Testing the Hypothesis: Kettlewell’s Experiments on Industrial Melanism in Moths


Peppered Moth

Kettlewell’s hypothesis was based on the fact that the peppered moth had evolved in response to the changes in the environment caused by the industrial revolution. The peppered moth is a common moth found in England, but during the late 19th and early 20th centuries, its appearance changed dramatically. At this time, factories and mills emitted large amounts of soot and other pollutants into the air, which covered trees and other surfaces with a dark layer of soot.

Kettlewell hypothesized that the increase in soot on trees and other surfaces made the lighter colored peppered moths vulnerable to predation by birds, because they were more visible against the dark background. However, he also hypothesized that the darker colored peppered moths were better camouflaged against the dark surfaces, and thus they were more likely to survive and reproduce.

This hypothesis had significant implications for our understanding of evolution, as it suggested that natural selection could operate over relatively short periods of time, and that the environment could be an important driver of evolutionary change.


peppered moth evolution in England

The peppered moth (Biston betularia) was first described by J.E. Borkhausen in 1792, but it was not until the mid-19th century that the moth was recognized by entomologists as one of the most common species in Britain. Charles Darwin himself mentioned the peppered moth in his book On the Origin of Species, published in 1859.

However, it was not until Cyril Clarke, an amateur lepidopterist, observed the peppered moths resting on the trunks of trees near his home in Dorset, that the idea of natural selection and evolution came into play. Clarke reported that he observed more dark-colored moths on the trunks of trees that had soot deposits from nearby factories. This led him to hypothesize that the darker moth coloration was advantageous in areas with high levels of industrial pollution, as it would provide camouflage from predators.

Bernard Kettlewell, an English geneticist, who had previously spent time studying the genetics of blackfly populations, conducted a series of experiments to test Clarke’s hypothesis. He was interested in explaining the changes in the frequency of two color morphs in the peppered moth population that had been observed in the United Kingdom.

Prior to the industrial revolution, the majority of peppered moths in the United Kingdom had been light-colored, with only a small proportion of dark-colored ones. However, after the increasing levels of air pollution through the industrial revolution, the frequency of dark-colored moths began to increase, with over 90% of some populations being the dark phenotype.

Kettlewell’s obsession with the peppered moth began in 1953 when he published a paper outlining the scientific procedure he planned to take to test Clarke’s hypothesis. He used the methodology of natural selection to test his hypothesis, which was that the change in the color of the moths was due to environmental selection pressure from predators rather than the direct effect of industrial soot.

Kettlewell carried out a systematic field experiment, by releasing marked moths of different morphs onto tree trunks in polluted and non-polluted areas throughout England. He then documented this process over several years, monitored which moths were eaten by predatory birds, and identified the frequency of different colored moths in different regions. He observed that moths resting on tree trunks that were darker were more likely to be eaten by predators, which supported his hypothesis.

Kettlewell’s Hypothesis

Peppered Moths Evolution

Kettlewell’s Hypothesis is centered in the concept of natural selection. He proposed that the color variation between light and dark peppered moths was a result of the selection pressure on their appearance from predators. In his hypothesis, he explained how the lighter colored moths were favored in the early 1800s due to the greater abundance of lighter colored lichens on the trees. This resulted in the environment of these moths becoming lighter over time, selecting advantageously for melanic (dark) moths in the later 1800s.

Kettlewell’s Experiment

Kettlewell Experiment

To test his hypothesis, Kettlewell conducted research on how peppered moths behaved in natural environments. In his experiments, he created two scenarios; one in an area with light bark trees and one in an area with dark bark trees. Kettlewell placed equal numbers of light and dark moths in each of these areas, allowing them to blend in with their surroundings. He monitored which moths were eaten by birds in each area. The results of his experiments were conclusive; in the area with light trees, the lighter moths were favored, while in the area with dark trees, the darker moths were favored.

Kettlewell’s experiments were important because they helped provide evidence for natural selection. It showed that predators were selective in their predation and that environmental change could shift the balance of favor for one color phenotype over another in a species. This echo the observations of Charles Darwin, who described selection pressures acting on species to form new varieties over time.

Criticism and Confirmation of Kettlewell’s Hypothesis

Oil Pollution Moth Evolution

Kettlewell’s hypothesis has been met with some criticism. The most significant issue has been the debate over the actual frequency of dark versus light moths existing in these environments prior to the industrial revolution in Europe. In 1986, the scientist Michael Majerus attempted to replicate Kettlewell’s experiments. He discovered that Kettlewell’s methods of observing moths in natural environments failed to replicate realistic patterns of predation, calling into question the authenticity of Kettlewell’s results.

Apart from criticisms, Kettlewell’s hypothesis has been supported by many other experiments In particular, more recent studies have found that predator-selective predation is more powerful than previously assumed. Other studies have also found that similar mechanisms are responsible for color variation in other species affected by environmental change, such as the darkening of the peppered moth from industrial soot pollution.

In conclusion, Kettlewell’s experiments on peppered moths were instrumental in shaping the early understandings of natural selection and the role of environmental change in the evolutionary process.

Testing Kettlewell’s Hypothesis

Kettlewell's experiments in the wild and in captivity

In the 1950s, Bernard Kettlewell proposed the hypothesis that the color of peppered moths changed from light to dark in response to pollution in England. Kettlewell believed that darker moths had an advantage in polluted areas because they were less visible to predators against urban backgrounds. Similarly, lighter moths had an advantage in rural areas where trees and other light-colored surfaces made them harder to spot.

Kettlewell’s hypothesis was challenged by other scientists who claimed that his experiments were flawed and that the data did not support his theory. One of the main criticisms was that Kettlewell’s experiments were carried out in artificial conditions, such as captivity, and did not reflect natural selection in the wild. So how did Kettlewell test his hypothesis?

Experiments in the Wild

Kettlewell's experiments in the wild

Kettlewell conducted extensive field experiments in order to test his hypothesis in the wild. He collected moths from both polluted and non-polluted areas and counted them in situ. He noted that the number of dark moths was higher in polluted areas, while the number of light moths was higher in non-polluted areas. Kettlewell concluded that the moths had adapted to their environment through natural selection and that the color change was a direct response to pollution.

Kettlewell used a variety of techniques to collect moths in the wild, including light traps, netting, and visual inspections. He also compared the coloration of moths to the background to determine how well the moths were camouflaged.

Experiments in Captivity

Kettlewell's experiments in captivity

To further test his hypothesis, Kettlewell conducted experiments in captivity. He placed moths in a variety of environments, ranging from clean backgrounds to heavily polluted backgrounds, and recorded their survival rates. He found that dark moths had a higher survival rate on polluted backgrounds, while light moths had a higher survival rate on clean backgrounds.

Kettlewell also conducted experiments in larger enclosed spaces, such as tents, to further simulate natural conditions. He placed moths on tree trunks and watched as birds tried to eat them. He found that dark moths were less visible to predators against a polluted background and were therefore less likely to be attacked.

Criticism of Kettlewell’s Experiments

Criticism of Kettlewell's experiments

Kettlewell’s experiments have been criticized by some scientists who claim that his methodology was flawed and that his results were inconclusive. One of the main criticisms is that Kettlewell may have selectively picked moths for his experiments, leading to a bias in the results. Others have argued that the pollution levels in Kettlewell’s experiments were not representative of the levels in natural environments.

Despite these criticisms, Kettlewell’s experiments were an important contribution to the field of evolutionary biology and helped to establish the role of natural selection in the adaptation of species to changing environments. Kettlewell’s experiments also helped to draw attention to the effects of pollution on wildlife populations and led to increased environmental awareness and conservation efforts.

Results of Kettlewell’s Hypothesis Tests

Peppered Moths

During the early 1950s, Bernard Kettlewell conducted a series of experiments to test his hypothesis concerning the adaptive melanism of peppered moths. It must be noted that Kettlewell’s hypothesis was developed during a time when air pollution was rife in England, and the dark, soot-covered trees in industrial towns provided the ideal habitat for darker colored peppered moths to thrive, while the lighter colored ones struggled to blend in and had higher predation rates.

To test his hypothesis, Kettlewell applied several methods to measure the ratio of light and dark-colored peppered moths in different locations and at different times. His hypothesis predicted that in areas with high levels of pollution, dark-colored moths would be more prominent, while light-colored moths would dominate in clean areas.

Kettlewell carried out two primary experiments: The first experiment involved capturing light and dark peppered moths in different areas, marking them then releasing them in similar ratios as they were caught. The second experiment involved placing the same moths on both clean and contaminated trees and recording how often they were predated on. To Kettlewell’s surprise, the results of both experiments were consistent with his hypothesis.

In areas with high levels of pollution, darker moths were found to be more prevalent, whereas in cleaner areas, lighter moths were predominant. Surprisingly, a higher percentage of dark-colored moths were preyed on by birds when resting on light-colored trees, while a higher percentage of light-colored moths were preyed on by birds when resting on dark-colored trees.

The experiments conducted by Kettlewell and his associates became regarded as one of the marvels of twentieth-century biology, particularly since it was one of the earliest examples of natural selection being studied and documented in a controlled manner.

It is noteworthy to mention that Kettlewell’s experiments and his conclusions were not beyond criticism. Some researchers held that he had overplayed the significance of his evidence, while others supported the experimental findings but offer different interpretations of the results. Several subsequent studies have been carried out since Kettlewell’s findings, but the overall consensus is that his analysis of the peppered moths’ evolution and their adaptations to their surroundings was accurate and groundbreaking for the time.

Criticism of Kettlewell’s Hypothesis

Criticism of Kettlewell Hypothesis

Kettlewell’s experiments on industrial melanism of peppered moths have been criticized since its inception. Some of the criticisms are:

Small Sample size

Small Sample size

Kettlewell’s first experiment used only 47 moths, which is not enough to represent the total moth population. Such a small sample size can fail to represent the genetic diversity of the population. Therefore, the results may not be reliable.

Unnatural setting

Unnatural Setting

Kettlewell conducted his experiments in a highly controlled setting, where he artificially introduced moths onto trees using forceps. This kind of artificial setting may make the moths behave in an unusual way than they would in the wild, making the results not appropriate for comparison.

His methodology was Muddled

Muddled Methodology

Kettlewell’s experiments have been criticized for having a muddled methodology. He did not record the number of moths he used during the experiments, nor did he state any criteria beforehand to pick one tree over another. This lack of clarity in his methodology can lead to questions about the accuracy of his results.

Data Manipulation

Data Manipulation

Critics have also accused Kettlewell of manipulating his data to support his hypothesis. Some claim that he painted moths and glued them onto the trees before photographing them to make it appear as if they were naturally camouflaged. These accusations, however, are not backed up by any concrete evidence.

Failed Replication

Failed Replication

Several scientists have attempted to replicate Kettlewell’s experiments, but with little success. In 2016, researchers from the University of Liverpool repeated the experiment under the same conditions, but they did not find any significant differences between the moth populations on the tree trunks and nearby branches. This suggests that Kettlewell’s findings may not be as consistent as initially thought.

Moths Do Not Rest on Tree Trunks

Moths Do Not Rest on Tree Trunks

Some have argued that peppered moths do not rest on tree trunks, where Kettlewell conducted his experiments. Instead of such a place, they rest on branches. Therefore, Kettlewell’s ‘selective against moths on light bark’ theory may be flawed as it assumes that moths generally rest on tree trunks.

In conclusion, while Kettlewell’s work has its merits, it also has its criticism. The criticisms, along with other similar issues, are concerns to consider while studying peppered moths.

The Impact of Kettlewell’s Hypothesis on Education

Kettlewell's Experiment

Kettlewell’s research on the evolution of peppered moths has been a significant contribution to the field of evolutionary biology. However, his experiment has had a disproportionate impact on the teaching of science and evolution education.

The experiment stands out in science education as an example of a well-designed and well-executed scientific investigation that clearly demonstrates the process of natural selection. As a result, it has become a staple in classroom discussions of Darwinian theory, and has been featured in countless textbooks, educational documentaries, and popular science books.

The demonstration of the influence of environmental factors on changes in the moth’s coloration illustrates the essential principles of Darwinian evolution and shows the impact of natural selection. The students can both observe and understand the mechanism of natural selection that operates within a population, and facilitates a conceptual transition from simply memorizing key facts to understanding the philosophical underpinnings of the theory of evolution.

Kettlewell’s experiment has provided scientists and science educators with a valuable case study while teaching the difficulties of experimental design. It has also highlighted some of the limitations in conducting scientific research, such as potential impact of human intervention in the experiment.

The impact of Kettlewell’s experiment in the education sphere is not limited to biology classrooms, it is widely applied in science and evolution education. The experiment has become a valuable resource for teachers struggling to explain the complex process of natural selection to their students. With its well-documented results and easy-to-understand methodology, Kettlewell’s experiment has become an archetype for how to conduct science experiment and has helped to solidify the importance of experimental design.

In conclusion, Kettlewell’s work on the peppered moth experiment has had a substantial impact on science education. It has helped teach future generations the necessary concepts of natural selection while inspiring budding scientists to design experiments that explicitly test their evolutionary hypotheses.

Kettlewell’s Hypothesis

Kettlewell's Hypothesis

Kettlewell’s hypothesis proposed that the color of peppered moths in England changed due to pollution. He believed that the light-colored moths were becoming easier for birds to spot against the blackened trees and buildings that were exposed to industrial pollution. Commonly, this made them more readily available to predators such as birds, and so the darker-colored, or melanic, moths survived and reproduced, leaving more and more melanics in the population.

Kettlewell’s hypothesis brought light to the idea that living organisms can adapt to their surroundings by changing their physical appearance. It demonstrated the role of natural selection in bringing about such adaptations and became a landmark experiment in evolutionary biology. It was also a significant discovery for scientists working in fields beyond biology as it showed how the surroundings could be linked to gene expression susceptibility of characters, which would affect the entire science field going forward.

Kettlewell’s Experiment

Kettlewell's Experiment

To test his hypothesis, Kettlewell set up a series of experiments in which he placed moths on a surface that was either light or dark. He then timed how long each moth took to be spotted by a bird. The results showed that dark-colored moths were much less visible on dark surfaces as compared to light-colored moths.

Kettlewell’s classic experiment involved gluing moths onto trunks, catching birds in the area as they tried to prey on them in daylight and checking what color they were. He carried out his most famous experiments in industrial areas, where moths resting on trees that had been darkened by pollution had a higher number of melanistic forms because they had greater camouflage.

Further Studies and Criticisms

Further Studies and Criticisms

After Kettlewell’s studies, the experiment continued to be replicated and investigated by other scientists. Some critics raised questions about the validity of the experiment, suggesting that the population of peppered moths was not solely driven by natural selection but had other intervening variables.

There were also questions raised about whether or not the moths actually “rested” on the trunks of trees during the day – as Kettlewell’s methods had implied – and debates raged over whether predation was in fact, the sole factor that could have lead to the final outcomes observed.



Kettlewell’s experiments demonstrate how environmental changes can lead to genetic variations in species and also how natural selection operates, allowing the fittest to survive and reproduce. The hypothesis, results, and conclusions that Kettlewell drew from his research remain foundational today, with the experiment serving as a prime example of the fundamentals of evolutionary science. Conclusively, the impact of the experiment on the scientific community and on students in education globally is highly significant and furthered the understanding of evolutionary biology.

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