Selection in action – peppered moths
Class practical or Demonstration
The case of the peppered moth (Biston betularia) is a commonly reported example of the change in a species from one form (or morph) to another, as a result of natural selection in different environments. It serves as a good introduction to the process of evolution by natural selection and a good example of population dynamics.
Students will need access to the internet to carry out a websearch for images. Or you could download suitable resources to a slide presentation. This is a text-based activity, based on some of the results of extensive practical work done by researchers over the years.
Students may hold different views about evolution. Be sensitive to their concerns, but maintain a focus on scientific evidence.
Apparatus and Chemicals
For each group of students:
Copies of the student worksheet and maps.
For the class – set up by technician/teacher:
Slide presentation (if preferred to websearch options)
Health & Safety and Technical notes
a Decide whether to carry out the websearch yourself and provide additional resources, or make that part of the students’ work.
b Work through the information on the student sheet and answer the questions.
Many species of European moths are active at night and rest immobile on tree trunks and branches during the day. Such moths are usually well-camouflaged on the bark of the species of tree on which they commonly rest. The camouflage of bark-resting moths depends on the colour of tree bark and the presence of lichen on the bark. Pale speckled moths are much less conspicuous when lichen is present, and are most conspicuous when the bark is darkened with sooty deposits.
During the first half of the 19th century, industrialisation based on coal increased greatly in many parts of Britain. Smoke from burning coal results in deposits of soot and tar on buildings and the bark of trees. Most coal contains some sulfur; this is converted to sulfur dioxide by burning, and released in the smoke. Lichens are extremely sensitive to sulfur dioxide pollution; they are rapidly eliminated near areas where coal is burned and in areas to which prevailing winds carry the smoke.
During the 19th century, collectors began to capture increasing numbers of dark-coloured or melanic mutant forms of many species of moth. The first melanic form of the peppered moth was taken near Manchester in about 1850. Melanic mutants became more frequent in some 70 species of bark-resting moth during the following hundred years. There is good evidence for the spread of these polymorphisms because the Victorians were keen collectors of butterflies and moths. Collectors seek out new and unusual variants, and many extensive Victorian collections are still preserved.
This phenomenon has been termed ‘industrial melanism’, because the occurrence of melanism is linked to industrialisation. In peppered moths (Biston betularia) industrial melanism has been closely studied by EB Ford, HBD Kettlewell and MEN Majerus. They have described three recognisable morphs: the pepper-coloured form typica, a much darker but still mottled form insularia, and a black form carbonaria. The genetics controlling the production of these forms is complicated. In Melanism: evolution in action (1998: p119ff), Majerus describes how these forms are produced by the action of at least five alleles operating at a single locus, influenced by modifier genes. The carbonaria morph is usually dominant to typica and insularia, and insularia is usually dominant to typica. However, the dominance is often incomplete.
In 1956, Kettlewell reared many carbonaria and typica moths in captivity and released equal numbers of the two forms in a woodland near Birmingham with dark, lichen-free trunks; and in a Dorset woodland where the trunks were covered in a rich flora of lichen. Data from the two experiments are given here:
Since the 1950s, smokeless zones have spread throughout Britain, and the use of coal as an energy source in the home and in small factories has declined. Large modern power stations have very high chimneys and any sulfur dioxide they produce is carried away. Since this time, the incidence of melanic mutants has declined and the typica morph is the most common.
Predation by birds causes differential survival between light and dark coloured forms of Biston betularia, depending on the environment.
Your students may encounter the anti-evolution agenda when carrying out a websearch, so it is important to be able to deal effectively with any questions they may have.
There has been some criticism of the peppered moth example by those with an anti-evolution agenda. They say that classic textbook photographs were staged – set up to show what the moths look like and not representative of their behaviour; from this they claim that deductions about the moths are unfounded. However, many textbook pages state that the purpose of the photographs is to show the forms side by side on different kinds of bark, rather than being indicative of moth behaviour. In the wild, the moths do seem to settle more commonly under branches and in leafy canopy – positions where effective cryptic colouration would still be useful. However, workers in the field report up to 30% of sightings on open bark, and also report sighting of birds taking moths from these positions.
Some anti-evolution postings state that the melanic form existed before the industrial revolution, so it is not a mutation in response to changing environment. Students need to understand that preferential selection of existing mutations is the normal mechanism proposed for evolutionary change by natural selection. In the timescales we can see (or keep records of) we will never see dramatic change from one species to another. Change from one form of a species to another is the only example we will ever see in documented history. Evolution from one species to another takes a very much longer time.
Polymorphism in the peppered moth has been subject to many hours of research study, and there have been studies of another 70 species of melanic moths in Europe. All this evidence leads to the conclusion that predation by birds in changing background environment is the selective agent resulting in change of populations from melanic to non-melanic form or vice versa.
Health & Safety checked July 2010
Download the student sheet Selection in action - peppered moths (449 KB) with data, instructions and questions.
A detailed rebuttal of the anti-evolutionary position on the melanic moth Biston betularia.
Keywords “Majerus” and “melanism” produce extensive extracts from Michael Majerus’ 1998 book Melanism: evolution in action – a comprehensive review of the research into natural selection in the peppered moth and other melanic moths.
(Websites accessed October 2011)