Question

The fossil record shows that the average size of black bears was large during cold glacial periods and got smaller during warmer time periods. What is this an example of?
directional selection
disruptive selection
ecosystem selection

Answer 1

Directional selection.

Explanation: the mean body size shifts toward one extreme (larger in cold, smaller in warm) as different sizes are favored under different climates. (This pattern also fits Bergmann’s rule: larger bodies in colder climates reduce heat loss.)