[Campbell Biology P.536] Based on the text and Figure 23.2, what was the primary selective p... | Practice Question

Based on the text and Figure 23.2, what was the primary selective pressure that led to the change in average beak depth in the finch population after the 1977 drought?

  • A: A sudden increase in the number of predators targeting small-beaked finches.
  • B: The shift in available food sources due to the drought.
  • C: A new genetic mutation occurring in finches with deeper beaks, making them more adaptable.
  • D: Increased competition for nesting sites, favoring finches with larger territories.

Explanation

The text states that during the 1977 drought, 'small, soft seeds were scarce, while large, hard seeds were more plentiful.' This led to birds with 'larger, deeper beaks' surviving at a higher rate because they were 'better able to crack and eat the larger seeds.' Figure 23.2 visually confirms the increase in average beak depth after the drought from 8.9 mm in 1976 to 9.7 mm in 1978 . Therefore, the primary selective pressure was the change in food availability.