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Natural selection in black and white: how industrial pollution changed moths

The classic peppered moth. Martinowksy, CC BY-SA

Changing wildlife: this article is part of a series looking at how key species such as bees, insects and fish respond to environmental change, and what this means for the rest of the planet.

The world is constantly changing around us. However, we might not realise the importance of a change until we see its effects on another animal.

The peppered moth will always hold a special place in the annals of evolutionary biology. Its story is a classic example of natural selection, and of how animals can act as indicators of environmental change.

One of two colours

The peppered moth is found throughout Eurasia and North America and can be either white or black. When black and white morphs breed, their offspring are also either black or white (rather than grey), indicating that colour is controlled by Mendelian segregation in this species.

Another notable thing is that their survival tends to rely on them tricking their enemies with camouflage; juvenile caterpillars masquerade as twigs, while the adult moths blend into their surroundings. However, when a species relies on a strategy like this, they become vulnerable when the world changes without them.

peppered moth caterpillars masquerading as twigs. Noor MAF, Parnell RS, Grant BS - A Reversible Color Polyphenism in American Peppered Moth (Biston betularia cognataria) Caterpillars. PLoS ONE 3(9): e3142. doi:10.1371/journal.pone.0003142

Adult peppered moths are most active at night and hide (or try to hide) from predators in plain sight during the day. A typical place for them to hide is on trees, and the more common white morph tends to blend in better with the white bark birch trees in England than their black counterparts.

Pollution drives natural selection

Throughout the industrial revolution, people noticed that white moths became much rarer while black moths became much more common. Bernard Kettlewell, a research fellow at the University of Oxford, was among the first to investigate this puzzling change in abundance.

He conducted an elegant set of experiments during the 1950s to test whether the soot and pollution from industrialisation made it easier for black moths to blend in with their newly dirty surroundings, while white moths were less able to blend in and more vulnerable to predation by birds.

A black morph peppered moth. Jerzy Strzelecki

He conducted a series of observations, fieldwork and aviary experiments, and found that his data fit his predictions: black moths were becoming more abundant because they were being better-camouflaged and predated less by birds in soot-covered areas compared to their newly exposed and vulnerable white counterparts.

Understandably, this set of experiments became a classic example of Charles Darwin’s theory of natural selection that was used by teachers throughout the world.

However, valid critical discussion by fellow scientists of Kettlewell’s experimental methods, such as that moths rarely rest of tree trunks and unnatural numbers of moths being used in experiments, was exploited by anti-evolutionists who turned molehills into mountains and succeeded in casting doubt on the validity of his work.

Silencing the critics

One of the critics, Michael Majerus from the University of Cambridge, conducted a series of follow up studies in the early 2000s to address his, and others’ criticisms of Kettlewell’s work.

Similar to Kettlewell, he used a combination of natural history observations and a series of field experiments which are among the most substantial of their kind ever completed.

A white morph peppered moth on a tree. Gilles San Martin

His experiments were conducted at an unpolluted site, which led to the prediction that white moths should be eaten less than black moths. His results were exactly what were expected: black moths were eaten more often in a clean forest than the background-matching white moths.

Majerus’s experiments addressed his own, and other peoples’ criticisms of Kettlewell’s work. In doing so, he vindicated Kettlewell’s original experiments and convincingly demonstrated that predation by birds is selecting colour morph in peppered moths, and that predatory preference depends on the local environmental conditions.

Unfortunately, while he presented his work at a conference, he did not live to publish them. The work was published by Cook and co-authors in 2012.

Together, their work comprises perhaps the most elegant example of natural selection in the world.

We will be publishing more articles in this series in the coming days.

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