Peppered Moths Did Evolve Through Natural Selection, Study Confirms

The darkening color of the peppered moth during the nineteenth century, often used by high school textbooks as a case study for adaptation, was confirmed as an accurate example of natural selection in a paper co-authored by a Harvard faculty member.

During the Industrial Revolution, English scientists noticed an increasing number of darker colored British peppered moths, an observation that has since been attributed to an adaptation to the soot-covered trees of the industrial era. As air quality improved in later decades, lighter colored moths regained their previous prevalence.

However, this finding garnered some controversy in the scientific community during the 1980s and 1990s, according to the paper’s co-author James Mallet, a lecturer in Organismic and Evolutionary Biology.

“It was just too good of a story, so scientists began looking for possible problems with it,” Mallet said.

The late Michael Majerus, a professor at University of Cambridge who was one of the first researchers to raise these concerns, set out on a six-year project to conduct experiments and make observations to improve on the previous work, according to Mallet’s paper. Using data from Majerus’ experiments, the paper evaluated the evolution of peppered moths.

“Majerus felt some of the experiments hadn’t been very well done, though he believed natural selection was the main factor that explained the change in color of peppered moths,” Mallet said.

Majerus utilized a random release procedure that allowed moths to land on branches and twigs in addition to the tree trunk itself. He also released the moths in smaller numbers and at night, according to the study.

“The peppered moth experiment is the biggest of its type,” Mallet said. “It was done in the most careful way that Michael Majerus, who is really an expert on the moths, could devise—and therefore it answers many of the possible criticisms that he and others had on the previous experiments.”

Mallet and three other professors from British universities used Majerus’ experimental data to write the paper that confirmed what most scientists have long accepted.

“With this new evidence added to the existing data, it is virtually impossible to escape the previously accepted conclusion that visual predation by birds is the major cause of rapid changes in frequency of melanic [dark] peppered moths,” the study reported.