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Peppered Moth

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Moth study backs classic 'test case' for Darwin's theory

By Steve Connor, Science Editor Published: 25 August 2007

For more than a century it has been cited as the quintessential example of Darwinism in action. It was the story of the peppered moth and how its two forms had struggled for supremacy in the polluted woodlands of industrial Britain.

Every biology textbook on evolution included the example of the black and peppered forms of the moth, Biston betularia. The relative numbers of these two forms were supposed to be affected by predatory birds being able to pick off selectively either the black or peppered variety, depending on whether they rested on polluted or unpolluted trees.

The evolution of the peppered moth over the last two hundred years has been studied in detail. Originally, the vast majority of peppered moths had light coloration, which effectively camouflaged them against the light-colored trees and lichens which they rested upon. However, due to widespread pollution during the Industrial Revolution in England, many of the lichens died out, and the trees which peppered moths rested on became blackened by soot, causing most of the light-colored moths, or typica, to die off due to predation. At the same time, the dark-colored, or melanic, moths, carbonaria, flourished because of their ability to hide on the darkened trees.

Since then, with improved environmental standards, light-colored peppered moths have again become common, but the dramatic change in the peppered moth's population has remained a subject of much interest and study, and has led to the coining of the term industrial melanism to refer to the genetic darkening of species in response to pollutants. As a result of the relatively simple and easy-to-understand circumstances of the adaptation, the peppered moth has become a common example used in explaining or demonstrating natural selection to laypeople and classroom students.

It became the most widely cited example of Darwinian natural selection and how it affected the balance between two competing genes controlling the coloration of an organism. Then the doubts began to emerge.

Peppered moth.jpg

Critics suggested that the key experiments on the peppered moth in the 1950s were flawed. Some went as far as to suggest the research was fraudulent, with the implication that the school textbooks were feeding children a lie.

Creationists smelt blood. The story of the peppered moth became a story of how Darwinism itself was flawed - with its best known example being based on fiddled data.

Now a Cambridge professor has repeated the key predation experiments with the peppered moth, only this time he has taken into account the criticisms and apparent flaws in the original research conducted 50 years ago. Michael Majerus, a professor of genetics at Cambridge University, has spent the past seven years collecting data from a series of experiments he has carried out in his own rambling back garden. It has involved him getting up each day before dawn and then spending several hours looking out of his study window armed with a telescope and notepad.

He wanted a definitive test of the idea that selective predation by birds really was responsible for the differences in the chances of survival among black and peppered varieties of B. betularia. His garden outside Cambridge is in an unpolluted area so in this setting it should be the typical or peppered variety of the moth that has a better chance of survival than that of the black or carbonaria form; it is unlikely to be seen by birds against the mottled background of the lichen-covered trees.

In a seminal description of his results to a scientific conference this week in Sweden, Professor Majerus gave a resounding vote of confidence in the peppered moth story. He found unequivocal evidence that birds were indeed responsible for the lower numbers of the black carbonaria forms of the moth. It was a complete vindication of the peppered moth story, he told the meeting.

"I conclude that differential bird predation here is a major factor responsible for the decline of carbonaria frequency in Cambridge between 2001 and 2007," Professor Majerus said.

"If the rise and fall of the peppered moth is one of the most visually impacting and easily understood examples of Darwinian evolution in action, it should be taught. It provides after all the proof of evolution," he said.

Criticisms of the 1950s experiments with the peppered moth, carried out by the Oxford zoologist Bernard Kettlewell, came to the fore in a 2002 book by the American author Judith Hooper. Hooper's book, Of Moths and Men, suggested that the scientists at the centre of these experiments set out to prove the story irrespective of the evidence.

While the professor has also described drawbacks to Kettlewell's methodology, he was able to address all of these concerns and even tested an idea that Hooper had raised in her book - that it was bats rather than birds responsible for moth predation - a suggestion he dismissed altogether.

Professor Majerus compiled enough visual sightings of birds eating peppered moths in his garden over the seven years to show that the black form was significantly more likely to be eaten than the peppered.

A statistical analysis of the results revealed a clear example of Darwinian natural selection in action.

"The peppered moth story is easy to understand, because it involves things that we are familiar with: vision and predation and birds and moths and pollution and camouflage and lunch and death," he said. "That is why the anti-evolution lobby attacks the peppered moth story. They are frightened that too many people will be able to understand."

Natural selection in action

The peppered moth comes in two distinct, genetic varieties: the black, melanic form (carbonaria) and the mottled form (typica). Against the background of a lichen-covered tree growing in unpolluted countryside, the typica form is well camouflaged. But in polluted areas where lichens do not grow, it is the melanic form that is difficult to see.

The Victorian naturalist J W Tutt noted that 98 per cent of peppered moths caught near Manchester at the end of the 19th century were the melanic variety. He was the first to suggest that it was the result of higher predation of typica by birds. With cleaner air in the late 20th century, it was the turn of the melanic form to suffer from bird predation. Now it is the typica form that is more common in most areas of Britain.

Source: [The Independent on Sunday (UK)]


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