Q.The evolutionary story of moths in England during industrialisation reveals, that 'evolution is apparently reversible'. Clarify this statement.

The evolutionary story of moths in England during industrialisation provides a clear example of natural selection in action and its apparent reversibility.

• Initially, before industrialisation, light-coloured moths were predominant because they blended effectively with the light, lichen-covered tree trunks, avoiding predation.
• As industrialisation progressed, pollution darkened the tree trunks with soot, making the light moths highly visible to predators. Dark-coloured moths, which were previously rare, now had a survival advantage as they were camouflaged against the darkened bark, leading to a significant increase in their population.
• Later, with improved pollution control measures, the tree trunks gradually lightened again as lichens returned. This shift in the environment reversed the selection pressure; light-coloured moths once again gained a survival advantage due to better camouflage, and their population began to increase, while dark moths declined. …

The evolutionary story of the peppered moth (Biston betularia) in England during the Industrial Revolution provides a compelling real-world example of natural selection in action, and how this process is intimately tied to environmental conditions. The observation that 'evolution is apparently reversible' stems from how the moth populations responded to both the rise and fall of industrial pollution.

Before the advent of industrialisation in the mid-19th century, the English countryside was relatively pristine. Tree trunks were often covered in light-coloured lichens, providing excellent camouflage for the light-coloured, speckled variety of the peppered moth. In this environment, any darker, melanic moths stood out against the pale background, making them easy targets for predatory birds. Consequently, the light-coloured moths had a survival advantage, and their population dominated. This is a clear demonstration of natural selection favouring traits that enhance an organism's ability to survive and reproduce in its specific habitat.

As the Industrial Revolution progressed, factories proliferated, burning vast quantities of coal. This released immense amounts of soot and smoke into the atmosphere, leading to widespread air pollution. The consequences for the moths' habitat were profound:

• Darkened Tree Trunks: Soot settled on tree trunks, turning them dark.
• Lichen Decline: The pollution killed off the sensitive lichens that once covered the trees.

This drastic environmental change reversed the selective pressure. Now, the light-coloured moths, once camouflaged, became highly conspicuous against the darkened, bare tree trunks. Conversely, the rare dark-coloured, melanic moths, which were previously at a disadvantage, now found themselves perfectly camouflaged. Predatory birds began to preferentially prey on the easily visible light moths. Over generations, the frequency of the dark-coloured moths rapidly increased, while the light-coloured variety became scarce. This phenomenon is known as industrial melanism.

The statement that 'evolution is apparently reversible' comes into play with the subsequent environmental changes. In the latter half of the 20th century, particularly after the implementation of the Clean Air Act in Britain, efforts were made to reduce industrial pollution. As air quality improved, the amount of soot in the atmosphere decreased. This led to:

• Lighter Tree Trunks: Trees gradually shed their soot layers and became lighter.
• Return of Lichens: Lichens began to recolonise the tree trunks. …

Trace the moth population as a three-stage timeline (pre-industrial to industrial to post-clean-up) and show that the dominant colour at each stage simply tracked which coloration was camouflaged against the bark of that period — 'reversibility' then follows automatically, since re …