Industrial melanism in Biston betularia

1.  Industrial melanism - the phenomenon

Many moths in Britain come in two different colours, a lighter "natural" form, and a darker, melanic form.

The melanic form is in all ways similar to the natural form except that it produces a much greater amount of melanin, the pigment that gives its wings their dark colouration. 

In Biston betularia, Kettlewell and his group have performed crosses that have demonstrated that a single gene controls the difference between natural and melanic morphs.

2.  Historical records

In the last 150 years, the relative preponderance of the two morphs has changed dramatically.

Old collections from the early 1800's show that the pepper morph was far and away the most abundant morph. 

Melanic morph was collected only very rarely.

Beginning in the early to mid 1800's, however, melanic form increased in abundance dramatically, until today, in some areas virtually all moths are melanic.

There has thus clearly been a dramatic change in gene frequencies, i.e. evolution has occurred. 

3.  Observation:  the change in morph frequencies corresponded with the onset of the industrial revolution in Britain.

One consequence of the industrial revolution was that the smoke and soot put out by all of the factories that sprung up across Britain caused a darkening of the tree trunks in many areas, particularly near urban industrial centers. 

Originally, tree trunks had been covered with light-coloured lichens, but after the industrial revolution, these lichens and the tree trunks they covered became dark in these areas; many likens were killed.

4.  Kettlewell's hypothesis:  Because moths spend a great deal of time during the day resting on tree trunks, Kettlewell reasoned that they are probably exposed to a great deal of predation by birds and other animals.  It would therefore benefit a moth to be cryptically coloured, so that it will blend in well with the background of the tree trunk on which it rests.

Individuals that were more cryptically colored would tend to escape notice by predators to a greater extent, and thus would tend to have a survival advantage over non-cryptically coloured individuals. 

Kettlewell postulated that before the industrial revolution, the pepper form blended in with the light-colored background of tree trunks, whereas the black morph was conspicuous, leading to selection against the melanic morphs.  This genotype would thus be held at very low frequency. 

With the change in tree trunk color associated with industrialization, however, the melanic forms became cryptically colored and the pepper forms became conspicuous.  Under such situations, the rare melanic mutants would enjoy a survival advantage and increase in frequency until the pepper forms were eliminated from the population.

Visual inspection of the degree of crypsis on dark and light tree trunks supports this hypothesis.   

5.  Experimental test of the hypothesis. 
I. Compare survival of melanic and pepper moths.

Capture-Mark-Release-Recapture experiment. Released several hundred marked individuals of both morphs into two types of woodlands:                  
 --  One woodland near a big city; polluted; trunks darkened with soot.
 --  One woodland more rural, relatively unpolluted; tree trunks not darkened.

Recaptured using caged females emitting pheromones. 

Reasoning:  if dark morphs have survival advantage in polluted woods, should recapture more dark morphs than pepper morphs.  By contrast, if pepper morphs have survival advantage in unpolluted woods, should recapture  more pepper moths there. 

Results:

                                          %Recaptured
                    Woodland    Melanics    Pepper
                    Urban            27.5           13.0
                    Rural               6.3           12.5

This experiment showed that the melanic form did indeed enjoy a survival advantage in polluted woodlands, while the reverse was true in unpolluted areas.  However, Kettlewell still had not shown that this differential survival was caused by differential susceptibility to predation. 

6.  Experimental test of the hypothesis. 
II. Differential predation

Placed recently-killed moths of each morph on tree trunks in the two types of woodland. Sat in blind and observed what happened to moths placed out.  In particular, counted the number of individuals of each morph eaten by birds. 

Results

                                            No. of moths eaten by birds
                    Woodland            Melanics        Pepper
                    Urban                     15                  43
                    Rural                     164                 26

Interpretation:  Kettlewell's hypothesis is confirmed.  In the polluted environment, more pepper morphs are eaten, while in the unpolluted environment, more melanic morphs are eaten.

7.  Summary:

Industrial melanism is genetically controlled by a single locus in B. betularia. 

Populations have undergone evolutionary change in color pattern. 

That change is consistent with the interpretation that it was due to natural selection, in that there is differential survival of the genotypes caused by differential predation on a particular background. 

Results confirm qualitative prediction of equation for gene frequency change.

8.  Additional point

Industrial melanism is seen in more than 70 species of British moths, and all show patterns similar to that seen in B. betularia.

At least one prediction based on Kettlewell's work has come true: since the imposition of pollution control devices on many of Britain's factories in the 1950's, pollution has decreased markedly in many areas.  As a result, many of the formerly polluted woodlands have returned to their original condition, in which the colour of the tree trunks is light and mottled.  As would be expected from Kettlewell's work, the melanic forms that were once so prevalent in these areas have almost disappeared.

(see H.B.D. Kettelwell.  1973.  Industrial Melanism.  Oxford Univ. Press, Oxford, U.K.)