- The moth population is shifting from predominantly dark-colored moths to predominantly light-colored moths over time. In 1959, when tree trunks were dark with soot, dark moths were more common because they were better camouflaged. By 1979 and 1995, as tree trunks became lighter due to cleaner air, light-colored moths became more common because they were now better camouflaged against the lighter background.
- This shows natural selection because the environmental change (cleaner air leading to lighter tree trunks) altered which moth color had a survival advantage. Birds preyed more on the moths that stood out against the tree trunks, so moths with better camouflage (light-colored moths after 1959) survived and reproduced more successfully, passing their advantageous traits to the next generation.
- If there were no birds to eat the moths, there would be no selective pressure based on color. Both light and dark moths would survive and reproduce at similar rates, regardless of tree trunk color, so the population would not shift dramatically in favor of one color over time.
- If England maintains clean air, I would expect the moth population to remain overwhelmingly light-colored in 25 and 75 years. Since the environment remains stable with light tree trunks, light-colored moths will continue to have the survival advantage. Natural selection will continue to favor them, and without a change in environmental conditions or predation pressure, the frequency of dark moths will remain very low.
Parent Tip: Review the logic above to help your child master the concept of evolution natural selection worksheet.
