The Men Behind the Moth

How a tiny speckled moth destroyed reputations, built legacies and changed the world

Photo: LucasVphotos on Unsplash

In 1878, a few years before Charles Darwin died of a mysterious tropical disease, a letter arrived on his desk. It was a letter from a fan. The writer was a British naturalist named Albert Brydges Farn, and he had noticed a species of moth that seemed to vary widely in colour depending on where it lived. In particular, he said, there seemed to be a lot of darker moths in places where the lime kilns had blanketed the countryside in soot. “Do these variations point to the ‘survival of the fittest’?” Farn wrote. “I think so.”

If you took a biology class in high school, then you probably know the tale of the peppered moth. It is a simple case study that has given generations of biology students a framework for understanding Darwin’s theory of evolution by natural selection. It goes like this: the peppered moth had white wings and black spots until the Industrial Revolution covered England in soot, peeled the lichen off trees and turned a clean country black. At this point, peppered moths in polluted areas had mostly black wings with white spots. Before humans were pumping fumes out of factories, white moths would hide from birds by camouflaging against the light-coloured lichen of trees—but when the trees went black, only the rarer black moths would survive long enough to reproduce.

But the real story of the peppered moth is far more complicated than textbooks will tell you. Today we accept natural selection in peppered moths as a scientific given, but it took 134 years of thinking, arguing, experimenting and re-experimenting for the moth to become the textbook example that it is. The tale of the peppered moth is the story of men who spent years of their lives studying the secrets buried in their speckled black wings. It is the story of feuds lasting generations, legacies that bloomed and shattered, and ideas that take hold of you.

The moth idea made its way off Darwin’s desk and into the mind of a diehard Darwinian schoolteacher who was moonlighting as an entomologist. By the time James William Tutt was hunting and pinning Lepidoptera in the Yorkshire countryside, Darwin had been dead fourteen years, England was getting darker, and the world had still not seen natural selection with their own eyes.

The schoolteacher was no casual moth-watcher. He had a sharp wit, a face like Dickens and an incessant work ethic. He had climbed inside the moth world at the age of thirteen. He published prolifically, joined the Entomological Society of London and rose to the ranks of editor of a prominent entomology journal. He gave lectures and addresses, he penned papers and published books and poured over the oldest history and the latest research. And he too had noticed the rise of the black peppered moth.

He had a theory that the black moth was camouflaging better against soot-black surfaces, hiding from birds and living long enough to reproduce. So it made sense, Tutt said, that the darker moths were getting luckier. “So it really is,” he said in 1896. “The paler ones the birds eat, the darker ones escape.”

Tutt wasn’t the first to suggest something like this. All through the nineteenth century, moth enthusiasts had been commenting on how the black moths were on the rise—but they couldn’t quite agree on why. Some said it was the climate; others put it down to diet. But it is Tutt who is generally recognised as the first person to pin natural selection as the reason for the rise in black peppered moths.

But Tutt was not a scientist. He was a schoolteacher who collected moths when the bell rang. His peppered moth hypothesis wasn’t backed by experimentation or published in the important scientific journals—it was based on observation and speculation. In the end, Tutt’s ideas were rejected for a simple reason: scientists didn’t believe that birds were key predators of moths.

It was a long time before anyone tested the hypothesis.

Three decades later, a British professor claimed he had proof to explain why the moths were turning black. The professor’s name was John Heslop Harrison and he was a tall, stooping professor who had lived almost his whole life among the black byproducts of the North England coal mines. He wore black suits, black ties, combed his black hair and trimmed his black mustache. He believed the peppered moth was turning black because the caterpillars that came before them were eating the soot, which in turn was triggering a chemical mutation in their genetic material.

The food idea itself is not astounding. Most animals do get their colour from food—flamingos are pink because of the beta carotene in seafood and algae; the bright blue feet of boobies get their colour from the carotenoids in fish (the same pigment that, remarkably, makes carrots orange); even blue hydrangeas turn pink in aluminous soil.

But Harrison was talking about a change to an organism’s genetic material not through survival of the fittest, but through direct mutation. This is a surprising idea because it suggests that evolution—a change in a species’ genetic characteristics—doesn’t need generations to unfold. It is like saying that if your father is one in a long line of thin men but lifts weights before you are conceived, your genes will predispose you to muscularity. Darwin himself had suggested that something like this could play a role in evolution, but only put forth a “provisional hypothesis.” If Harrison could prove his hypothesis, it would mean that organisms could alter their genomes within their own lifetimes, a timeline far shorter than that of Darwin’s natural selection. So far, no one has produced enough evidence to prove that a species could change their heritable characteristics in a single generation, and most of the scientific world has given up on the idea.

Harrison said that he had fed polluted leaves to white moth caterpillars and watched them emerge as black moths. But the rate at which Harrison’s moths were supposedly mutating was absurd. It was far higher than anything anyone in the science world had ever recorded—higher, even, than a group of fruit flies that had been blasted by radiation.

Other scientists couldn’t seem to get the results Harrison had. Was he a genius? Or was he something else? It turns out that Harrison, who had been one of Britain’s most lauded botanists, was a fraud. He had forged evidence, lied about his work and faked discoveries that won him prizes. In fact, he was the ultimate trickster: dubious moth experiments aside, he famously had the nerve to plant rare seeds and grasses on a Scottish island and took an expedition back there to ‘discover’ them.

It was 1955 when things really heated up. Inside the brand-new Genetics laboratory at the University of Oxford’s Department of Zoology was an excitable naturalist named Dr Henry Bernard Kettlewell. By the time Kettlewell joined the laboratory in 1952 he had been a medical doctor, an anaesthetist, and an emergency war physician. He had moved to South Africa to study locusts, ventured out on expeditions in the desert and forestlands, and was not, as his critics made clear, a trained scientist. But Kettlewell—like Tutt—believed the peppered moth had turned black because generations of black moths had survived by camouflaging against soot-black trees, but he needed more than ideas to prove it

This is why scientists conduct experiments: to witness the macro by recreating it in micro. There is no way to accurately record how many moths exist in England over the space of decades, let alone how many are black, how many are white, and which are being rubbed out by birds. But there is a way to accurately record how many black and how many white moths exist in a very small and very tightly controlled environment.

Which is exactly what Kettlewell did. Over two long experimental summers he placed moths in both clean and polluted forests and tracked which variations disappeared from which environments. If it sounds obvious, that’s the point. The natural world is anything but simple. But experiments? The very best of them are fundamental.

Kettlewell recruited his test subjects by luring them into box traps, using virgin female moths (heavy on the pheromones) as bait. He painted the underside of the moths’ wings to mark his own moths from wild ones, shook them out of the box traps and onto the trees, let them settle into their daytime positions, and watched as they were picked off or passed over by woodland birds. At the end of each experiment he recaptured the moths and tallied up blacks and whites.

Kettlewell’s experiments proved several things: that birds are the primary predators of moths; that moths rest on colours close to their own; and that moths that camouflage better avoid being eaten. He had seen the birds eat the moths, watched them settle in resting positions that matched their colourings, and tallied up more surviving white moths in the clean forest and more black moths in the polluted one.

Naturally, the scientific community was sceptical of Kettlewell’s results, so as soon as he had finished his second experiment, he travelled back to the site of the first one and did it again—this time, with a camera.

It seemed that Kettlewell had done it: he had proven that the peppered moth had turned black by natural selection. Kettlewell’s experiment catapulted the peppered moth to evolutionary fame, and secured Kettlewell’s own position as the world’s leading expert on moth melanism. Kettlewell, in his own words, had found “Darwin’s missing evidence.”

Kettlewell—admittedly or not—had set out to prove what Tutt couldn’t. The problem here was that Kettlewell’s papers made almost no mention of Tutt, even though the experiment was clearly founded on Tutt’s ideas, and there was no way someone as entrenched in the moth world as Kettlewell hadn’t heard of Tutt. This is where Kettlewell’s critics accuse him of sloppiness and even academic dishonesty for failing to credit Tutt. Still others were slandering him as a fraud.

But an experiment is generally only considered credible if it can be replicated—and get the same results. The result of a one-off experiment can be a fluke; anything can happen once. I can flip a coin once and turn up tails, but it won’t tell the whole story.

Ten years after Kettlewell’s famous experiment, a zoology professor from Massachusetts named Theodore Sargent tried to replicate the results of the moth test. But Sargent’s birds seemed to eat black and white moths in equal amounts no matter the colour of the tree and his moths wouldn’t settle on matching background colours. He argued that Kettlewell had put the moths on trees when trees weren’t their natural resting place, suggested that Kettlewell had used dead moths in his photographs, and implied that he might even have trained his birds to pick only the uncamouflaged insects off the trunks.

Kettlewell’s reputation in the scientific world nosedived. After years of moth-hunting and days and nights spent in cold wet fields, Kettlewell laboured through bronchitis and pneumonia. His heart pumped poorly, he was struck down by the flu, his chest was permanently inflamed. In 1978 he fell from a birch tree on an expedition and never recovered. The following year, he overdosed on painkillers and died.

But right around the time Kettlewell was painting moth wings and setting birds after them, a little boy was born in the southeast of England. By his fourth birthday, Michael Majerus was obsessed: he would catch butterflies in his net, take field trips on weekends and wait for his father to arrive home from his travels with more specimens. At ten, he was reading thumb-thick books on ecological genetics.

Majerus grew up and became a geneticist. He taught evolution at the University of Cambridge and in 1998 he published a book about evolution that outlined the peppered moth story, and dove into the virtues of and problems with Kettlewell’s experiment. The paper made a splash in evolutionary circles, and was reviewed by a prominent biologist named Jerry Coyne.

Coyne was dismayed after reading the book. Seeing the peppered moth proof under the microscope, he said, was like finding out that Santa was actually his father. “For the time being,” he said, “we must discard [the peppered moth] as a well-understood example of natural selection in action.” He stopped teaching the peppered moth to his students, and the scientific world began to turn up its nose at any mention of the moth.

Majerus was upset. He had meant only to discuss some of the flaws in Kettlewell’s experiment and suggest that the moth story might be more complex than we want it to be. He had not meant to denounce Kettlewell and the peppered moth entirely. He respected Kettlewell. He was passionate about the mechanisms of evolution. And he still believed the peppered moth held the keys to Darwin’s universe.

The review broke through the bubble of academia and into the mainstream, where the fall of the peppered moth story made headlines across the world. The bigger problem was the creationists: they took hold of the review and ran with it, tearing apart the moth in print, spreading it across the web faster and attacking it in the anti-evolution curriculum of religious schools. The peppered moth, and the people who believed in it, seemed beyond repair.

Majerus went all in. He found a hectare of rural garden space in Cambridgeshire, and handpicked 103 tree branches for a new kind of moth study. He numbered the branches, measured their angles and fixed climbing structures to the tall ones (some of them eight storeys high). Every night, he slid long black netting sleeves around a handful of branches, holding them in place with wire rings, and releasing a single black or white peppered moth into the cylinder.

Before sunrise the next morning, Majerus would check on the moths and note their positions. He would pull off the cylinders and then watch the moths—sometimes with binoculars, sometimes with a telescope. If a bird swooped in on a moth, he wrote it down. If a moth changed position, he wrote it down. He continued this for three or four hours every morning, and then plucked off the rest of the moths, ready to start fresh the next day.

By 2007 Majerus had been releasing and watching moths on tree branches every night of peppered moth season for six years running. All in all, he had released 4,864 moths into his experimental hectare. It was—and still is—the single largest experiment of its kind ever attempted. In August he travelled to Sweden and presented the findings of the world’s longest, largest and most elaborate peppered moth experiment: the peppered moth had turned black because of the moth’s ability to camouflage from birds. In short: what Tutt had been saying all along. And Kettlewell. And the others.

But it would be Majerus’s last experiment.

In 2009, Majerus died of mesothelioma. He was fifty-four, and he had still not published the findings that would change everything. After his death, a team of four scientists spread across England assessed the data that the moth man had left behind.

In 2012, they published a small, four-page paper titled Selective bird predation on the peppered moth: the last experiment of Michael Majerus. The paper proved that Majerus’s epic experiment had produced indisputable evidence for the peppered moth story. Unlike Tutt, Majerus was a career scientist with the means to prove an idea. And unlike Kettlewell, Majerus had plugged every experimental hole you could think of.

He had dismissed the sceptics, denounced the anti-evolutionists, silenced the pseudo-scientists ready to pounce at any opportunity. He had proven what Tutt couldn’t, finished what Kettlewell hadn’t, and demonstrated what Darwin was unable to witness in his own lifetime.

Historians believe that Darwin ignored the letter about the moth. He certainly never pursued it publicly. We don’t know why—maybe he was sick of fan mail, maybe he really didn’t believe it possible. Either way, Darwin went to his grave without seeing a species change in his own lifetime. As it turned out, it had flown right by him.

Evolution is the kind of scientific breakthrough that changes everything. But it is so large, so complex, and so removed from the realities of our daily lives that it is hard to get your head around. That’s why the humble peppered moth is so important. It is easy to grasp and easy to see; it is a case of black and white. Kettlewell himself said that the morphing colours of moths and butterflies throughout the 19th and 20th centuries was “the most striking evolutionary change ever actually witnessed in any organism, animal or plant.”

But getting there wasn’t easy. From a letter on Darwin’s desk to a tiny paper printed in 2012, the legacy of the peppered moth has been polished, restored, and placed back in the centre of the mantlepiece. It is back in biology textbooks and back in favour. In the end, the tale of the peppered moth is more than a story about the mechanics of evolution: it is a story about the evolution of scientific thought.