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The World of Darwin
Biston betularia
Biston betularia in Birmingham
Around the beautiful countryside of Devon, England, lives Biston betularia, the peppered moth. Normally this moth is hard to find because it rests on the bark of lichen-covered tree trunks and has wonderful cryptic appearance (it is well camouflaged).

Blending in with its greyish background, the moth, being good to eat, avoids predation by birds and survives thanks to a very effective defensive adaptation; it cannot be seen by creatures who want to eat it!
In 1937, E. B. Ford at Oxford University noted that a new variety of moth, which was very much darker, predominated around the industrial areas of Birmingham and Manchester. Here the trees were all covered in soot from the surrounding factories, and Ford suggested that the darker moth, against this darker background, was better adapted for survival.
In fact, birds were removing the lighter moths in favor of the darker variety. Natural selection was at work.

To prove this theory, H. B. D. Kettlewell, also of Oxford University, collected samples of both types of moth, the normal peppered moth from the clean fields of Devon, and the darker moths from the polluted trees of Manchester.

He then released 64 light moths and 154 dark moths into the sooty countryside around Birminghan. Nature went to work, and the birds began to catch their prey, at least the ones they could find.

Later Kettlewell was able to recapture 16 light moths (16% of the original release) and 82 dark moths (53% or the original population).

It certainly looked as if the darker moths had a higher survival rate. This was confirmed by direct observation. Using binoculars, Kettlewell saw that in the sooty barked woods, birds took 43 peppered moths from the trees, and only 15 dark moths.

Repeat the Experiment
Kettlewell then repeated this experiment in the clean atmosphere and grey lichen covered trees of Devon.

He released 393 light moths and 406 dark moths. Later he was able to recover 54 light moths (13.7%) and 19 dark moths (4.7%). In the Devon countryside, the peppered moth had the advantage.

Taken together, these experiments of Kettlewell show that cryptic coloration (camouflage) is a good, but not foolproof, defensive mechanism.

Under both sets of circumstances, birds were able to see, capture and eat both types of moth, but moths with the better adaptation (less easily seen on the tree trunks) were caught less frequently than were those that could be easily seen resting on the tree trunks.

Survival was a statistical probability, not an absolute certainty.

In the presence of predators
In the presence of predators, genes in the moths for cryptic coloration (camouflage) are an adaptation that increases the chances of survival. No doubt, in every generation of these moths, some gene combinations of new or old mutations produced dark moths, but they rarely survived.

Birds spotted them at once and they were quickly eaten before they had a chance to reproduce.

When the environment changed, however, and the trees became darker from soot deposits, the normal (lighter) peppered moth was at a disadvantage; it could be spotted by any passing bird. No longer concealed among the lichen on the tree bark, these moths were eaten at a higher and higher frequency, while the darker moths now survived.

Over time, the darker moths predominated in the populations of Biston betularia around Birmingham. Variation of genes and gene combinations followed by natural selection were working in the first steps in the evolution of a new kind of moth.

© 2001, Professor John Blamire