The Industrial Revolution (18th and 19th centuries) caused rapid environmental changes, particularly in Great Britain. Widespread coal burning in factories and homes released immense quantities of pollutants, primarily smoke and soot, into the atmosphere. This sudden contamination created environmental pressure that resulted in one of the most documented examples of rapid evolutionary change. This transformation was first noticed in the appearance of trees, specifically the pale bark of the birch, which indicated the changing air quality.
The Pre-Industrial Birch Environment
Before heavy industrialization, birch bark in the British Isles was typically a pale, mottled canvas of light gray or white. This light appearance was largely due to the organisms growing upon it: lichens. Lichens are symbiotic partnerships between a fungus and an alga or cyanobacterium. Highly sensitive to air purity, lichens thrived in clean environments, forming extensive, pale, speckled colonies across the tree trunks. These colonies created a bright, textured background that blended seamlessly with the bark, giving the tree a pale, uniform appearance.
How Pollution Darkened the Bark
Industrial pollutants drastically changed the birch bark through physical staining and biological elimination. The physical change resulted from the direct fallout of fine carbon particles, or coal soot, from factory smokestacks. This black soot settled heavily on exposed surfaces, staining the naturally pale bark dark gray or black, especially near industrial centers.
The biological change involved the death of lichens, which are vulnerable to acidic air pollution. Coal combustion released high concentrations of sulfur dioxide (\(\text{SO}_2\)), which formed sulfurous and sulfuric acids in atmospheric moisture. This acid exposure was toxic, causing lichen colonies to die off rapidly. The loss of the light-colored lichen layer, combined with the layer of black soot, resulted in a widespread darkening of the forest landscape.
The Evolutionary Shift of the Peppered Moth
The darkening of the birch bark established the necessary condition for a rapid evolutionary event observed in the Peppered Moth (Biston betularia). Before industrialization, the common form (typica) was pale with black speckles, camouflaged against the light, lichen-covered bark. A rare genetic mutation produced a dark, or melanic, variant known as carbonaria.
As tree trunks became dark with soot and devoid of lichens, the survival advantage reversed. The light typica moths were now highly visible to avian predators, contrasting sharply with the blackened background. Conversely, the previously rare dark carbonaria morph blended effectively with the soot-stained bark, avoiding predation. This difference in predation pressure—natural selection—drove a swift shift in the moth population’s color frequency. Dark moths survived and reproduced at a much higher rate than light moths. In industrial areas like Manchester, the dark morph exploded from being extremely rare to accounting for approximately 98% of the population by 1895, an example of industrial melanism.
Modern Environmental Cleanup and Reversal
The environmental conditions causing this evolutionary shift began to reverse in the mid-20th century with the implementation of clean air legislation. These policies mandated a change in fuel use away from high-sulfur coal, significantly reducing atmospheric sulfur dioxide and soot levels. As air quality improved, the direct physical staining of the bark diminished, and lichens began to recolonize the tree trunks.
Reversal of Camouflage
The return of the pale, speckled lichen layer lightened the appearance of the birch bark, reversing the camouflage advantage. With the background color predominantly light again, the dark carbonaria moths became vulnerable targets. The light typica form regained its survival edge. This environmental reversal led to a corresponding decline in the dark morph’s frequency, a process sometimes called de-melanization. The light-colored moth has since become the dominant form in many formerly polluted regions, demonstrating the link between air quality, tree bark appearance, and natural selection dynamics.