Question

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

Answer 1

In a collection of moths made in the 1850s, i.e., before industrialisation set in, it was observed that there were more white-winged peppered moths (Biston betularia) on trees than dark-winged or melanised moths (Biston carbonaria). However, in the collection carried out from the same area, but after industrialisation, i.e., in 1920, there were more dark-winged moths in the same area, i.e., the proportion was reversed.

• Before industrialisation set in, thick growth of almost white-coloured lichen covered the trees in that background the white-winged moth survived but the dark-coloured moth were picked out by predators.

Lichens can be used as industrial pollution indicators. They will not grow in areas that are polluted. During post-industrialisation period, the tree trunks became dark due to industrial smoke and soots.

• Under this condition the white-winged moth did not survive due to predators, dark-winged or melanised moth survived. Hence, moths that were able to camouflage themselves, i.e., hide in the background, survived. This understanding is supported by the fact that in areas where industrialisation did not occur, e.g., in rural areas, the count of melanic moths was low. This showed that in a mixed population, those that can better adapt, survive and increase in population size.