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A population of finches lives on an island some distance from the mainland, where another population of finches of the same species resides. In both populations, the size of the birds' beaks is influenced by two alleles at the beak locus, B and b. Finches with the BB genotype have the largest beaks, bb have the smallest beaks, and Bb individuals have a beak size intermediate between the two homozygotes. On the island, the frequency of the B allele is 0.3, and the frequency of the b allele is 0.7. On the mainland, both alleles are equally frequent (0.5).
(a) If the migration rate from the mainland to the island is 0.1 (meaning that 10% of individuals in the island population migrate in from the mainland every generation), what is the change in the frequency of the B allele on the island in one generation due to migration? Assume there is no migration from the island back to the mainland.
(b) The genotypes at the beak locus have the following fitness on the island: wBB = 0.95, wBb = 0.98, wbb = 1. Which allele is the most fit? What size beak is selectively favored on the island? What do you predict will happen to the frequencies of these alleles in the long term?
(c) The island experiences a severe drought for two consecutive years, which kills a plant species on the island that makes small seeds eaten by the finches. As a consequence, the food available decreases and the average size of edible seeds on the island increases greatly. Birds with large beaks are much better able to crack large seeds and thus better able to survive the drought. Which allele is the most fit? How do you predict the allele frequencies of B and b will change during this drought?
18. The longest field study in evolutionary biology (35 years) is being conducted on Darwin's finches by Rosemary and Peter Grant (Princeton University) on the Galapagos Island of Daphne Major. The study has provided compelling evidence for which of the following? a. Convergent evolution of beak morphology b. Speciation resulting from geographical isolation c. Natural selection in the wild d. Sexual selection on adaptive traits
During his 5-week voyage to the Galapagos Islands, Darwin noticed that several closely related species of finches had slightly different beak structures, each of which favoured the use of different food sources. Could Lamarck have used this observation to support his own theory of evolution by use and disuse? What evidence would need to be observed in order to decide which theory was more accurate?
