Peppered Moth

• It is the most celebrated example of natural selection observed within a human lifetime: industrial melanism ran its course in about 50 generations. The first dark specimen was caught near Manchester in 1848, and by 1898 dark moths already made up 95% of the local population.
• In 2016 the mutation responsible was identified: a transposable element inserted into a gene, published in Nature.
• The dark form is in retreat today because cleaner air has brought the lichens back to the tree trunks: natural selection working in reverse.
• It is the most-cited example of natural selection in all the textbooks. During the Industrial Revolution, soot darkened the tree trunks of English woodlands and the black form, called carbonaria, went from exceedingly rare to dominant in polluted areas, because birds spotted it less often.
• With the clean air laws of the 20th century the process reversed: the lichens returned, the trunks lightened and the pale form regained ground. It is evolution observed in real time, over a century and a half.
• The genetic cause was identified in 2016: a transposable element, a jumping fragment of DNA, inserted into the first intron of the cortex gene. Statistical analysis places that mutation around 1819, with the Industrial Revolution already under way.
• The caterpillar mimics a twig, and not only in shape: it adopts the posture of a dry branch and can change colour to match the tone of the one it is sitting on.
• There is a third form, insularia, with intermediate patterns, genetically distinct from the other two.

Skeleton type

Invertebrate - chitin exoskeleton (insect)

Habitat & environment

Floodplains, wetlands and mixed deciduous woodland and their fringes. It also occurs on cultivated or abandoned farmland and even in densely built-up residential areas

Litter / clutch size

A single, very drawn-out generation a year. The females lay eggs in great numbers and the newly hatched caterpillars disperse by spinning silk threads on which they let the wind carry them

Sources consulted: Kettlewell 1955, Heredity 9(3); Kettlewell 1973, The Evolution of Melanism; Majerus 2008, Evolution: Education and Outreach 2(1); Van't Hof et al. 2016, Nature 534; Walton et al. 2018, Communications Biology 1; Cook 2003

Best time: From early May to August