Organisms adapt to their surroundings through changes in traits that allow them to thrive. When an environment changes, certain characteristics can become highly beneficial, influencing survival and reproduction. This dynamic process highlights how organisms interact with habitats, leading to shifts in populations over time.
The Environmental Shift
The Industrial Revolution, which began in Britain during the 18th century, profoundly altered the natural landscape. Factories powered by burning coal released thick smoke and soot into the atmosphere, leading to significant air pollution in industrial centers.
As a result, soot and grime accumulated on surfaces, darkening tree trunks, buildings, and other parts of the environment. This blackening transformed formerly light-colored surfaces, including tree bark that previously hosted pale lichens. This drastic environmental change created a new backdrop for local wildlife.
The Advantage of Dark Coloration
Amidst this darkened environment, a notable change occurred in populations of the peppered moth, Biston betularia. This moth typically exhibited a light, speckled coloration that provided camouflage against lichen-covered tree trunks. As pollution intensified, a darker, melanic form, known as carbonaria, became more prevalent.
Dark moths were better camouflaged against soot-blackened trees and buildings, making them less visible to predators. Birds, primary predators, found dark moths harder to spot. Conversely, light moths stood out starkly against the sooty background, becoming easy targets. This difference in visibility directly influenced survival; dark moths had a higher chance of surviving to reproduce, passing on their advantageous trait.
Genetic Inheritance of Moth Color
The color variation observed in peppered moths is a heritable trait, passed down from one generation to the next. Scientists determined that moth coloration is controlled by specific genes. The dark (melanic) form is typically dominant over the lighter form, meaning an individual moth needs only one copy of the gene for dark coloration to exhibit the dark phenotype.
Research pinpointed a mutation in a gene called cortex as the cause of this dark coloration. This mutation involves the insertion of a transposable element into the cortex gene. This genetic change is estimated to have occurred around 1819, predating the first recorded sighting of a dark peppered moth in Manchester in 1848.
Adaptation in a Changing World
The dynamic relationship between the peppered moth and its environment continued to evolve. Beginning in the mid-20th century, cleaner air legislation in the United Kingdom led to declining air pollution. This reduction in soot allowed tree trunks to gradually lighten as lichens returned and accumulated grime dissipated.
As the environment became cleaner, the advantage of dark coloration diminished. Light-colored moths became better camouflaged against lighter tree bark, while dark moths became more conspicuous to predators. Consequently, the light form increased in frequency, becoming the predominant form in many areas. This reversal illustrates that advantageous traits are context-dependent and adaptation responds to environmental shifts.