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Transcript
http://www.streaming.mmu.ac.uk/cook/
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 19821984. 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 3allele 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.