Peppered Moths: Pollution's Deadly Impact

The peppered moth is a well-known example of evolution by natural selection, often referred to as Darwin's moth. During the Industrial Revolution, pollution from coal-burning factories killed off the light-coloured lichens that grew on tree trunks, leaving the bark darkened. This made the previously well-camouflaged light-coloured moths more visible to predators, while the darker variety became more camouflaged. As a result, more dark-coloured moths survived and produced offspring, leading to an increase in the population of dark-coloured moths over time. However, with the introduction of clean air legislation and a reduction in pollution, the light-coloured form of the peppered moth once again became predominant. While pollution has been observed to impact the survival and reproduction of peppered moths, leading to changes in their coloration over time, there is no direct evidence to suggest that it causes their death.

The impact of pollution on the peppered moth's habitat

The peppered moth is a widespread species in the UK, Ireland, Europe, and North America. Its name comes from the typical patterning of dark speckles across its white wings, which camouflage it against lichen-covered tree trunks in its habitat.

During the Industrial Revolution, pollution from coal-burning factories darkened the trees in the peppered moth's habitat with soot and killed off the lichens that grew on them. This pollution had a significant impact on the peppered moth population. The dark-coloured or melanic form of the peppered moth, which was previously rare, became better camouflaged against the darkened trees. As a result, birds were more likely to catch and eat the light-coloured moths, which now stood out in their polluted environment. This selective survival is an example of natural selection, with the dark-coloured moths having a higher survival rate and passing their dark-coloured genes on to their offspring. By the end of the 19th century, the dark-coloured moths almost completely outnumbered the original light-coloured type, with a record of 98% in Manchester in 1895.

In the mid-20th century, clean air legislation was introduced to reduce air pollution. As a result, tree trunks became cleaner, lichen growth increased, and the light-coloured peppered moths were once again better camouflaged. Consequently, the light-coloured moth population increased, as they were less likely to be eaten by birds. This reversal in the peppered moth population further supports the theory of natural selection, demonstrating that the moths' colouring changed in response to their environment.

While pollution has had a significant impact on the peppered moth population, it is not clear whether it has directly caused the moths to die. However, the dark-coloured moths had a higher survival rate in polluted environments, indicating that the light-coloured moths may have been dying at a higher rate due to increased predation.

The Industrial Revolution's role in the peppered moth's evolution

The Industrial Revolution's role in the evolution of the peppered moth is a clear illustration of the effects of industrialisation and Charles Darwin and Alfred Wallace's theory of evolution by natural selection.

During the 1700s, the Peppered Moth was widespread across the UK. The name comes from the typical patterning of dark speckles across its white wings, which allowed it to be very well camouflaged against the lichen-covered tree trunks of the mainly rural landscape of the time. This protected it from being spotted and eaten by predators. However, some peppered moths had a naturally occurring genetic mutation, resulting in dark wings. This variety was highly visible to predators and so was much less common than its lighter cousins.

During the 1800s, the Industrial Revolution brought about significant changes to the environment. Pollution from dense industrial smoke and soot killed off lichens and darkened tree trunks and walls in towns and cities. As a result, the lighter-coloured moths, which had previously been well-camouflaged, now stood out against the darkened backdrop, making them easier targets for predators. On the other hand, the darker variety became more camouflaged and was better able to hide from predators.

As the darker moths survived and reproduced more, while the lighter moths were picked off in greater numbers, the common, pale variety was gradually replaced by the darker variety in urban areas. The first dark Peppered Moth was recorded in Manchester in 1848, and by 1895, this variety accounted for 98% of the moths in the city. This shift in the moth population towards darker colouring is known as industrial melanism and provided early support for Darwin's theory of natural selection.

Since the introduction of clean air legislation in the mid-20th century, pollution levels have decreased, and the lichen has started to grow again. As a result, the light-coloured Peppered Moth population has once again increased in urban areas.

How pollution affects the peppered moth's camouflage

The peppered moth is a well-known example of evolution by natural selection. Before the Industrial Revolution, the light peppered moth was common, while the dark form was very rare. The light moths blended in with the light-coloured trees and lichens. However, pollution from the Industrial Revolution killed the lichens and darkened the tree trunks and walls in towns and cities with soot. As a result, the light-coloured moths were easier to spot by predators, while the darker variety became better camouflaged.

The dark-coloured or melanic form of the peppered moth (var. carbonaria) was first recorded in 1848 in Manchester, England. By the end of the 19th century, it had almost completely outnumbered the original light-coloured type (var. typica), with a record of 98% in 1895. This increase in dark-coloured moths was due to selective survival—birds easily caught the light-coloured moths on soot-darkened trees, while the dark moths were camouflaged.

The evolution of the peppered moth is a clear instance of directional colour change in the moth population due to air pollution during the Industrial Revolution. This phenomenon, known as industrial melanism, was an early test of Charles Darwin's theory of natural selection. The frequency of dark-coloured moths increased during the Industrial Revolution, and later, when pollution was reduced in response to clean air legislation, the light-coloured form once again predominated.

The story of the peppered moth illustrates the effects of pollution on camouflage. Before the Industrial Revolution, the light-coloured peppered moth was well-camouflaged against the lichen-covered tree trunks. However, as pollution from industrial smoke and soot killed the lichens and darkened the trees, the light-coloured moths became more visible to predators. The dark-coloured moths, on the other hand, were better camouflaged in the polluted environment, which led to their increase in population.

Since the mid-twentieth century, controls have been implemented to reduce air pollution. As a result, tree trunks have become cleaner, and lichen growth has increased. With the improvement in air quality, the normal pale peppered moths have regained their camouflage, while the black forms have become more noticeable. This has led to a resurgence in the population of light-coloured peppered moths, as they are better suited to the environmental conditions.

The genetic basis of the peppered moth's colour change

The peppered moth's colour change is a result of a genetic mutation. The mutation, called "carbonaria", is a "jumping" piece of DNA, or a transposon, that inserted itself into a gene called cortex. This gene does not have a known role in pigmentation, and it is still unclear how the mutation causes black colouring. However, it is known that the dark-coloured or melanic form of the peppered moth was rare before the Industrial Revolution. The first black specimen was collected before 1811 and kept at the University of Oxford. The first live specimen was caught in Manchester, England, in 1848, and by the end of the 19th century, it almost completely outnumbered the original light-coloured type.

The increase in dark-coloured moths was due to industrial pollution darkening tree trunks, which provided better camouflage for the dark-coloured moths and made the light-coloured moths more visible to predators. As a result, more dark-coloured moths survived and passed on their genes to the next generation. This is an example of natural selection, as the dark-coloured moths were better suited to their environment and thus more likely to survive and pass on their characteristics.

Since the Clean Air Act was introduced in 1956, the light-coloured peppered moth population has increased again in urban areas due to the reduction in pollution. This further supports the idea that the colour change in peppered moths is due to genetic variation and natural selection.

The role of natural selection in the peppered moth's survival

The peppered moth is a classic example of natural selection in action. Before the Industrial Revolution, the typical patterning of the peppered moth was dark speckles across white wings, which served as effective camouflage against the light-coloured lichen-covered tree trunks in rural areas. This protected the moths from being spotted and eaten by predators.

However, during the Industrial Revolution, pollution from coal-burning factories darkened tree trunks in towns and cities with soot. As a result, the once-camouflaged light-coloured moths became more visible to predators, while the dark-coloured variety, previously less common due to their visibility, now blended in better. The dark-coloured moths had a higher survival rate, and their numbers increased drastically. By 1895, the dark-coloured variety accounted for 98% of peppered moths in Manchester. This directional colour change in the moth population was a direct consequence of air pollution.

The story of the peppered moth is often used as evidence of natural selection, with Charles Darwin noting its evolutionary importance during his lifetime. The process by which the moths changed colour is a clear demonstration of natural selection: individuals with certain traits (in this case, colouration) survive better or worse and reproduce more or less successfully than others in certain conditions.

In the mid-twentieth century, clean air legislation was introduced, and pollution levels decreased. As a result, the light-coloured lichens returned, and the tree bark lightened. Once again, the light-coloured moths were better camouflaged, and their numbers began to increase. This cycle of colour variation in response to environmental changes illustrates the role of natural selection in the peppered moth's survival.

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