The Pre-Industrial Landscape

Before the Industrial Revolution, most peppered moths in England were light-colored with black speckles. This coloration provided excellent camouflage against the lichen-covered, light-colored tree trunks, protecting them from predatory birds. The dark-colored (melanic) form of the moth existed but was rare, as it was more visible to predators and thus less likely to survive and reproduce.

The Impact of Industrialization

With the onset of the Industrial Revolution, massive amounts of soot and pollution were released into the environment. This pollution killed off many lichens and darkened the tree trunks. Suddenly, the once-camouflaged light-colored moths became easy targets for predators, while the dark-colored moths were now better hidden against the soot-blackened trees. As a result, the frequency of the melanic form increased dramatically in urban and industrial areas.

Natural Selection at Work

This shift in moth coloration is a textbook case of natural selection. The environmental change altered the selective pressures acting on the moths:

• Light-colored moths, once favored, became more visible and were eaten more frequently.

• Dark-colored moths, previously at a disadvantage, now had a survival advantage and reproduced more successfully.

Over several generations, the population shifted from being predominantly light-colored to predominantly dark-colored. When air quality improved in the mid-20th century and lichens returned, the selective pressure reversed, and light-colored moths once again became more common.

Why Not Other Explanations?

• New mutation due to pollution: The dark form already existed in the population before the Industrial Revolution; it was not a new mutation caused by pollution.

• Macroevolution: The change observed is a shift in trait frequency within a species (microevolution), not the emergence of a new species.

• Natural selection of the camouflaging mechanism: While camouflage is the mechanism, it is the natural selection of moth coloration (the trait) in response to environmental change that is the evolutionary process at work.

Conclusion

The prevalence of dark-colored peppered moths during the Industrial Revolution is best explained by natural selection of dark colored moths which were initially present in fewer numbers. As the environment changed, so did the selective pressures, leading to a rapid and observable evolutionary shift.

Correct answer:
(1) natural selection of dark coloured moths which were initially present in fewer numbers.