Natural selection is a process where organisms better suited to their environment are more likely to survive and reproduce. This leads to changes in a population’s heritable characteristics over generations. A trait is a distinguishing characteristic, like color or size, influenced by genes, environment, or both. Natural selection favors advantageous traits, altering their prevalence and distribution within a population over time.
The Classic Case of the Peppered Moth
The peppered moth, Biston betularia, provides a classic example of natural selection influencing trait distribution. This moth species has two main forms: a light-colored, speckled form (typica) and a dark, nearly black form (carbonaria). Before the Industrial Revolution in Great Britain, light-colored moths were predominant.
These lighter moths were well-camouflaged against pale, lichen-covered tree trunks, making them difficult for predators to spot. The dark form was rare, easily preyed upon against the light bark. This changed dramatically with the onset of the Industrial Revolution in the 19th century.
Widespread burning of coal released soot and smoke, which darkened tree trunks and removed light-colored lichens. In this altered environment, light moths became conspicuous against the soot-stained bark, making them easy targets. Conversely, the dark moths now blended in with the blackened trees.
This created strong selective pressure, as dark moths had a higher survival rate in polluted areas. Dark moths were more likely to survive, reproduce, and pass on their genes. By 1895, the dark carbonaria form accounted for approximately 98% of peppered moths in Manchester, a significant change from its earlier rarity.
This phenomenon, known as industrial melanism, demonstrated how environmental changes can drive shifts in a population’s trait distribution. The trend reversed in the latter half of the 20th century with cleaner air acts, such as the UK’s Clean Air Act of 1956. Reduced pollution allowed tree bark to lighten and lichens to return, favoring light moths. As the environment recovered, the light form resurged, while the dark form became less common, illustrating the link between environmental conditions and trait distribution.
Understanding the Mechanisms of Change
The peppered moth story illustrates the mechanisms through which natural selection alters trait distribution. First, a population must exhibit variation in a trait, like the moth’s color forms. These variations arise from genetic differences among individuals.
Next, these traits must be heritable, passed from parents to offspring. The color of peppered moths, for instance, is genetically determined, ensuring offspring inherit the color. This inheritance allows advantageous traits to persist across generations.
Differential survival and reproduction then occurs. Individuals with traits better suited to their environment are more likely to survive, reproduce, and produce more offspring. In polluted environments, dark moths survived predation more often than light moths.
Over generations, this leads to an increase in the advantageous trait’s frequency. Conversely, the less advantageous trait becomes rarer. This shift changes the trait’s distribution, demonstrating how natural selection drives evolutionary change in response to environmental pressures.