Sunday, March 7, 2004

Lesson excerpt: The peppered moth



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This excerpt is from a proposed 10th-grade biology lesson on how students can critically analyze evolution. Students are encouraged to use additional research resources.

PEPPERED MOTHS

Brief supporting sample answer: During the industrial revolution in England, more soot was released into the air. As a result, the tree trunks in the woodlands grew darker in color. This environmental change also produced a change in the population of English peppered moths (scientifically known as Biston betularia). Studies during the 1950s have suggested a reason for this change. It was observed that light-colored moths resting on dark-colored tree trunks were readily eaten by birds. They had become more visible by their predators compared to their dark-colored counterparts. This different exposure to predation explained why the light-colored moths died with greater frequency when pollution darkened the forest. It also explained why light-colored moths later made a "comeback" when air quality improved in England. This whole situation demonstrates how the process of natural selection can change the features of a population over time.

Brief challenging sample answer: English peppered moths show that environmental changes can produce microevolutionary changes within a population. They do not show that natural selection can produce major new features or forms of life, or a new species for that matter - i.e., macroevolutionary changes. From the beginning of the industrial revolution, English peppered moths came in both light and dark varieties. After the pollution decreased, dark and light varieties still existed. All that changed during this time was the relative proportion of the two traits within the population. No new features and no new species emerged. In addition, recent scientific articles have questioned the factual basis of the study performed during the 1950s. Scientists have learned that peppered moths do not actually rest on tree trunks. This has raised questions about whether color changes in the moth population were actually caused by differences in exposure to predatory birds.




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