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The Peppermoth: A Case of Evolution in Action

Updated on April 1, 2017
The typica form of the peppered moth seen here was the most common form of the peppered moth in Britain prior to the industrial revolution.
The typica form of the peppered moth seen here was the most common form of the peppered moth in Britain prior to the industrial revolution. | Source

The peppered moth, a nocturnal creature by nature, spends its night foraging for food and its day sitting on the barks of trees. As you can imagine, its choice to spend its daylight hours, when it is inactive and unable to spot oncoming danger, perched on the bark of trees makes it particularly vulnerable to any predator looking for a quick, easy snack.

Luckily for the peppered moth, it has a defense mechanism that lowers the chances of the moth being spotted while it naps. The moth, whose wings are white and dotted with black specks (typica), easily blends into the lichen background of light-coloured trees.

However, there were times when its speckled body was not so easily camouflaged. During the late 1800s, population growth in Britain and expanding foreign trade led to an increased demand for British goods. The system, at that time, that was used to manufacture goods relied heavily on animal and water power and was not cost effective. It was at this time that inventor James Watt made improvements to the already developed steam engine.

The newly improved steam engine would then go on to power machinery, locomotives, and ships. Most of the machinery that was used to mass produce goods relied on steam power, which was produced using coal as a fuel source. The widespread use of the steam engine to provide steam power allowed Britain to increase the volume of goods it could produce, but the resulting smog and soot from coal use painted the normally light-coloured indigenous trees black.

The white speckled body of the moth that was so easily camouflaged before against the normal pale color of most trees now stood out like a beacon against the newly blackened background. Predators had little trouble spotting them, and this led to increased predation rates. Fortunately, nature had afforded the peppered moth a means to adapt to this change. Prior to industrialization, a mutation had arisen that made the bodies of a small percentage of the moths black.

This mutant form (carbonaria) was first spotted in 1848, a mere 10 years before Darwin and Wallace had formally outlined the theory of natural selection. When this mutation first arose, the black body it afforded the moths that possessed it put them at a disadvantage. Their bodies stood out like a sore thumb against the lichen-covered pale trees of Britain. With the coming of the industrial revolution, however, the black bodies of the mutants allowed them to blend in with the soot covered barks of trees.

The black moths, which were once quite rare in the population, began increasing in frequency as predation drove the numbers of the speckled moths down. Naturalists were quick to point out that the changes seen in the peppered moth were as a result of environmental changes caused by industrial activity.

Kettlewell's Classic Experiment

In the 1950’s, one scientist by the name of Kettlewell set out to find answers to the peppered moth phenomenon. Were the differences in the amount of camouflage that the two forms afforded its possessors (and subsequent differences in rates of predation) coupled with environmental changes causing the changes in the frequencies of the speckled and black moths?

He attempted to answer this question by performing a series of experiments in which he captured, marked, and released peppered moths. The moths were then recaptured the following night, and the results of his experiments showed that black moths had higher rates of survival in polluted areas with darkened trees and speckled moths had higher rates of survival in unpolluted areas with pale trees.

At this point, it was easy to conclude that natural selection had caused the increase in the frequency of black moths in the soot covered areas. However, Kettlewell’s experiment was flawed. The inability to replicate the experiment and the less than favorable methods used to conduct the experiment earned Kettlewell scathing criticism.

For instance, he placed the moths on the sides of tree trunks that weren't their natural resting position. He also neglected to account for the degree to which migration of moths from surrounding areas affected the results of his experiment. The discrepancies in his experiment were used by creationists as a part of their anti-evolution crusade and even biologists, at the time, were skeptical of his work. He was later accused of being a fraud and his experiment was eventually disregarded.

Speckled moths blend in with pale white backgrounds while black moths blend in with dark backgrounds.
Speckled moths blend in with pale white backgrounds while black moths blend in with dark backgrounds. | Source

Majerus Finally Links Changes with Differential Predation

In the years that have passed since the industrial revolution, the passage of laws for cleaner air standards and the use of cleaner forms of energy has led to the decline of black moths and the resurgence of speckled moths. This was the background in which geneticist Michael E. N. Majerus would be working in when he launched a study in 2001.

Majerus, like many others before him, thought that the design of Kettlewell’s experiment was flawed. Nevertheless, he believed the inferences that were made by Kettlewell were valid. So he set out to at last prove that the differences in the level of predation of the two forms led to the changes seen in the frequencies of black and speckled moths during and after the industrial revolution.

Over the course of six years, Majerus released a total of 4864 moths. The moths were allowed to take up natural resting positions on trees and were counted four hours after sunrise each day. Since moths are not known to move from their resting positions unless disturbed, any moth that was unaccounted for was presumed to have been eaten by predators.

Approximately 26 percent were seen being eaten by birds via binoculars. The results of his study showed that black moths were 9 percent less likely to survive than their speckled counterparts. And with that, the inadequacies of the previous experiment were corrected and the accuracy of it claims verified.

And the Gene that Caused the Mutation is.....

Even though there was all this research out there on peppered moths and the changes in the frequency of black and speckled moths, there was none that sought to determine the genetic basis of the mutation that led to the existence of the mutant, black moth.

That is until a team of scientists led by Ilik Saccheri began sequencing the genomes of both black and speckled moths. They examined the genetic markers on each chromosome, and in 2011 narrowed down the potential location of the gene to a portion of the moth genome containing 400, 000 bases.

They would later begin the process of meticulously determining every single difference between both types. Eighty-seven DNA differences in total were identified and with the search now narrowed they began searching for these differences in the wider population. They found that a single gene variation present in black moths was missing in 95% of the pale moths that they tested.

To their surprise, the mutation turned out to be a transposon, a gene that jumps to another position in the genome. And that wasn’t the only surprise, the transposon had inserted itself in the cortex, a gene that is known to control cell division and egg development in fruit flies, but does not play a role in pigmentation.

Scientists only explanation for this odd occurrence is that the gene may control the developmental rate of the differently hued scales that covers its body. They also estimated that the mutation occurred in a 10 year period around 1819.

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