50 min version Activity - Darwin's Finches Natural Selection

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Feb 20, 2024

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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, and measured hundreds of ground finches, tiny birds about the size of sparrows, and recorded their diets. The diet of ground finches consists mostly of seeds. Among ground finches, coloration, body size, wing length, beak length, and beak depth 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: 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 depth changed because of changes in food availability. 1 Modified and Adapted from Delaware Science Coalition Assessments Beak Depth Ground Finch

2 Modified and Adapted from Delaware Science Coalition Assessments = mean beak depth Figure 1 = mean beak depth Figure 2

Figure 3 Figure 4 Use Information in the Introduction and Figures 1-4 in answering the following questions: Questions in blue are recommended to be completed on notebook paper BEFORE you meet in active learning. 3 Modified and Adapted from Delaware Science Coalition Assessments Changes in Seed Abundance Before and After the Drought of 1977 0 2 4 6 8 10 12 Seed Abundance (g/m 2 ) Jan 1976 Jul 1976 Jan 1977 Jul 1977 Jan 1978 Jul 1978 Jan 1979

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1. Using data in Figure 1, what is the range in beak depth in the finch population in 1976? Using data in Figure 2, what is the range in beak depth in 1978? Did variation in beak size increase or decrease after the drought? 2. How does mean beak depth before the drought compare with mean beak depth after the drought? 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.) 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.) 5. Is natural selection occurring in the finch population during the drought? Explain. 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. 4 Modified and Adapted from Delaware Science Coalition Assessments

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 big- seed plant populations crashed. With plenty of food available, the size of the finch population increased dramatically over the next several years. a) How will the rainy weather influence the large beaked birds? b) How will the rainy weather influence the small beaked birds? c) 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. d) Explain your prediction. 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 Genotype Number of Individuals Observed Genotype Frequency Low Bmp4 expression AA 491 0.36 Low Bmp4 expression Aa 336 0.24 High Bmp4 expression aa 552 0.40 In 2009, allele frequencies were measured again on Daphne major for this particular gene. Phenotype Genotyp e Number of Individuals Observed Genotype Frequency Low Bmp4 expression AA 706 0.53 Low Bmp4 expression Aa 493 0.37 High Bmp4 expression aa 129 0.10 5 Modified and Adapted from Delaware Science Coalition Assessments

If evolution is not occurring, we expect that allele frequencies stay the same over time (Hardy- Weinberg equilibrium). For example, we would expect the allele frequencies in 2001 to be the same as those for 2009 if the Bmp4 locus is not affected by evolution. This section explores this option. a) Determine the allele frequencies for both A and a in 2001 and 2009. Allele Frequency in 2001 Frequency in 2009 A a b) Genotype frequencies can be calculated by multiplying the frequencies of both alleles together (using the formula p 2 + 2pq + q 2 = 1). For example, if the frequency of the A allele is 0.36, the frequency of the AA genotype is A 2 (0.13). Under Hardy-Weinberg equilibrium, these frequencies represent the predicted genotype frequencies of future generations. Calculate the expected Hardy-Weinberg genotype frequencies using allele frequencies from 2001. f(AA)= f(Aa)= f(aa)= Fill in the table with your answers from above for a side by side comparison. Genotype Expected Frequency in 2001 Observed Frequency in 2009 (See pg. 5) AA Aa aa c) Do these data suggest that the population, relative to this gene, has evolved from 2001-2009 in this population? Combining what we learned from the Grants’ 1976-1978 data, explain what may be driving this change. 6 Modified and Adapted from Delaware Science Coalition Assessments

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