Question

figure 1. proboscis length distribution in a population of tobacco hawk moths
over the next year, a virus attacks and rapidly kills almost all plants of the tobacco species. a closely related species of tobacco that is resistant to the virus becomes established during that same year. the nectary tube in the resistant species averages 7.5 cm. scientists monitored the area for the next five years to determine how this change affected the moth population.
which of the following best describes the expected distribution of proboscis size of the moths at the end of the five-year study?
chart a: x-axis proboscis length (cm) 1.00–10.00, y-axis number of hawk moths (n) 2–10, bars from 2.00–6.00 cm; chart b: bar near 7.5 cm area (partial display)

Explanation:

The original tobacco plants die, and a resistant species with a 7.5 cm nectary tube becomes established. Natural selection will favor moths with proboscis lengths close to 7.5 cm (to access nectar). So the distribution should shift toward longer proboscis lengths, with more moths having lengths around 7.5 cm. Option B (assuming its bar is around 7.5 cm, though the diagram's B likely shows a peak near 7.5) fits, as A has peaks at shorter lengths. The key is directional selection favoring longer proboscis to match the new plant's nectary.

Answer:

B (assuming the diagram for B has a peak near 7.5 cm, consistent with natural selection favoring proboscis lengths matching the new tobacco's nectary tube length of 7.5 cm)