Mighty Moth Man

An evolutionary biologist’s posthumous publication restores the peppered moth to its iconic status as a textbook example of evolution.

May 1, 2012
Cristina Luiggi

 

SALT AND PEPPER: Camouflage is the key to peppered moth predator avoidance.
SALT AND PEPPER: Camouflage is the key to peppered moth predator avoidance.
MICHAEL MAJERUS

It must have seemed terribly ironic to late University of Cambridge evolutionary biologist Michael Majerus, after dedicating nearly half a century to the study of peppered moths (Biston betularia), that in the late 1990s his name became central to an increasingly contentious campaign to strip the peppered moth of its status as the prime example of Darwinian evolution in action.

It’s a well-known story: The moth’s ancestral typica phenotype is white with dark speckles. In the decades following the Industrial Revolution, a new, soot-colored form, known as carbonaria, flourished and displaced the typica moths in the heavily polluted woodlands of Europe.

Although scientists hypothesized as early as 1896 that the increase in carbonaria frequency could be explained simply by the fact that soot-covered tree barks camouflaged the dark-colored moths against predation by birds, it wasn’t until the 1950s that an Oxford University lepidopterist named Bernard Kettlewell performed the key experiments that provided persuasive evidence that bird predation was indeed the selective agent at work.

Kettlewell placed live carbonaria and typica moths on tree trunks in polluted and unpolluted woodlands in the U.K. and counted how many of each type survived predation. Moths with a coloring that blended better with the tree trunks survived in greater numbers.

But beginning in the 1980s, peppered-moth experts, including Majerus, began noting flaws in Kettlewell’s experimental designs. The most serious of these was that limited research into peppered moth behavior seemed to suggest that tree trunks were not the insect’s preferred resting place. That alone threatened to put a serious dent in the validity of Kettlewell’s setup—and in the bird predation theory itself.

In his 1998 book, Melanism: Evolution in Action, Majerus discussed these shortcomings in the context of a critical dissection of all the peppered-moth case evidence that had accumulated.

MOTHS ON TREES: Majerus confirmed that wild moths (typica above and carbonaria below) did indeed rest on tree trunks.
MOTHS ON TREES: Majerus confirmed that wild moths (typica above and carbonaria below) did indeed rest on tree trunks.
MICHAEL MAJERUS

For Jerry Coyne, a University of Chicago evolutionary biologist who had been teaching the case to university students for years, learning that “Kettlewell’s experiments weren’t really that carefully done” came as a shock, he says. In a review of Majerus’s book, published in a November 1998 issue of Nature, Coyne concluded that “for the time being we must discard Bistonas a well-understood example of natural selection in action.”

Coyne’s review dismayed Majerus, who, despite his criticisms, did believe there was strong evidence to back the case for the peppered moth as evolution’s poster child.

In no time, the popular media as well as the creationist movement pounced on the review, calling the classic peppered moth story “fraudulent,” a “blunder,” and an “embarrassment” to science. Even The Scientist contributed to the hysteria, publishing a 1999 opinion piece by then-cell biology postdoc and current intelligent design advocate Jonathan Wells on rethinking the peppered moth story.

“Through innuendo and through public smear campaigns, this case study was put into disrepute undeservedly,” says Ilik Saccheri, an evolutionary biologist at the University of Liverpool.

Not one to shy away from controversy, Majerus was nonetheless distressed by the fallout from Coyne’s book review. “[Majerus] was a cocky bastard,” says Laurence Cook, a retired geneticist from the University of Manchester. “He liked to have arguments of this kind around. But I think he did feel, because what he had written had been misinterpreted, that it was up to him to try and put it right.”

And that’s exactly what, in 2001, Majerus set out to do.

Over the course of 7 years, Majerus systematically recorded the fates of 4,864 peppered moths released into a 2.5-acre plot of unpolluted rural land in Cambridgeshire, U.K.

This time, instead of artificially placing the moths on tree trunks during the daytime, as Kettlewell did, Majerus released the moths within the hour before sunset into netting sleeves set up around the lateral branches of trees—which, through years of observation, he had identified as the moth’s preferred resting site. (He somewhat vindicated Kettlewell by observing that wild moths could in fact be found resting on trunks at least a third of the time.)

Majerus would then leave the moths to flutter about inside the enclosure throughout the night, removing the netting only after sunrise, when the moths were already settled in their resting positions for the day. After 4 hours or so, he counted the number of moths that were still at their resting sites. Those that were missing were presumed eaten. He also saw some of them being eaten by birds.

The results of his ambitious predation experiment showed that, in the unpolluted parkland of Cambridge, carbonaria moths had a daily survival rate 9 percent lower than that of their light-colored kin. This was significant enough to explain rapid directional changes in color frequencies.

In 2007 Majerus presented his results at a conference in Uppsala, Sweden. But before he could publish them, he passed away from a sudden and aggressive case of mesothelioma in 2009.

After realizing that Majerus’s results would not be published, Cook, Saccheri, and evolutionary biologists James Mallet from University College London and Bruce Grant from the College of William and Mary published the detailed account of Majerus’s work in the journal Biology Letters in February (Biol Lett, doi:10.1098/rsbl.2011.1136, 2012). “We really felt that we had a duty to try and make sure that all his efforts were not wasted,” Cook says.

Upon seeing Majerus’s results in a peer-reviewed journal, Coyne resumed teaching the peppered moth story after a 14-year hiatus. “He did everything he needed to do to take care of the problems in Kettlewell’s experiment,” he says.

Correction (May 4, 2012): This story has been updated from its original version to correctly state that Kettlewell used only live moths in his experiments, not live and dead moths as originally stated. The Scientist regrets the error.