"One Of The Most Beautiful Experiments In Evolutionary Biology": What The Peppered Moth Taught Us About Evolution

Despite the wealth of evidence supporting it, its obvious sense as an explanation, and its endorsement by many and varied religious institutions, there are some people out there who still don’t accept the theory of evolution. Sure, they sometimes say, fossils might be real; there are animals that once existed that no longer do – but if one species could really become another, wouldn’t we be able to see it happening?

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Lesser advocates might point out that no, evolution is a long-term process built off the back of myriad imperceptible changes at the genetic level, eventually building up enough to affect an entire population. But we’ve got a better counterargument, and it’s this: that you can see it happen – and what’s more, somebody kind of did.

The hypothesis

Britain has, throughout the centuries, been home to two things: bugs and biologists. In the mid-1800s, the country gave the world worm enthusiast Charles Darwin and Alfred Russel Wallace; then, a century later, it brought forth Henry Bernard Davis Kettlewell.

The latter name may not be as well known as the former ones – and that’s fair, since Kettlewell didn’t come up with or present the entire theory of natural selection. But what he did do, in a famous series of experiments in the 1950s, was demonstrate it.  

“H. B. D. Kettlewell’s work on the phenomenon of industrial melanism is widely regarded as the classic demonstration of natural selection and one of the most beautiful experiments in evolutionary biology,” wrote David Wyss Rudge, a professor in the Department of Biological Sciences at Western Michigan University, in a 2005 article for the journal BioScience.

“Kettlewell’s life work centered on the phenomenon of industrial melanism,” Rudge explained – that is, the phenomenon of animals rapidly evolving darker coloring, seemingly in response to rising industrial pollution. In particular, as a lepidopterist, he was interested in peppered moths (Biston betularia) – a species which has always come in white (typica) or black (carbonaria) versions, but in very different ratios at different times.

In the period between Darwin and Kettlewell, the dark moths were on the up – and, conventional wisdom said, that was because of all the soot in the air. “E. B. Ford, Kettlewell’s mentor at Oxford University, thought the spread of the dark form reflected a physiological advantage conferred by the gene responsible for the dark form,” explained Rudge. “To account for why the spread was limited to manufacturing areas, Ford drew attention to the obvious disadvantage of dark coloration against avian visual predators when the moth rests on pale lichen-covered trees in unpolluted environments.”

Peppered moths are what’s known as a “cryptic” insect – one whose defenses rely on an ability to disguise itself as something else. For the moths, it’s tree trunks: look at a typica peppered moth on a lichen-covered branch, and you’ll be hard-pressed to pick it out; similarly, a carbonaria will blend in perfectly against a blackened, soot-covered tree in an industrial area.

But while the theory seemed sound, nobody had ever proved it evidentially. Seriously, it’s hard to overstate how much we “knew” about the life of wild insects was pretty much vibes-based at this point. As Rudge pointed out, not only were there no field studies into bird predation on industrial melanistic species, but apparently, nobody had ever even confirmed that birds ate cryptic insects at all.

Evidently, Kettlewell had work to do.

The experiments

If you’re going to prove whether or not industrial melanism helps moth species elude birds, you’re going to have to start small. Like, really small: Kettlewell’s first step was “a preliminary experiment […] to determine whether birds ate moths at all,” wrote Rudge.

Perhaps unsurprisingly to modern audiences, the answer was “yes, they do” – but now here’s the follow-up: do they eat moths in a way that suggests the moths’ disguises are working?

Well, this one is easy, you might say: just put some white moths and some black moths on the same bark, and see whether the birds spare the ones that match their background. Kettlewell had the same idea – and it failed spectacularly. “During the first two hours, the birds ignored the moths entirely,” Rudge wrote, “and then proceeded to take nearly all of the moths, regardless of the background against which they rested.”

So much for that hypothesis. But then, Kettlewell had a brainwave: perhaps, he thought, the birds were particularly tuned in to mothkind right now. They were, after all, in a moth-eating experiment – maybe they were good at finding the moths because they were simply looking for them especially hard, searching each trunk systematically as soon as they entered the enclosure and “thereby defeating the object of the experiment,” Kettlewell noted.

Like the moths, he had to adapt. Rather than only moths, he filled the birds’ enclosure with a smorgasbord of insect life. “This proved successful,” recorded Kettlewell; the birds were less laser-focused on peppered moths, and as a result, certain biases started to manifest. The birds “took [the insects] in an order of conspicuousness similar to that gauged by the human eye,” Kettlewell confirmed, and “if the same could be shown in the field, it would provide the missing data needed to substantiate the current explanation of industrial melanism.”

The theory was sound. But how would it play out in the real world?

The field test

If you aren’t familiar with the Black Country, in the Midlands of England, you can get a sense of their one-time effect on the surrounding countryside by imagining Mordor. That’s not hyperbole: it’s been suggested that Tolkien was not just inspired, but directly based the “barren wasteland, riddled with fire and ash and dust” where “the very air you breathe is a poisonous fume” on the real-life area.

It was, in other words, the perfect place to test whether black peppered moths had an advantage over their paler cousins. In a wood just outside of the industrial city of Birmingham, Kettlewell used a dab of paint to mark hundreds of peppered moths – 447 carbonaria and 137 typica – and set them loose.

“The position of the paint marks, being on the underside which is not exposed by day, excluded the possibility of an additional risk of attack by predators,” recorded Kettlewell. 

“The released insects were recaptured by assembling to females and at light traps,” he then noted. “Of 149 which were recovered, the proportions were carbonaria 27.5 percent, [typica] 13.0 percent.”

Evidently, the darker moths had a more-than-doubled chance of survival over their paler compatriots – with the local birds “act[ing] as selective agents,” Kettlewell concluded, “as postulated by evolutionary theory.” And this was backed up by the local wildlife, too: it wasn’t just Kettlewell’s own moths who got lured into his traps, and out of the non-daubed specimens he found that an incredible 93 percent were the carbonaria variety.

A second experiment in the same wood provided similar statistics, and a reverse experiment – releasing a bunch of peppered moths in a non-polluted wood – resulted in exactly what you’d expect if Kettlewell’s hypothesis was correct: from a group of 393 typica and 406 carbonaria, he was able to recapture 54 and 19, respectively. 

“It must be accepted, therefore, that the carbonaria has an approximate 6:1 advantage in Birmingham, and the reverse was true for Deanend,” Kettlewell concluded.

It seemed the theory was sound. But would it hold up against criticism?

The controversy

For many, Kettlewell’s experiments weren’t just convincing, but genuinely beautiful. But at some point in the 1990s, people – and it was some certain people in particular – started to question the result. Why?

Well, the problem with such a clear demonstration of natural selection in action is that it makes it really hard to defend pure creationism – so, rather than adjust their worldview based on evidence, fundamentalist Christians attacked the peripheries.

“Open any biology textbook published before about 2000 and you’re likely to find photos of typica moths and carbonaria moths against either ‘clean’ or sooty tree trunks,” wrote Stephanie Keep, former Editor of Reports of the National Center for Science Education, in 2014. “The moths in these photos are dead – preserved and stuck onto the tree trunks as part of an experiment to assess the effect of moth density on the feeding behavior of the birds that feed on them.”

“Unfortunately, some people seized on this perhaps not-exactly-elegant experimental set-up, and cried foul,” Keep explained. “Kettlewell was called a fraud and a fake, and scientists and textbook developers alike were accused of participating in a conspiracy to trick our children into believing in natural selection. The moths became pariahs in some circles.”

But this and other objections weren’t enough to sway scientists in general. Not because they were being “dogmatic”, as creationists (ironically enough) accused them – but because they, to quote Rudge again, “rest[ed] on […] fundamental misunderstandings about the nature of science.”

The conclusion

We don’t need to only hand-wave away arguments against Kettlewell’s work to defend it. Since the original experiments, his methods have been fine-tuned and his results replicated, again and again… and again and again.

And the smoking gun for the idea that it’s genetic adaptations we’re looking at? Well, we now know the exact genes responsible for the melanism of the carbonaria moths.

It was undoubtedly a mam-moth (sorry) undertaking. “We knew that within [the] 400,000 bases [of the moth DNA], there was some sequence that had to […] cause the actual difference between the black type and the typical type,” explained Ilik Saccheri, a professor of evolutionary biology at the University of Liverpool, in 2016. “So we went about an excruciatingly tedious process of identifying every single difference between the two types.”

“After a long time, we eventually managed to get down to a single one, which then had to be the causal mutation,” he told the BBC at the time. “To our surprise, it also turned out to be a rather unusual type of mutation.”

Not only do we know how the change happened, we also know when. The first black peppered moth likely hatched in 1819 – and from there, its genetic camouflage allowed the new coloring to sweep the country.

Today, Kettlewell’s work and reputation are safe once again – and so is the theory of natural selection. 

“Had Darwin observed industrial melanism, he would have seen evolution occurring not in thousands of years but in thousands of days – well within his lifetime,” Kettlewell wrote in 1959. “He would have witnessed the consummation and confirmation of his life's work.”