Laurence Cook

The Rise and Fall of the Peppered Moth

The earliest record that I know of peppered moths is 1776, when an entomologist called Moses Harris described the black and white insect he could find near London. He also pointed out that the larvae came in different colours. Since he noted the variation in the larvae, he was likely to note variation in the adults if there were any.

I thought I would just show you some pictures.

There are about 200 moths that show something you could call industrial melanism, that is, the presence of dark forms at higher frequencies in industrialised areas. But the one I am going to talk about exclusively is the peppered moth (Biston betularia), which is the most famous and obvious example. I also propose to describe how ideas on the subject have developed, both among scientists and the general public.

The speckled form is the sort that Harris would have seen. In the early nineteenth century there are one or two examples of black forms in collections but they only became well established by the middle of the nineteenth century, and the first black forms which have actually been located are from Manchester, in the Cheetam Hill area. They look like the next one.

On a pinned specimen there is a little bit of the typical black and white pattern, but that part of the wing would not be seen in life. The rest of the insect is uniform black

These forms started to appear in the second half of the nineteenth century.

This is a picture of a collection taken in Manchester in the early 1970s. In about 120 moths there are four typicals. The rest are the melanic form called carbonaria.

There are some other forms, normally fairly rare, which are intermediate in colour (called insularia), such as these in the centre.

This picture was published in 1908 by a Manchester man, carbonaria at the bottom, typical at the top and insularia between.

The different forms are produced by a single locus. The darker are dominant to the paler forms. There seem to be three insularia alleles. They are difficult to distinguish in the field, but sometimes you can separate collections into the different categories.

So that is the variation that developed in the 19th century, and excited people’s attention.

The first people to discuss or write about the variation were amateur entomologists in places like Manchester. They wrote, in amateur entomological journals, fairly anecdotal remarks such as “I went out and made a collection and what do you think I found? Several species that contained unusual black individuals”.

The interest, and comments that it elicited, was not really to do with ideas about evolution, it was to do with variation. People were concerned about what species they had. They were interested in variation because it shed some light on whether or not they were dealing with a new species.

The new black forms excited some attention in the amateur entomological literature. People asked themselves why the change had happened and, basically, there were three kinds of answer.

1. It may because the newly industrialised towns are warmer than the surrounding rural areas, and the dark forms like warm places.
2. It may be that the larvae are eating something poisonous (e.g. lead salts) deposited on the trees, that turns them dark.
3. Or, it may be that there is a difference in camouflage and dark colour is a protection against predators.

All of those ideas were floated in the nineteenth century.

This picture shows what Manchester used to be like at the time we are talking about. When I came to Manchester the Dunlop factory building visible though window, which is now apartments, looked like this. It didn’t have smoke coming out of the top but we had the chimney and completely black-walled buildings. The area we are now in was shops. Just behind were narrow streets with two-up, two-down houses, all of them at that time with domestic grates burning coal. So there was a tremendous amount of smoke and general dirt, which was obviously noticeable to nineteenth century people.

The interest in this topic from an evolutionary point of view came from the scientific establishment, which was conscious of a problem. The Darwinian idea was that evolution took place through adaptation and selection producing imperceptible changes; very slow progressive modifications in organisms occurred to suit their environment. The inherited component, which was carried forward in evolution, was thought at first to be due to some kind of blending of material that was passed on from one individual to another. By the 1880s or so, several difficulties were seen with this explanation.

One is that, if the inherited material is blended, the distinctions would disappear, so you had to have some kind of discontinuous inheritance. This was understood well before Mendel’s results were discovered by the general scientific public. The other problem was that species could be seen not blend into each other; they were discontinuous in characters.

About 1880 Francis Galton and Frank Weldon set up a committee of the Royal Society to study, statistically, the continuous variation that they observed in organisms, with a view to supporting general evolutionary ideas according to the Darwinian imperceptible change model.

A fellow student of Weldon’s, William Bateson, became involved. Basically what impressed him was that there is a fundamental discontinuity between species, so we have to look for something that causes us to jump from one species to another. There were also other kinds of evidence that the situation is more complex than the continuous change model. For example, if you look at pairs of closely related species you often find one of them is very variable, while the other is not.

Yet they live in the same kind of environment doing the same kinds of thing. He quoted various moth species for which that is true. The implication is that species are organised in some kind of physiological sense, he said. In other words, to do statistical measurements alone is to bark up the wrong tree.

To cut the story short, Bateson became involved with the Royal Society group. It began as the Committee for the Study of Statistical Variation in Species; he turned it into the Evolution Committee. He commissioned people to look for examples of discontinuous variation, and wrote to amateur entomological journals saying “we need data, get it together and send it to me”.

Bateson was therefore the person who founded the scientific study of peppered moths: a nice example for him because big changes were taking place and he thought that this may be important from a speciation point of view. At about the same time it was demonstrated that there was a Mendelian basis to the variation, so there was no need to keep considering polluting lead salts as a direct environmental effect changing the phenotype. The forms segregated when you bred them.

Gradual evolutionary change and blending inheritance versus discontinuous change, both genetically and in evolutionary terms, was the original issue that people argued about in connection with the peppered moth.

Having got the information together Bateson had it published, and if you examine it you get a picture which looks like this.

Individual records were dotted around the country. People sometimes said “where dark forms were once rare they are now very prevalent”, information as loose as that. But, if you interpret these records it looks as if there was a spread from somewhere in north west England, where there was quite a high frequency of melanics in the 1860s. To north and south of the country the first records were not until the 1890s. So the pattern looks like a classical migration outwards of the melanic form.

By the time melanics were being noticed in the southern counties the frequency around Manchester had got to 90% or more, so the frequency of melanics here continued to rise as the spread took place in the south. This is interesting because London had been a very large conurbation and extremely polluted for hundreds of years. So, it may be simply by chance that a rare mutation got going in the Manchester area and spread.

It could equally well have started in London, I suspect. A lot of seventeenth and eighteenth century literature recorded how terrible the atmosphere was in London, and it was noted how much clearer it became once when the Dutch blockaded the east coast and stopped coal boats coming down from Newcastle.

So London had the sort of problems we are used to associating with the north, yet the melanic form was late in increasing in frequency in the south east.

There was a gap in peppered moth studies in the 1920s, when people lost interest in the mechanism of evolution as a subject in itself – they became interested in phylogenies and function instead. This gap takes us up to about 1950 when the principal person to enter the field was Bernard Kettlewell.

He had a grant to study industrial melanism in general and the peppered moth in particular, to see what the pattern had become and why it was brought about. At that time he again assembled data on frequency. They were originally published as pie diagrams, which show the limits of the records better than a contour map, but contours provide a neater pattern to look at.

The Kettlewell data come from three surveys made between 1952 and 1972 - about 20 years. It didn’t look as if any change had occurred over those 20 years. The environment did not change much either, so the whole data set looks like a static pattern with high melanic frequency in industrial regions and a very steep cline going into rural N. Wales.

The change to the south is less steep. The whole of the Liverpool and Manchester area was over 90% melanic [carbonaria] phenotype frequency. There was an area of high insularia in south Wales [lower left], but this map just shows carbonaria frequencies.

Kettlewell got together a large collection of records for the middle of the twentieth century; a much better set, but similar in pattern to that collected at the end of the nineteenth century. It suggested that, while in the nineteenth century it had been changing, by the beginning of the twentieth century the pattern had become a fixed.

He also decided that one of the important things to look at, in trying to explain why the pattern occurred, was camouflage and selective predation. There are various reasons for choosing this approach that we won’t go into at the moment, but one important point was that camouflage and selective predation were susceptible to experimental work.

Kettlewell started a series of experiments. First he put insectivorous birds in cages and fed them moths of different kinds. While doing so, he judged whether he could see the moths or not, and the ones that he couldn’t see well, the birds couldn’t see very well either. This gave him confidence in the story.

These experiments showed that birds actually will eat moths, something which had been argued about previously. That suggested to him that it was worthwhile carrying out a field experiment. This he famously did, with one set of trials in Birmingham and one in Dorset. The atmosphere of Birmingham was extremely polluted at the time, that of Dorset unpolluted. Both experiments were in woodland. In Birmingham most wild peppered moths were melanic and in Dorset he found no carbonaria at all.

He released mixtures of different forms of live moths and recaptured them. When you do that you know the frequencies released and the frequencies that you recapture. It is then possible to compare the one with the other to get an indication of the relative survivability of the two types.

The conditions involved in the tests were extreme – this is a black and white story. A picture of moths in North Wales on a characteristic piece of tree trunk shows clearly that the carbonaria form is more obvious than the typical.

The contrasting picture of moths on an ash tree in my back garden in Rusholme in the early 1970s shows that there typicals were much more conspicuous.

Not surprisingly, given the differences involved, carbonaria was twice as likely as typical to survive per day in Birmingham and half as likely to survive per day in Dorset. Niko Tinbergen took a photograph, in Birmingham, of a cock redstart with a typical in its beak that was graphic evidence of predation. So ‘Hey Presto' we have the answer to why the frequency increased! That very much became the accepted, the ‘text book’, story.

Of course, there was a context within which this experimental work was carried out. Other people, such as Tinbergen, were doing similar experiments. Tinbergen was interested in studying ethology, the behaviour of animals in the field. He introduced an experimental approach that showed you could learn a tremendous amount about behaviour patterns in species such as gulls and sticklebacks

In addition to his direct interests, he was also opening up a type of experimental procedure that showed that simple experiments could produce quite profound answers. It allowed judgements to be made about evolutionary processes. Before that time people thought of evolutionary studies as measured by phylogenetics or geological change. The evidence involved slow processes from which you could make inferences, but with which you could not interact.

People felt empowered by the new type of experiment, to find things out in the field in quite simple ways. It was a ‘heady’ feeling at the time. Kettlewell belonged to this group of experimentalists and worked directly with Tinbergen. That is one of the reasons why the peppered moth story became such an important example. It was topical in the way it was handled.

This map shows survey work carried out by Clarke and Sheppard and Bishop in the Liverpool, N. Wales area, and by myself and others in Manchester, in the late 1960s and early 1970s. The trouble with Manchester as a study site at that time was that nothing much happened. The frequencies were all very much the same, whereas in N. Wales we have this very steep cline. It is useful because it allows us to do experimental manipulations. You can imagine that the selection is likely to vary considerably across short distances there, and quite a number of experiments, along the lines of Kettlewell’s, were carried out.

A couple of general surveys have been done since. The first was undertaken by Open University students. The students were asked to collect moths in traps and send examples to be identified. Once we had checked them and thrown away those that weren’t peppered moths it produced this picture in 1982-1984. The central core of high frequency had contracted and the area to south was broken up into more irregular patches, but the original Kettlewell pattern can still be seen.

Subsequently, in the mid 1990s, there was another survey using data obtained by the Rothamsted Insect Survey. They had been collecting moths all over the country and by the mid 1990s we had a pattern like this, where the 90% frequency patch has disappeared. The highest frequencies are 50% - 60% and the whole pattern is flattening. There is still a cline in N. Wales, with higher frequencies to the N.E, but the whole plateau, which once looked permanent, is now disappearing rapidly. It looks as if we are dealing with a process that is a blip in the life of the peppered moth. Melanics rose in frequency during the industrial period based on coal burning, then went down.

I am running ahead slightly so far as the development of ideas about the subject is concerned. The declines in frequency in the peppered moth, which took place in concert with manifest changes in the environment, occurred at a time when there was a lot of interest in the environment, particularly with problems of increasing pollution.

Many people (some of them in the 1970s with long hair, flares and flowered shirts), were deeply pessimistic about the future of the environment. But here was a storey to be optimistic about in the field of environmental biology. It showed you could improve the environment and that there could be a response as far as a species was concerned.

To illustrate the change, this is a picture of Manchester University tower in the early 1970s. When the smoke control legislation was established and all of the small houses around here were pulled down, the university decided to clean its buildings.

This is a test area on the stone work. Subsequently the authorities went ahead and to produce a beautiful, clean, optimistic looking, environmentally friendly, seat of learning.

The peppered moth story therefore showed that environmental improvements could be followed through with responses by natural organisms. This was a matter of public interest, which generated quite a lot of excitement at the time.

This is a summary diagram of change in frequency over time. There are two ways in which we can try to measure the forces that must operate to produce the changes that have taken place. One is directly, as Kettlewell did, the other is to look at points in a sequence. If you have a series of values then you can estimate the amount of selection required to cause the change. This is a summary for the Manchester situation.

The figures in blue at the left are from general impressions people had: “when I was collecting at this date I never saw any melanics, later they were nearly all black”. That is the sort of evidence we have from the nineteenth century. The points in red are better records that we have collected over the twentieth century, including Mike Dockery’s [in the audience]. His is a small sample with a big standard error. Nonetheless, it fits in the right place as far as the general trend is concerned.

The diagram shows a longitudinal section through a hundred and fifty years of change. It really does show a dramatic change.

One of the most intriguing developments recently is the way the story has been picked up by the public and press.

There have been nice improvements to the environment accompanied by changes in the peppered moth, then we get this type of thing from the Daily Telegraph. “Scientists pick holes in Darwin Moth Theory”. Although Darwin had nothing to do with the moths his picture was attached. The article says “scientists admit they do not know the real explanation for the fate of Biston betularia”. Well, they never said they knew all of it. They just thought they had made some progress. The article describes the work as worthless, and other hostile and violent words have been used in other comments. To find out more you are directed to a creation web site.

Such articles present a very different picture from the one that I have put forward. I got this from one such web site – for the Institute for Creation Research. It says about the peppered moth, “what a wonderful time to be a creationist when even the supposed best proof of evolution in action is so flimsy it cannot stand the test of truth”. What are they talking about?

There have been arguments between different biologists about Kettlewell’s experiments. For example they have been criticised because he put the experimental moths low down on tree trunks. Well, he put them where he could see them and photograph them. When you study them in detail you realise that a naturally settling moth goes through quite a complex behavioural pattern, as you would expect.

It will land on a tree, tend to walk up it, come to branches and either settle under a branch or walk along the branch. It is not then sitting where the photographs were taken, but in a more protected position. This is the information that the press interpreted as indicating that the whole subject has collapsed.

The creation press has also described photographs of moths on tree trunks as fakes, because they were taken in more exposed places than those where the moths often rest.

The attacks make one ask why the idea of demolishing this topic should have such an appeal today. You could argue that demonstration is simple, even almost boring, yet some people feel a need to destroy it.

I put in this slide because, although the critics attack Kettlewell’s experiments, he was not the only person to carry them out. The slide shows published data from the literature. The fitness of the melanic carbonaria (measured as w, the frequency remaining divided by frequency presented) is plotted against estimated frequency of typicals where the experiment was carried out.

The open square in the bottom right is Kettlewell’s Dorset result, his figures for the industrialised area (Birmingham) are open squares at top left. Later studies were done at different times, and frequencies in the areas will have changed, so the pattern is relatively complicated. But fitnesses (w) are independent of the frequency estimates. The diagram shows that, overall, the estimated fitness of carbonaria was highest in places where melanic frequency was high, and vice versa.

The people who write the critical literature say that Kettlewell's results are worthless because they were done in the wrong place. But, there is a whole series of people who have done other experiments which, to me, support the original argument.

One criticism of Kettlewell’s results claims they are worthless because they were done on the wrong part of the trees. But there is a whole series of other experiments which support the original argument. Some were by Howlett and Majerus (open circles), two researchers who studied natural settling of the moths and queried the original experiments. They carried out experiments in the “wrong” and the “right” place – which produced the same results. Two other tests by Lees and Creed, (crosses on the graph) show that melanics are better protected on dark, wet tree trunks than pale, dry ones. Such results support the Kettlewell argument.

Black squares to left of the graph represent tests carried out by Clarke and Sheppard in Liverpool. They used dead moths, which allowed them to classify the sites where individuals were attached. Dark and light moths were placed in dark and light patches in all possible combinations.

When both types were on pale backgrounds, carbonaria was lost more than typical. For the others carbonaria had the advantage. There is little doubt looking at the graph that where the melanic frequencies were high there is an advantage overall to carbonaria. The reverse is true where melanic frequency is low. Do you believe it? This evidence was simply rejected by Judith Hooper, for example, who has written a book effectively libelling Kettlewell.

The points differ in their accuracy and information content. Since it is important to establish the probability of the trend I decided to carry out a significance test that was as robust as possible. Because figures on both coordinates are subject to error I decided that the safest approach was to convert fitness values into normal deviates (by dividing the deviation of each estimate from equal fitness by its standard error). You get the set of points in the next graph.

These points are significant if they are more than two standard deviations from the 0 value, where there is no selection. At top left selection is in favour of carbonaria, at bottom right selection is against it. The mean deviates for the left hand third and the right hand third of points are about +3 and -3, giving a Chi squared value of about 9 for each group. To estimate the significance of the trend we should add them together, giving a Chi squared of about 18 with 2 degrees of freedom. That is a highly significant result. It is clear that the trend cannot simply be rejected, as the ‘anti’ lobby has done.

Part of the criticism may be due to a contemporary reaction to reductionist science. To try to understand the problem we could describe the Kettlewell programme as in some respects naïve. It says.
• You can measure selection.
• What you measure matters.
• What you measure is going to be important in evolution.
• Change in gene frequency, as seen here, is fundamental to evolution.
• Once it is demonstrated that changes in gene frequency can occur rapidly then all the other evolutionary processes follow.

But this is a limited view of evolution. To understand the full range of phenomena we observe and which amaze us we need much more complex models. It may be that the approach crudely described here as a list of bullet points is perceived as arrogant and limited in its explanatory power. Even so, the validity of the evidence is unaltered by changing attitudes to scientific methodology.

I want to consider now some research that has been carried out since the classical peppered moth story was established. The two later maps of morph frequency that I have shown you are part of the programme. We can take a look a few other examples.

Sir Cyril Clarke and his wife collected the most complete longitudinal set of records of frequency available. They worked on the Wirral peninsula, near the edge of the N. Wales cline, sampling every year from 1959 (black points). A less good data set from Manchester shows similar changes to theirs (red points). There are one or two other results like this.

The green ones come from Kent (SE England, near London), the blue crosses show figures from Nottingham (E. Midlands). They all present similar pictures of sharp decline in melanic frequency. When you look in more detail, the curves with lower starting points appear to be shallower than those with higher starting points. This is not easy to see on the curves, but when we examine all available figures comparing samples from Kettlewell’s time to recent ones there is a convincing trend.

I worked out the selective values (w). There is a problem looking at morph frequencies because the selective value will produce a different change in morph frequency depending on the start point. Therefore, the selective values have to be standardised. The ones which started out with the highest frequency of melanics [left side] are the ones with the current greatest disadvantage to carbonaria.

This shows that selection is not uniform over the whole of the country. It is most intense in the industrial regions where melanics reached their highest levels and where the environmental clean-up effort has been greatest.

Finally, we can look at some evidence relating to insularia. These are the collecting sites used by the Rothamsted Insect Survey.

I can use the map to point out that in S. Wales and Gloucestershire there has always been quite a high insularia frequency. We don’t know why that is. S. Wales is industrialised and Gloucestershire relatively non-industrialised. The frequency of insularia is sometimes higher than that of carbonaria in these areas, compared with the 1% - 5% found in the northern ‘carbonaria’ regions. Why is that?

One possible reason for the difference is that special conditions in the south west favour insularia rather than carbonaria, but there is no evidence what these conditions might be. Another possible reason relates to the dominance relations of the alleles concerned. If insularia is intermediate in fitness between typicals and carbonaria, then at the beginning of industrialization both carbonaria and insularia would have an advantage over typical, and increase in frequency. But as the change proceeds carbonaria becomes the most common morph, against which insularia is disadvantageous and therefore decreases. So, the expectation is that curves of changing insularia frequency have a hump in them. Different starting frequencies can lead to different patterns of change in different places. Here is a little bit of theoretical representation.

On this graph I have plotted the insularia frequency (y) on the carbona frequency (x). The typical frequency is what is left. The insularia form is assumed to have a fitness intermediate between the other two.

At bottom left there is 100% typical, and on the diagonal there is 0% typical. If you start with the same insularia frequency but different carbonaria:typical ratios the trajectory that insularia follows varies widely, as shown by the dotted green lines. It may rise to a high frequency before dropping off, or it may hardly change at all. That is simple due to the dynamics of a 3-allele system.

The amount of selection determines the rate at which the curves are followed, but not their position, which is determined by the frequency of the other two alleles against which insularia is compared.

So, it is possible that the reason why there is high insularia in S. Wales is simply that insularia became established before carbonaria did. If conditions remained constant insularia would eventually decline, to be replaced by carbonaria.

We are looking at a non-stable situation, however, and all melanics have subsequently declined.

This is just an example of the type of inference that can be extracted from the available data – so long as you do not believe that the whole story is a hoax!

Thank you very much.

Martin Jones: Would you like to answer some questions? Well, you will answer some questions.

Laurence Cook: I want an answer to the question why people hate so much these days. There have been two books written recently, one of which is entirely about how scientists have tried to perpetuate a fraud, the other with similar allegations. Why?

The following is a more comprehensive coverage of this topic.
Cook, LM 2003 The rise and fall of the carbonaria form of the peppered moth. Quarterly Review of Biology 78, 399-417.