5.2 Applications and skills

5.2.1 Case study: Industrial melanism – polymorphism in the peppered moth

As you know, evolution is the change in frequency of genes in a population over time. A well-known example of evolution is the case of industrial melanism in the peppered moth, Biston betularia, circa 1850. Colouration in the moth is polymorphic – that is, there are different phenotypes of peppered moth, as shown in Figure 5.2.1a.

Polymorphism in Biston betulariaFigure 5.2.1a – Polymorphism in Biston betularia
The carbonaria form is shown on the far right against a soot-covered tree, and again on the far left against a lichen-covered tree; the typica form is shown at the centre right and centre left.

Before the proliferation of coal-fired factories in the Manchester area, the most common form of the peppered moth was the typica form. With this colouration, the moth was well camouflaged against the lichens growing on tree bark in public parks and nearby woodlands. However, by 1895 almost all of the moths sighted had carbonaria colouration.

A British lepidopterist, J.W. Tutt (Figure 5.2.1d), hypothesised that the pollutants from nearby factories were killing lichens and depositing black soot on the bark of trees, making the dark phenotype less conspicuous to daytime predators (i.e. birds). In other words, a change in the natural environment caused an increase in the fitness of one genotype over another – the very definition of natural selection.

The hypothesis was largely ignored in the 19th century, but during the 1950s Bernard Kettlewell (Figure 5.2.1e), a British ecologist working in Birmingham, investigated the hypothesis in two areas of England.

Kettlewell bred both forms of the moth and released them into polluted and unpolluted woods. He then attempted to recapture the moths, in order to determine the proportion that had been picked up by predators from each site. His summarised results are shown below:

Kettlewell’s experimental results
Form Polluted wood 
recapture rate (%)
Unpolluted wood 
recapture rate (%)
typica 13.0 14.6
carbonaria 27.5 4.7

Activity 1: Does the data support the hypothesis?

Answer the following questions:

  • Does this data support the hypothesis that the melanic form offers an adaptive advantage against predation?
  • What assumptions was Kettlewell working with when he performed this experiment?
  • Which variables would he have needed to control to get reliable results?
  • What other information would you like to know about Kettlewell’s experiment before drawing conclusions?

Transient polymorphism – the moth story continues

Although scientists disagree on the reliability of Kettlewell’s original data and conclusions, the peppered moth story continues to be used in biology classes to illustrate the idea of transient polymorphism. When the environment changes, the fitness of one genotype increases over the other. If the change is transient, when the environment changes again there should be another predictable shift in gene frequencies.

During the 1960s and 1970s, anti-pollution legislation was passed all over the developed world. Analyse the data in Figures 5.2.1b and 5.2.1c and complete the activity.

comparison of atmospheric sulpher dioxide concentrations

Figure 5.2.1b – A comparison of atmospheric sulphur dioxide concentrations in different states (1964–96)

sulpher dioxide concentrations

Figure 5.2.1c – Decrease in the melanic form of B. betularia in different states (1959–2001)

Activity 2

  • Sketch a new graph to show the relationship between pollution level and gene frequency of the melanic form.
  • Explain the trend in your graph.
  • Comment on the hypothesis that industrial melanism is an example of transient polymorphism, based on the data in the figures above and from your graph.

Did you know?

A lepidopterist studies butterflies and moths and other insects from the family Lepidoptera.

In the lab

The Capture-Recapture technique is a common way to estimate population size. Learn more about fieldwork on Page 4.1.

J.W. TuttFigure 5.2.1d – Practical hints for the field lepidopterist by J.W. Tutt

KettlewellFigure 5.2.1e – Photograph of Bernard Kettlewell

International mindedness

Learn more about the human face of science in:

Hooper, J. (2004) Of Moths and Men: An Evolutionary Tale: The Untold Story of Science and the Peppered Moth. London: W.W. Norton.