Science Interviews


Sun, 29th Jun 2008

Updating Evolution Evidence - The Peppered Moth

Dr Remy Ware, Cambridge University

Listen Now    Download as mp3 from the show Evolution and Natural Selection

Dave - Dr Remy Ware is from the Department of Genetics at Cambridge University where they’ve been going through the experiments that got the Peppered Moth in the textbooks. First of all, can you tell us what the original story was?

The Remy - Yeah sure. There was an original observation made in 1848 in Manchester in which there was a dark form of the so-called Peppered Moth recorded.  The Peppered Moth, its normal form is light in colour with a black speckling which gives it its name.  In 1848 the first black or melanic form of this moth was recorded.  Towards the end of the 19th century, towards 1895 or so, a very large proportion of the moths found in Manchester were of this darker form.  A very distinguish Victorian lepidopterist called JW Tutt proposed a hypothesis for why this was occurring.  His was of differential bird predation.

The idea was that with the industrial revolution the pollutants produce such a sulphur dioxide and soot fallout had darkened the surfaces of the trees on which these moths rest.  A combination of killing off the foliose lichens on the trees and actually darkening the surfaces themselves meant that what was previously a well-camouflaged light moth on a light surface now stood out very conspicuously to predators and suffered a higher predation rate such that a new mutation causing a melanic form of the moth was at a greater advantage of being more cryptic on the surface. 

Following this hypothesis a chap called Bernard Kettlewell in the 1950s tried to really test what was going on here.

The Dave - So basically the birds – if you’re flying along and you see a black moth on a black tree you’re not going to see it, whereas all the white ones are going to get eaten really quickly.

Remy - Simply a case of looking cryptic against your background so being light and camouflages on a light surface or being black and camouflaged on a blackened surface.

Dave - So the more black ones survive the more will be in the next generation.  There’ll be more and more of them around for future generations.  There’s been some criticism of this experiment.  What was that?

Remy - Ok.  This chap Kettlewell started trying to test the hypothesis of this bird predation.  More recently over the past decade or so his work’s been criticised.  Mainly because of issues to do with it being very artificial.  What he actually did was he had an experiment in which he looked at an oak woodland in Dorset and a polluted woodland in Birmingham.  He looked at the levels of predation of the different forms of the moth: the light form versus the dark form.

He found a reciprocal result in his data.  More of the dark form were predated upon in the lighter area.  The criticism was that it was rather artificial for many reasons.  Firstly he was using a mixture of lab-bred moths and wild caught moths so they may not have been behaving naturally.  Also he was placing them in very conspicuous places on the tree.

Dave - So naturally moths would have hidden themselves?

Remy - Yeah.  Later experiments would involve gluing dead moths onto the tree in positions that were rather conspicuous.  We know now that moths naturally rest under twigs and under branches and things.  Also he was releasing moths in very, very high densities so essentially he was creating what we would call a bird-table effect.  The birds were learning that they could come to this site and they would have a good lunch straight away.

It’s really these criticisms of artificiality which have been at the forefront of the arguments against the Peppered Moths case.

Dave - I guess he was still showing that there was a selection pressure towards the dark ones in the dark trees and the light ones in the light trees but people weren’t entirely convinced.

Remy - Yeah so whatever the criticisms are of it being artificial, it was the reciprocal nature of the results of the two areas that was so convincing.  Never mind the nitty-gritty of the quantitative nature of the case, quantitatively it seemed very convincing.  Subsequent evidence came from a reduction in the dark form of the moth following anti-pollution legislation later on.  This was rather convincing and shows, importantly, that evolution was not a one-way process – it can go back.

Dave - So you’ve now been looking at trying to fix some of the problems with this experiment to try and answer its critics. What have you been doing?

Remy - Some work led by my colleague, Professor Michael Majerus, he wrote a book on melanism in 1998 addressing this point and since then had a number of criticisms.  He set about systematically trying to correct the problems with the original story mainly removing these issues to do with it being a very artificial type of experiment.  He tried as much as possible to make it a natural experiment.  He was releasing moths in natural frequencies, very low frequencies, and this experiment took over seven years to complete because it was trying to be so realistic.

Dave - Very patient!

Remy - Yep!  Importantly he also allowed the moths to choose their resting positions naturally.  He released them overnight and allowed them to select their positions as they would do in the wild whereas Kettlewell was releasing them during the day.  It was more likely they would select natural positions.  Various things like this and natural density could be why it took such a long time to do.  It sort of addressed each point in turn.

Dave - What was the result of the experiment?

Remy - Similarly to what was found by Kettlewell, again he found – this work was done in Cambridge – it was found incredibly convincingly that there was a very strong correlation between observed declines in the dark form in Cambridge between 2001 and 2008.  It’s just been published now.  He found there was a very close correlation between the decline of the dark form of moth during this time and the actual predation that he observed by eye.  He observed various birds taking these moths from the trees, differentially with respect to colour: with respect to black form or the light form. He found a very close correlation between the predicted decline in the dark form as a result of this predation compared to what was actually observed.

So it was very strong evidence that differential bird predation was responsible for this.

Dave - Brilliant. Thanks very much Remy. That’s Remy Ware from Cambridge University on how experiments in evolution can themselves evolve and improve.


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