The Evolution of the Peppered Moth

29 April 2007

Interview with

Mike Majerus, University of Cambridge

Kat -   So you are, I suppose, a lepidopterist?Peppered mothMike -   Well I'm just a bug man, I do lepidoptera, and I also do ladybirds.Kat -   So tell us a bit about your work in Moths, what can they tell us about evolutionMike -   well, I work with one moth in particular, a very famous moth, and then a couple of hundred other species which are very much less famous.  These are the British Moths that have black forms, sometimes called melanic forms, and some of those only have had black forms since the industrial revolution.  The most famous one is the peppered moth, the first black form of which was found in 1848 in Manchester.  It spread very rapidly so within 50 years, by the end of the 19th century, 98% of the Mancunian peppered moths were black.  There was a famous Victorian lepidopterist called Tutt who suggested in 1896 that the reason for that was because the tree trunks had all lost their lichens due to acid rain, and then soot fallout had blackened the tree trunks so the black form was harder to see on those trees. Birds therefore selected, naturally selected the black form because they ate the other form which was white with black speckling.Kat -   So going back a bit, where would this black form have come from?Mike -   Just a chance mutation.Kat -   In one of the genes that are responsible for pigmentMike -   The lepidoptera have lots of different melanic (pigment) genes, and this is just a particular mutation, a particular change in one of the wings which produces black all over the wings instead of just in a speckled pattern.Kat -   So once this had arisen randomly those insects weren't getting eaten so much, so then they could breed and so it would spread.Mike -   Yes, the idea is that this mutation would occur every now and then back over thousands of years, but usually it would be a disadvantage and so would be selected against.  When the environment had changed, with pollution, then it became an advantage and so it spread.Kat -   I understand that some creationists say that the original experiments that were done to prove this, as a lovely example of evolution at work, something changing in the genes in response to the environment; but some people say that the results weren't very well done, or that it's inaccurate.Mike -   There's a very interesting lepidopterist called Bernard Kettlewell who, in the 1950s, did some classical experiments that are now reported in all the school textbooks.  In two different woods, one polluted, one unpolluted, he released live moths of both white and black forms onto tree trunks.  In polluted  Birmingham he saw that birds took far more of the pale form, so the black form was more successful.  Down in Dorset it was the other way round, so on trunks with Lichen the black from was eaten more often than the pale form. 

Since the 1950s scientists have gradually tinkered with Kettlewell's experiments, said he did it at too high density, he moved moths from one location to another and so on.  So scientists have been saying this might not be absolutely accurate.  About 10 years ago, the creationists started not only saying that these experiments weren't valid, but some were actually saying that Kettlewell faked his results, he was accused of fraud.  Unfortunately both Bernard Kettlewell and his mentor E. B. Ford, who also has come in for a lot of criticism from the intelligent design and creationist people, are both dead.  I decided that because there's so much criticism (if you put Peppered Moth into google, there's actually more hits on creationist websites that on biological websites, that just seems to be the wrong way round) so since 2001 I've been doing a new predation experiment in which I'm trying to correct for every single one of the flaws that people have pointed to in the way Kettlewell did it.  Actually, peppered moths don't usually rest on tree trunks; they rest underneath lateral branches, in the shadows.  I put them in the right place, I let them, within a limited arena, choose their own resting sites. They can fly at night and choose where to rest, so I'm not telling them where they've got to sit, they do it themselves and so on.  Unfortunately, this takes a tremendous amount of time because I have to do it at the density that the moths occur in nature.  So I can only do rather few per day, but I'll finish this year, thank goodness!  Hopefully, that will either say that Kettlewell is wrong, in which case I'll probably get the front page of the Times and Nature, or it will turn out that he was right and then I hope some of the critics of Kettlewell and Ford would then admit that they were wrong.   Although given that if you prove one thing, creationists tend to just move the goalposts, which is why we now have this strange thing 'Intelligent Design', so I really doubt that we will change anyone's mind.  What I'm really trying to do is say that the peppered moth case is or isn't a good example of Darwinian Evolution in action, and try to keep creationists out of science class.

Kat -   That's a subject for a different show I think, but lets talk more widely about insect populations and how they respond to the environment.  Are there other examples of how insect populations have changed, are we seeing populations changing as climate is changing?

Mike -   Yes, populations change, whether this is actually affecting the genes so much... One of the strange things, that many people who think about evolution don't quite understand is that there's a huge difference between somewhere like Galapagos, where Darwin made many of his observations, and 'us'.  By 'us' I mean Britain connected to continental Europe until a few thousand years ago, and the great continents.  If climate is changing what will happen here, and in Europe, Asia and connections to Africa, is that things will simply move.  They move first, they adapt second.  If they can move they will simply move, so what we're really seeing is a lot of southern European species coming up here and being able to survive.

Kat -   We've had questions about Ladybirds that are black with red spots, but aren't they traditionally red with black spots?

Mike -   We have three native species that are always black with red spots, the Pine ladybird, the Kidney Spot and the Heather ladybird.  We have two other species, the Two-Spot and the Ten-Spot, that sometimes are black with red spots but they can also be red with black spots, or orange with black spots. That's one thing I'm working on because they're polymorphic, a bit like us having black or red hair, that's all genetically controlled.  

There's also a new one, the Harlequin ladybird which arrived in 2004 and is now spreading like mad, taking over from out native ones.  We need to study that because as it's just arrived we can study all sorts of things as it's having to adapt to the British climate and other environmental factors.  Unfortunately, although it's beating all our native ladybirds already, its going to get even better at doing it, so goodness knows what's going to happen in the next fifteen years.

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