Podcast: The evolution of the British peppered moth

Peppered moth

Peppered moth

19 August 2014 by Sue Nelson

This week in the Planet Earth podcast, Ilik Saccheri and Arjen van 't Hof of the University of Liverpool describe how the British peppered moth changed from peppered to black during the Industrial Revolution in northern England.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Sue Nelson: This time in the Planet Earth podcast, understanding evolutionary change with the British peppered moth. I'm Sue Nelson and I'm at the Institute of Integrative Biology at the University of Liverpool and I am in the evolutionary and ecology lab with Dr Ilik Saccheri and we're in front a bench, there are some large refrigerators at the side of us - that's the whirring sound you can hear. There are pictures of different types of moths stuck above the bench with the usual test tubes and equipment bottles and we're by what looks like the sort of fridge you would have in your house now, not one of those big fridges. So why have you got some moths inside this fridge?

Ilik Saccheri: Well these peppered moths they rest during the daytime and they don't actually feed as adults, they only feed during the larval stage, they over winter as pupae they merge around now, this is about the peak of the flight season in the wild and so when we bred them or when we catch them in the wild if we want to extend their life then we refrigerate them and they are perfectly happy at four or five degrees and they just live longer that way. So that's why we keep them in the fridge.

Sue Nelson: Okay, so let's get them out.

Ilik Saccheri: So what I'm taking out here are two specimens of typical moth, so we have different morphs. The light coloured forms as well as a malonic form here.

Sue Nelson: Hold on we've got three little boxes, they are little cardboard boxes with netting at the top so we can see through a little moth with its wings open in each of them.

Ilik Saccheri: The typical form looks like this-

Sue Nelson: You are taking out there... this is white and you can see why it is called peppered now, it looks like somebody has sprinkled ground black pepper on a piece of white paper. It's very pretty actually.

Ilik Saccheri: And so this is a male and it actually comes from a place on the Wirral not far from Liverpool. They have what is known as pectinate antennae - I don't know if you can see there. If you look closely there is a kind of like a moustache, a very long moustache with a central rib and then the fuss that you can see around that is actually very, very fine hairs coming off that central mid rib and they are designed to pick up single molecules.

Sue Nelson: It's not very big because some moths you can get are absolutely huge and this is, probably, half the size of the length of my little finger, about one and half widths of a two pence piece.

Ilik Saccheri: Most moths have this feature of staying quite flat against the surface that they are resting on. That is for concealment so they try and blend in with the background that they are resting on, because during the daytime they are just basically just waiting for night and just hoping that something doesn't find them and eat them.

Sue Nelson: And in the other one one of the other two you've got, the colouring is very different. There's no speckling, there's no peppering of the peppered moths name, it's almost black. And this is one of the reasons they are called Darwin's moths isn't it because of this history that goes back to the industrial revolution.

Ilik Saccheri: That's right and the black coloured form was unknown until the first record in 1848 in Greater Manchester. The heavy use of coal during the industrial period, the eighteen hundreds and through pretty much to the sixties and seventies where two Clean Air Acts were brought in precisely to control the smogs, they led to blackening of the surfaces across very large parts of industrial Britain, be they trees or be they buildings, all the surfaces that these moths rest on. This will have led to a much stronger selection against these light coloured forms resting on the dark background.

Sue Nelson: So, basically, during the industrial revolution when you had these predominantly white peppered moths they will have just been easy prey because you put them against the black sooty bark or what have you, their prey can see them. What happened then, was there an anomaly, a genetic anomaly and there was one all black peppered moth and then that thrived because no one could see it against the soot.

Ilik Saccheri: Well that was the hypothesis that we started out with but it wasn't really until we determined the underlying genetics which was only done very recently by our group that we could actually specifically say, yes there was a single mutation and it did happen around this time. This mutation was actually contemporary of the time when Darwin was alive and working but he didn't know about this particular example of evolution and natural selection.

Sue Nelson: So how did you do that then? How did you discover on a chromosomal level that there was this change and then going back to find out where it all originated from?

Ilik Saccheri: We did it using a method known as linkage mapping, so where you can cross a light coloured male with a dark coloured female or vice versa, in fact in Lepidoptera they are quite unusual in that recombination only happens in males, so only in the production of sperm do you get crossing over between homologous chromosomes, females just pass their whole chromosomes on to their offspring.

Sue Nelson: And where is all this analysis done?

Ilik Saccheri: We do it in different parts. We do it in the molecular genetics lab here mostly using various types of sequencing technologies. The main person who has been involved in that over many years is my collaborator, Dr Arjen van 't Hof who is actually in the lab at the moment probably working on something to do with this system.

Sue Nelson: Alright, well I think then we will go and pop along and see him.

Arjen van 't Hof: Arjen, sorry to interrupt your work but perhaps you could explain to me which chromosome did you find was responsible for these two different forms of the peppered moth?

Yes, there are two different forms, a malonic form and a typical form. The malonic form is dominant so if an individual has got one chromosome which is typical and one chromosome which is malonic then it will be a black one. In the offspring of such an individual half of the offspring will get the typical chromosome and the other half will get the malonic chromosome and by looking at these offspring and comparing it to the father, in this case we can trace back which chromosome does have this mutation on it.

Sue Nelson: Is there a specific number chromosome that has it?

Arjen van 't Hof: Chromosome number 17. So there are 31 chromosomes in those species and it is on chromosome number 17.

Sue Nelson: Is chromosome number 17 only responsible for wing colour or does it have other effects to on the moth?

Arjen van 't Hof: No visual characteristics as such, so we don't know. There are loads and loads of genes on that chromosome - I think about 12 megabase in size. So only a very small region of the chromosome is actually involved in the melanisation. So after finding this chromosome we have done several steps of narrowing down to the region on this chromosome that contained this mutation that is causing the melanisation.

Sue Nelson: At the moment are there more black versions of the peppered moth than white versions in the wild?

Ilik Saccheri: No, the malonic which in the UK is known as carbonaria and is virtually extinct in the wild in the UK now, so it went through a rise from this first record in 1848, it rose in the kind of in the Greater Manchester area and in the industrial heartlands of Yorkshire and Merseyside to close to 100% really. It almost completely replaced what we call the wild types, so the original type which was the typical form and stayed at that very high frequency all the way until the early seventies when, as I said before, these Clean Air Acts were brought in and quite quickly the coal pollution reduced and over a period of time the resting surfaces also changed and became less dark.

Sue Nelson: Arjen, do you have comparisons where you can compare the current modern specimens of moth with ones from the period of the industrial revolution?

Arjen van 't Hof: We've done a small pilot looking at a few samples that were a hundred years old to see if we can still get DNA out of it that we can use to find a sequence in and that was successful, so now we're going to try and do this on a much larger scale. So we've collected museum specimens and we're going to compare that to the modern situation to see if the gene is indeed the same, which it will be, but also how this selection affected the region around the mutation. There is usually a footprint around such a mutation on the chromosome that you can trace back in time and which narrows down over time.

Sue Nelson: So thank goodness then for Victorian gentlemen collectors, basically.

Arjen van 't Hof: Absolutely.

Sue Nelson: Dr Ilik Saccheri and Dr Arjen van 't Hof, thank you both very much indeed. That's the Planet Earth podcast from the Natural Environment Research Council. You can follow us on Facebook and Twitter where we will post some of the photographs that were taken today. I am Sue Nelson, thanks for listening.