Item EN029002: In a population of light and dark moths, if the number of dark trees for the moths to hide in suddenly increases, the number of dark moths will increase in each generation because more dark moths will survive and reproduce than light moths.

Moths are a type of insect similar to butterflies.  A species of moth lived in a forest made up of a mixture of dark- and light-colored trees.  Some of the moths were dark-colored and some were light-colored.  The coloring of these moths is inherited from parent to offspring.  The dark moths were hidden from predators on the dark trees, and the light moths were hidden on the light trees. 

When the light-colored trees were killed by a disease, the forest was suddenly made up almost entirely of dark-colored trees.  Generations later, almost all the moths had the dark coloring.  Which of the following gives a correct explanation?

  1. The light moths that survived did not mate and have offspring because they saw that their offspring would easily be seen and eaten by predators.
  2. The light moths that survived had dark offspring because the change in the environment caused the parents to have offspring that would do better in the changed environment.
  3. The light moths saw that the dark moths were less likely to be eaten by predators, so they made themselves dark as well.
  4. The dark moths were better hidden from predators, so more of them survived.  Because dark parents outnumbered light parents, there were many more dark offspring than light offspring in the next generation.

Distribution of Responses

Chart showing distrubtion of responses for Item EN029002

Percent of students responding correctly

Overall61%
Grades 6–857%
Grades 9–1268%
Male56%
Female65%
Primary Language is English61%
Primary Language is not English55%

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