Today’s post is by Bryony from the Visitor Team at Manchester Museum. We are not experts, but we are people with a passionate interest in the museum and its objects. We each bring our own insight into Manchester Museum and its collections.
Changes We Can See – The Peppered Moth
Manchester Museum has been invaded!
Peppered moths as a symbol for change throughout Manchester Museum, as part of the current Climate Control exhibition.
The peppered moth is so-called because of its peppered black-on-white wings, and was, originally, entirely coloured in this way. This provided them with extremely effective camouflage against tree bark and lichens (as you can see – or not)!
Camouflage is a widely used form of defence against predators – often the effort of finding something like this moth is just, well, not worth the effort.
But one day, things began to change. In 1848, near the centre of Manchester, Robert Smith Edleston, a nineteenth century naturalist, made the following quick note in his diary:
“Today I caught an almost totally black form of Biston betularia (peppered moth) near the centre of Manchester.”
He thought so little of it however that he didn’t mention it officially until many years later, when in 1864 he published his findings and noted that, by then, the black form had become the most common form in his Manchester garden.
But why the shift? Weren’t the white peppered moths perfectly camouflaged already?
Well, not here in nineteenth century Manchester. Because of its proximity to good sources of power, along with the great success of the Bridgewater Canal and the Manchester Ship Canal, Manchester had risen to become Britain’s first industrial city. Cotton mills abounded, with Manchester and Lancashire providing up to 32% of global cotton production. Of course, this had an impact on the city:
“A thick black smoke covers the city. The sun appears like a disc without any rays. In this semi-daylight 300,000 people work ceaselessly. A thousand noises rise amidst this unending damp and dark labyrinth … the footsteps of a busy crowd, the crunching wheels of machines, the shriek of steam from the boilers, the regular beat of looms, the heavy rumble of carts, these are the only noises from which you can never escape in these dark half-lit streets …”
— Alexis de Tocqueville, describing Manchester in Journeys to England and Ireland, 1835
Thick, poisonous fogs, known as ‘pea-soupers’, regularly descended upon the city. They were sometimes so bad that their consistency was more like slime, coating everything with soot and sulphur dioxide, causing horrific health problems and leading to deaths in thousands.
Lichen is an organism that is often very sensitive to environmental changes, and therefore wouldn’t have been able to survive this onslaught. The trees, of course, would have ended up black under the same coating as the rest of the city.
The really cool thing about the story of the peppered moth is that it visibly demonstrates evolution in action.
The genetic shift from typical colouration to black is simple enough for many organisms, which is why for example we have panthers as well as jaguars – one is the typical form, and the other is the all-black or melanistic form (although with the moths, the black form was labelled ‘carbonaria’ after, you guessed it, coal).
You see, evolution works like this: there’s no change in an individual organism over its lifetime that can get genetically passed down, it’s due to minute mutations in the DNA that happen completely by chance when they’re being conceived. Of course, only a small percentage of these changes actually do anything noticeable, and only a vanishingly small percentage of those actually benefit the organism enough to ensure they survive to pass on those slightly mutated genes.
And yes, that means that you’re descended from mutants. After all, if our ancestors had never mutated, we’d still be primordial slime, as would the beautiful peppered moth.
Luckily, since the Clean Air Act was passed in 1956 (yes, it was really that recently), the pea-soupers have stopped and our whole environment is a great deal cleaner – the shift of the peppered moth back to the white (‘typica’) form is a brilliant indicator that Manchester’s looking a lot healthier these days.
University of Manchester entrance, just beside us on Oxford Road, pictured in the 1950s compared to now. Available from: manchestereveningnews.co.uk
And that is the story in a nutshell of how Manchester’s environment drove the evolution of a species, and then drove it back again. What an amazing little moth!
P.S. Want to find out more?