Fig. 2 shows three members of a finch species captured on an island before being released. Their body size is similar, but the width and depth of their beak...
Question 1 Report
Fig. 2 shows three members of a finch species captured on an island before being released. Their body size is similar, but the width and depth of their beaks differ. For several years the birds mainly ate small soft seeds. A drought then killed many seed-producing plants. The seeds left on the ground were mostly large and hard. Scientists returned five years later and found that the average beak depth of the finch population had increased.
(a) Describe the variation in beak shape shown in Fig. 2. [2]
(b) Explain why finches with deeper, stronger beaks were more likely to survive during the drought. [3]
(c) Use natural selection to explain the increase in average beak depth in the population. [5]
(d) Suggest how a new disease that kills most of the finches could increase the chance that this species becomes extinct. [3]
Answer Details
(a) The finches show variation in beak width, depth and strength. For example, the finch identified as having the broadest beak has a deeper, broader beak than the finch with the narrower beak. [2]
(b) Deep, strong beaks are better able to crack large, hard seeds. These were the main seeds available during the drought. Birds able to crack them obtained enough food and energy, making them less likely to starve. [3]
(c) The original population had inherited variation in beak depth. During drought, deeper-beaked birds were more likely to survive and reproduce because they could eat the hard seeds. They passed alleles for deeper beaks to their offspring. Offspring with deeper beaks formed a greater proportion of the next generation. Over many generations, the mean beak depth increased. [5]
(d) A disease that kills most finches causes a large fall in population size. Fewer individuals remain to find mates and reproduce. A later environmental change, predation event or low genetic variation could then remove the remaining individuals, causing extinction. [3]
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