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 EVOLUTION

INDUSTRIAL MELANISM

The Case of The Peppered Moth (Biston betularia)

Melanin is a black pigment protein

Observation

19th century entomologists noticed that the melanic form of the moth was found in larger numbers in and around industrial regions and regions of dense human population.

(The first melanic form was thought to have been caught in Manchester in 1849)

Hypothesis

  1. That chemical pollution was affecting the moths turning them black.
  2. That the black variety of the moth was genetically determined. This variety was better adapted to survive in industrial regions.

Experiment

Breed moths of different varieties to establish whether the colour was genetically controlled.

Result

The moth colour is inherited.

(It is controlled be a polyallelic gene where the alleles for black colour are progressively more dominant to the allele for peppered colour.)

Conclusion

Melanism is genetic in origin and not caused by the environment.

What caused the melanic form to increase in industrial regions?

Normal form: Peppered

 

Variant: Black (melanic)

 

Distribution of melanics

(Adapted from Kettlewell Sci Amer March 1959)

 

Hypothesis

  • The selective pressure came from predators (insect eating birds). The moth’s colour provides camouflage.
  • Moths rest during the daytime on trees.
  • In rural areas where the air is clean the trees are covered in lichen. The peppered variety is better camouflaged when it rests on these trees. The melanic form is easily seen.
  • In industrial areas air is polluted and the lichens die.
  • The tree trunks may also be covered in soot giving a black background.
  • This favours the melanic form giving them a selective advantage.

Experiment

Transfer moths of both varieties to clean and polluted areas. Record the rates of predation on each variety.

Results

Variety

Predation Rates

Clean rural area

Polluted industrial area
Peppered form

Low

High
Melanic form

High

Low

Conclusion

Natural selection was operating through the predators, changing the proportions of the two varieties in the two areas.

Corollary

Since the 1950s air pollution caused by burning coal has been controlled more effectively. the trend in industrial areas been seen to reverse the moth populations. Less melanic forms are found and more peppered forms appear.

NB Because the melanic form is caused by a dominant allele it will appear and disappear quite rapidly form the population if conditions of natural selection change.

© Paul Billiet 2004