Evolution is a fundamental biological process where life forms change over generations. While evolution often spans immense geological timescales, scientists have documented instances unfolding within human lifespans. Moths offer compelling examples of rapid evolutionary change, providing clear insights into how populations adapt to shifting environments.
The Peppered Moth Story
Before the Industrial Revolution, peppered moths (Biston betularia) in Great Britain were predominantly light-colored. Their speckled pattern blended with lichen-covered trees, offering effective camouflage from predatory birds. As the 1800s progressed, coal-burning factories caused significant air pollution, blanketing the countryside with soot. This darkened tree trunks and killed lichens, making pale moths highly visible.
A genetic mutation resulted in some peppered moths having dark, or melanic, wings. Initially rare, these dark moths (carbonaria) became camouflaged against the soot-blackened trees. This provided a survival advantage, as predatory birds found them harder to spot. Dark moths survived in greater numbers, reproduced, and passed on their genes for melanism.
By the mid-19th century, dark moth frequency increased, reaching 98% in Manchester by 1895. This phenomenon, where industrial pollution drives color change, became known as “industrial melanism.” Following the Clean Air Act of 1956, pollution decreased, and lichens returned. Light-colored moths regained their camouflage advantage, and their numbers subsequently increased in UK urban areas.
How Scientists Tracked Moth Evolution
H.B.D. Kettlewell’s work in the 1950s rigorously investigated peppered moth evolution. He aimed to demonstrate how bird predation drove observed changes in moth populations. Kettlewell conducted mark-release-recapture studies in both polluted and unpolluted woodlands.
Kettlewell marked hundreds of light and dark peppered moths with cellulose paint. He released them into specific woodland areas, including a polluted wood near Birmingham and an unpolluted wood in Dorset. After several nights, he attempted to recapture the moths using light traps and traps with virgin females.
Kettlewell’s results provided concrete evidence for selective pressures. In the polluted Birmingham wood, he recaptured a significantly higher proportion of dark carbonaria moths (around 27.5%) compared to light typica moths (approximately 13.1%). Conversely, in the unpolluted Dorset wood, the light moths had a clear advantage, with a higher recapture rate. These findings, combined with observations of bird predation, strongly supported the hypothesis that birds acted as selective agents, preying more heavily on moths that contrasted with their background, thus driving the observed changes in moth coloration.
Diverse Evidence for Moth Evolution
Beyond the classic peppered moth example, other lines of evidence further confirm moth evolution. Genetic studies have revealed the underlying molecular changes responsible for adaptations within moth species. For instance, the dark coloration in peppered moths is linked to a “jumping gene” mutation, or transposable element, inserted within the moth’s cortex gene. This specific mutation, dated to around 1819, coincided with the onset of the Industrial Revolution, providing a genetic basis for the observed rapid color change.
The fossil record also offers insights into moth evolution over longer timescales. While the fossil record for Lepidoptera (butterflies and moths) is less complete than some other insect groups due to their delicate bodies, ancient wing scales and fragments have been discovered. Fossils from the Triassic-Jurassic boundary, approximately 200 million years ago, reveal primitive moth-like species with wing scales that share characteristics with living moths. These ancient fossils, including those showing evidence of a proboscis, suggest early evolutionary adaptations for feeding.
Observing resistance to pesticides in some moth populations also demonstrates rapid evolutionary adaptation. The codling moth, for example, developed resistance to the insecticide lead arsenate as early as 1914. More recently, the diamondback moth has evolved resistance to Bacillus thuringiensis (Bt) toxins through mutations affecting toxin binding sites. This rapid emergence of resistance highlights how strong selective pressures, like pesticide application, can quickly favor individuals with pre-existing genetic variations that confer survival advantages, leading to significant evolutionary changes within a relatively short period.