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The Peppered Moth Story

Contributed by Alice Howse on 19.01.2016

An article written by Henry McGhie, Head of Collections & Curator of Zoology, Manchester Museum, University of Manchester

Back in the early 1800s, a small black moth was found in Manchester for the first time. It wasn’t a new species, but an unusual black variety of the Peppered Moth, a fairly common insect that is usually pale grey with black speckles. The new black variety was named ‘carbonaria’. Through the century, more and more of these black Peppered Moths were found in the industrialised north of England, and by the end of the century they had more or less replaced the original paler moth. Fast forward to the 21st century and people are still interested in the story of the Peppered Moth.

From the 1950s, scientists hypothesised that the dark Peppered Moth had replaced the original pale form because birds had eaten the pale moths. The pale moth’s speckling makes it well camouflaged among lichen on the bark of trees where it rests during the day, but the black moths looked better camouflaged on the soot-covered bark of trees around smoky cities. Experiments with moths placed on tree trunks showed that birds did indeed find moths based on the colour of the bark of trees. The story became a classic example of evolution by natural selection.

Towards the end of the 20th century, scientists found that some of the assumptions in the original experiments weren’t borne out in nature, as many of the moths rested on tree branches rather than on tree trunks. Critics of evolution and natural selection latched onto these flaws to try to undermine the evidence of evolution.
Mike Majerus, a scientist at the University of Cambridge, studied Peppered Moths to try to better understand the behaviour of the moths and the birds that ate them. He released moths and allowed them to settle on trees by themselves, then studied which ones were eaten by birds. Unfortunately, he died shortly before he completed the study. His colleagues continued the work and confirmed that bird predation was sufficient to explain the spread of the dark peppered moth in industrialised areas.

The story of the Peppered Moth is particularly powerful as it is so easy to understand.  Dark moths that sit on dark trees are better camouflaged than pale moths. Birds eat the moths based on which ones they can find. As moth colour is genetically determined, this is the definition of evolution, namely change in gene frequency over time. As the change is due to different levels of predation on one particular colour variety, this is natural selection. Interestingly, after the Clean Air Acts of the 1950s, dark moths declined and were replaced by the ‘original’ pale moths, which were better camouflaged on the bark of trees not covered by soot.

Back in the 1800s, people who we might today call citizen scientists went out and looked at the animals and plants in their surroundings. Their curiosity led them to look closely, to record what they saw, and to share their findings. Their collections of pinned insects in museums such as Manchester Museum represent both the scientific data and the history of the Peppered Moth story.

The Peppered Moth story is also an affirmation of the scientific method: of asking questions, making predictions, testing ideas and accepting or rejecting predictions based on the evidence. Science doesn’t always need fancy equipment or lots of funding, just people who ask questions, and then set out to find the answer.
The story of the Peppered Moth works on many levels. It reminds us that people can have a great impact on their environment and that there can be the possibility of change. Can we use this story, a Manchester story, to help realise positive change? Change can be a frightening concept, but can we help promote change for the better? We know that our society can’t keep operating the way it has done without damaging the environment further. One of the most worrying challenges facing us is that our climate is changing, because people have altered the chemical composition of the atmosphere.  Climate change is very concerning and can seem too big to do anything about. Can we rethink the way we present it and connect people with the subject in positive ways? Maybe we need less doom and gloom and more excitement, enthusiasm and creativity. Can we use the story of the moth, in the world’s first industrial city, to inspire people about better futures?

Over the next year, the museum is interested in using the Peppered Moth story as the basis of exhibits, performances, discussions, and public art projects. We want as many people as we can reach to connect with the story, and to appreciate our connections with the world around us. By connecting with people’s hopes and dreams, we hope to promote a vibrant, rich future where people and nature can flourish together.