The Peppered Moth

At the beginning of the nineteenth century, most moths in the UK were an off-white color, highly adapted to hiding from predators, such as birds, by resting on pale, mottled grey tree trunks. But the Industrial Revolution polluted the environment, raining soot down on the countryside. As a result, white moths became highly visible on the now blackened tree trunks. Black moths, on the other hand, such as the peppered moth (which had previously been quite rare), suddenly had a competitive advantage. They were well camouflaged on black tree trunks, and their numbers grew exponentially. By the early twentieth century, they were the dominant moth form in polluted areas of the UK.

The above story is considered the classic example of "evolution in action." It shows how an environmental change can give one form of a species a selective advantage, leading to its dominance. Its validity rests upon experiments conducted by researcher H.B.D. Kettlewell during the 1950s, which demonstrated that white moths do have an advantage over dark moths on pale trees, and a disadvantage on dark trees, and vice versa.

But what made the story of the peppered moth particularly popular was the visual evidence. In 1955 Kettlewell published a pair of photos showing the relative camouflage of the black and white moth forms in the two settings. In the top photo it is easy to see the black moth on the pale, lichen-covered bark and to imagine how a bird could pick it off, but the white moth is almost invisible. In the bottom photo, by contrast, the black moth is almost invisible while resting on a soot-blackened tree.

Since the mid-1960s most Biology textbooks have included the story of the peppered moth, accompanied by Kettlewell's two photos (or ones very similar to them). The ubiquity of the images made it that much more shocking when the public learned the photos were staged. Finding black and white moths posed beside each other in a natural setting would have been almost impossible, so to create the photos Kettlewell pinned dead moths to tree trunks. Moth experts knew the photos were staged because live moths would not have had extended wings. But no textbook ever disclosed this detail to readers.

The staging of the photos was first raised as an issue by intelligent-design advocate Jonathan Wells in his 2000 work Icons of Evolution. But the controversy reached a more mainstream audience in 2002 when science writer Judith Hopper discussed it in her popular account of the science of the peppered moth, Of Moths and Men.

The staging was an issue, critics argued, because it over-simplified the peppered moth story and made it seem that the camouflage of the moths was a self-evident advantage. However, it wasn't clear that moths rested on tree trunks during the day, as the pictures implied. Some evidence suggested they preferred to remain higher in the tree canopy and beneath branches where their coloration would have been less of an advantage. Also, it wasn't clear that birds were the main predator of moths. Bats also ate moths, and since bats use echolocation to navigate, the coloration of the moths would not have made a difference. Critics also questioned the methodology of Kettlewell's experiments.

Scientists still vigorously defend the peppered moth story as an example of evolution in action. They also defend the use of the staged photos in textbooks, arguing that, although they're not entirely accurate, they offer an invaluable way of presenting the concept of natural selection to students in an easy-to-comprehend form.

Nevertheless, the pair of images has become one of the most famous and controversial examples of staged photographs in all of science.