Environmental Changes
Before the Industrial Revolution, the peppered moth was mostly found in a light grey form with little black speckled spots. The light-bodied moths were able to blend in with the light-coloured lichens and tree bark, and the less common black moth was more likely to be eaten by birds. As a result of the common light-coloured lichens and English trees, therefore, the light-coloured moths were much more effective at hiding from predators, and the frequency of the dark allele was about 0.01%.
During the early decades of the Industrial Revolution in England, the countryside between London and Manchester was blanketed with soot from the new coal-burning factories. Many of the light-bodied lichens died from sulphur dioxide emissions, and the trees became covered with soot. This led to an increase in bird predation for light-coloured moths, as they no longer blended in as well in their polluted ecosystem: indeed, their bodies now dramatically contrasted with the colour of the bark. Dark-coloured moths, on the other hand, were camouflaged very well by the blackened trees.
Although a majority of light-coloured moths initially continued to be produced, most of them didn't survive, while the dark-coloured moths flourished. As a result, over the course of many generations of moths, the allele frequency gradually shifted towards the dominant allele, as more and more dark-bodied moths survived to reproduce. By the mid-19th century, the number of dark-coloured moths had risen noticeably, and by 1895, the percentage of dark-coloured moths in the Manchester peppered moth population was reported at 98%, a dramatic change (by almost 1000%) from the original frequency. This evolved darkening in colour as a result of industrialization has come to be known as industrial melanism as a result.
While evidence of increasing frequency of melanic forms in the Lepidoptera was available during Darwin’s lifetime – the first observations were made in 1848 – current understanding is that it was not until 1896, 14 years after Darwin’s death, that Tutt explicitly linked melanism with natural selection. However, a recent article reports that melanism in the Lepidoptera had been linked to natural selection prior to Tutt. Albert Brydges Farn (1841–1921), a British entomologist, wrote to Darwin on 18 November 1878 to discuss his observation of colour variations in the Annulet moth (then Gnophos obscurata, now Charissa obscurata). In his letter, Farn mentions the existence of different colour morphs, describing how each is matched to the habitats in which they are found (dark morphs on peat, white morphs on chalk cliffs) and refers explicitly to this variability as pointing to ‘survival of the fittest’.
In modern times, because of cleaner air standards in Europe and North America, the dark-bodied moth is becoming less frequent, again demonstrating the adaptive shifts in the peppered moth population.
Read more about this topic: Peppered Moth Evolution