?
The mean beak depth before the drought was about 9.2mm. After the drought, it was about 10.0mm.
3. What do the data show about seed abundance before the drought compared with seed abundance after
the drought? (Note: Use numerical data in your response.)
The seed abundance before the drought ranged from around 7.7 to 10.6 g/m2. During the drought in
1977, the seed abundance decreased to about 2.8 g/m2 but then started to increase to about 3.5 g/m2
after the drought in July 1978.
4. Describe the relationship between the abundance of seeds and the size of the finch population. (Do
not explain. If you don’t know the difference between describe and explain, look it up prior to
completing this question.)
Seed abundance and finch population size are directly proportional. As one increases, the other
increases and as one decreases, the other also decreases.
5.
What type of natural selection would you say is occurring in the finch population during and after
the drought? Explain.
Directional selection for a larger beak is occurring in the finch population after the drought because
the average beak size is increasing (average phenotype changed in one direction).
Modified and Adapted from Delaware Science Coalition Assessments
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6. Propose an explanation that accounts for the change in the mean depth and the change in variation of
the finches’ beaks between 1976 and 1978 using your knowledge of natural selection, and the
information in the Introduction and Figures 1-4.
A good explanation might look like:
The increase in mean beak depth from 9.2 to 10.0 mm and the decrease in the variation of beak depth
from a range of 5.8 to 13.8 mm to 6.8 to 11.4 mm can be explained in terms of directional selection for a
medium to large beak depth as a result of food availability during the drought in 1977. The introduction
states that plants withered and seeds became scarce. Small soft seeds were quickly eaten by the finches
leaving mainly large, tough seeds behind. Figure 1 indicates that there was considerable variation in
beak depth in the population before the drought, from very small (5.8mm) to very large (13.8mm).
Eighty-four percent of the finch population disappeared as a result of the decreased food supply during
the drought. The data suggests that finches with smaller beaks are experiencing lower reproductive
success during the drought than finches with larger beaks. Finches with medium to very large beak
depths were probably able to eat the larger, tougher seeds while finches with smaller beak depths were
not able to eat the tough larger seeds. Therefore, many more smaller beak depth finches died than larger
beak depth finches since the larger beak depths were able to eat the larger, tougher seeds. In addition, the
introduction states that finches with larger beaks also have larger bodies. Since the range in beak depth
decreased at both ends (very small and very large), it may indicate that larger body finches with larger
beak depths also were at a disadvantage during the drought and were not as fit as finches with medium to
large sized bodies and beaks. This could be due to the fact that larger bodies require more food and since
food was scarce, very large beak finches (those in the 12-13.8mm range) also starved during the drought.
Assuming a genetic component to beak depth, surviving finches which happened to have medium to
large beak depths reproduced, passing on the medium to large beak depth genes to the next generation.
By 1978, mean beak depth had increased and the range of beak depths had decreased due to the better
reproductive success of finches with a medium to large sized beak.
Modified and Adapted from Delaware Science Coalition Assessments
5
7. Little rain fell on Daphne Major from 1979 through 1982. At the beginning of 1983 unusually heavy
rainfall (1359 mm) occurred on the island. Plants that produced small seeds flourished while bigseed plant populations crashed. With plenty of food available, the size of the finch population
increased dramatically over the next several years.
a. Predict how the rainy weather of 1983 affected mean beak depth over the next several generations
using your knowledge of natural selection. Make your prediction by circling one of the following
choices:
A. Mean (average) beak depth increased.
B. Mean (average) beak depth decreased.
C. Mean (average) beak depth remained the same.
b. Explain your prediction.
Unusually heavy rainfall increased the abundance of small soft seeds and decreased the abundance of
larger seeds. This would allow more finches with smaller beak depths to survive and reproduce,
decreasing the mean beak depth of the population.
8.
It was later discovered in Cliff Tabin’s lab that a gene called Bmp4 involved in cell-cell signaling
during beak development is partly responsible for beak depth in ground finches (Abzhanov et al.,
2004). Alleles that increase Bmp4 expression increase the depth of the beaks, and were most likely
selected for during the drought in 1977 (Abzhanov et al., 2004). In 2001, ground finches were
collected from Daphne major and genotypes were analyzed at this particular locus.
Phenotype
High Bmp4 expression
Low Bmp4 expression
Low Bmp4 expression
Genotype
aa
Aa
AA
Number of Individuals
552
336
491
In 2009, allele frequencies were measured again on Daphne major for this particular gene.
Phenotype
High Bmp4 expression
Low Bmp4 expression
Low Bmp4 expression
Genotype
aa
Aa
AA
Number of Individuals
129
493
706
a) If allele frequencies were at Hardy-Weinberg equilibrium, we would expect the allele frequencies
in 2001 to be the same as those for 2009 at this particular locus. Determine the allele frequencies
for both A and a in 2001 and 2009.
Allele
Frequency in 2001
Frequency in 2009
A
.48
.72
a
.52
.28
Modified and Adapted from Delaware Science Coalition Assessments
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b) Genotypic frequencies can be calculated by multiplying the frequencies of both alleles together.
For example, if the frequency of the A allele was .36, the frequency of the AA genotype would be
.13. Calculate the Hardy-Weinberg genotypic frequencies in 2001. Remember the sum of all
three genotypic frequencies should be 1.
f(AA)= .23
f(Aa)= .25*2=.50
f(aa)= .27
c) Calculate the Hardy-Weinberg genotypic frequencies in 2009.
f(AA)= .52
f(Aa)= .20*2=.40
f(aa)= .08
Fill in the table with your answers from b) and c).
Expected Frequency from
Genotype
2001
Observed Frequency
in 2009
AA
.23
.52
Aa
.50
.40
aa
.27
.08
d) Assuming Bmp4 is the only gene that controls beak depth, what do these data suggest about the
food availability between 2001-2009 on Daphne major? Explain.
The data suggests that small soft seeds were more abundant in 2009 compared to 2001. In 2001, the
frequency of the ‘a’ allele which causes high Bmp4 expression was much higher, suggesting that
there was a selective pressure for increased beak depth (perhaps another drought). In 2009, the
frequency of the ‘a’ allele had greatly decreased, suggesting that mean beak depth had decreased due
to a selective advantage of low Bmp4 expression (smaller beak depth), or a disadvantage of high
Bmp4 expression (larger beak depth).
Modified and Adapted from Delaware Science Coalition Assessments
7
References
Abzhanov A, Protas M, Grant B R, Grant P R, Tabin C J. Bmp4 and Morphological Variation of Beaks in
Darwin’s Finches, Science 2004; 305 (5689): 1462-1465.
Grant P, Grant B R. Unpredictable Evolution in a 30-Year Study of Darwin’s Finches, Science 2002; 296
(5568): 707-711.
Modified and Adapted from Delaware Science Coalition Assessments
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