Natural selection is a fundamental process in biology that explains how populations of organisms change over generations, driving the diversity of life. Moths, particularly the Peppered Moth, offer a clear illustration of natural selection in action. This article explores how environmental conditions influence moth evolution.
The Core Concept of Natural Selection
Natural selection operates on several basic principles. Within any population, individuals exhibit variations in their traits, such as size, color, or behavior. These variations are often heritable. Organisms typically produce more offspring than can survive, leading to competition for resources.
Individuals with traits that are better suited to their current environment are more likely to survive, reproduce, and pass on those advantageous traits to the next generation. Over time, these beneficial traits become more common in the population, while less favorable traits decrease in frequency. This selective survival and reproduction, driven by environmental pressures, results in gradual changes in the genetic makeup of the population.
The Peppered Moth: A Living Example
The Peppered Moth ( Biston betularia ) provides a classic example of natural selection, often referred to as industrial melanism. Before the Industrial Revolution, the majority of Peppered Moths in Britain had light, speckled wings, which provided effective camouflage against lichen-covered tree trunks. A rare dark-colored form, carbonaria, existed.
As the Industrial Revolution progressed in the 19th century, coal-burning factories released vast amounts of soot and pollution into the atmosphere, particularly in industrial cities like Manchester. This pollution darkened tree bark and killed off light-colored lichens, changing the moths’ resting backgrounds. The light-colored moths, once well-camouflaged, became highly visible to bird predators on the darkened trees.
The dark carbonaria form now had a selective advantage as it blended in with the sooty tree trunks. This led to a significant increase in the frequency of dark moths; by 1895, the carbonaria form constituted approximately 98% of the Peppered Moth population in Manchester. After clean air legislation was implemented in the late 20th century, pollution levels decreased, and lichens began to return, causing a reversal of this trend as the light-colored moths once again gained a survival advantage.
How Environmental Shifts Drive Moth Evolution
Environmental pressures strongly influence the direction of natural selection in moths. Bird predators play a significant role by preferentially preying on moths that contrast with their background. This selective predation creates strong pressure favoring camouflaged individuals.
Beyond industrial pollution, other environmental factors also shape moth evolution. Habitat changes, such as deforestation or urbanization, can alter the availability of food sources and suitable resting places, leading to shifts in moth populations over time. The presence of specific predators or changes in their abundance can also exert pressure, favoring moths with better defensive traits or more effective camouflage. These ongoing interactions between moths and their changing environments drive continuous adaptation.
Moths Beyond Melanism: Ongoing Adaptation
Moths continue to undergo natural selection in response to contemporary environmental challenges, beyond industrial melanism. For instance, some moth species, like the diamondback moth (Plutella xylostella), have developed pesticide resistance due to agricultural chemical use. Climate change also presents new selective pressures, as rising temperatures can alter the geographic ranges of moths, pushing some species northward in search of cooler areas.
Changes in climate can also affect the timing of moth life cycles, potentially causing them to emerge out of sync with their food plants or the availability of suitable conditions for reproduction. Some species, like the European corn borer moth, show genetic adaptations that allow them to adjust their internal biological clocks to synchronize with changing seasons, impacting their ability to enter or exit a dormant winter state. These ongoing adaptations demonstrate that moths serve as valuable models for studying evolutionary processes as environments continue to change.