The peppered moth, Biston betularia, is a species of nocturnal moths that can be found across the Northern hemisphere. In Britain, the two most common morphs are called typica (white with dark irregular patches and spots) and carbonaria (dark melanic). There is also an intermediate morph that is known as insularia.
Before the early 1800s, the melanic morph was unknown. About a hundred years later, this morph completely dominated the population. This coincided with the industrial pollution of forests, thereby depriving the white morph of its good camouflage and allowing the melanic morph to better hide in the surroundings, since trees had been darkened by soot and a lot of lichens died due to sulfur dioxide. This became known as industrial melanism. From the 1960s and to the present, this trend has been reversed. As the air and the environment became less polluted, the melanic morph fell in prevalence and the white morph rose again.
Research of the prevalence of different morphs at different places and times can be found in many scientific papers, such as Kettlewell (1956), Grant, Owen and Clark (1996) and Cook and Saccheri (2003).
Why are peppered moths a case of natural selection in the wild?
This is a fantastic example of natural selection in the wild, because there was a shift in the prevalence of the two forms due to selection from the environment and this occurred both during increased and decreased pollution. This has also been observed in many locations in both Europe and North America, further solidifying the case.
Later research showed that the major mechanism behind this change turned out to be bird predation. Before the pollution, white morphs had a good camouflage against light trees. During the strong pollution era, these were easier to spot by birds against the blackened background, whereas now the melanic morphs had a better ability to hide. As the environment improved, the situation was reversed again.
Creationists try to spread misinformation about these mechanistic details and how frequent these moths rests on different parts of the trees, but whatever they claim, their assertions do not disprove the fundamental fact that natural selection caused the shift in the prevalence of different morphs over time.
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