Genetic variation in the London moth population (the classic peppered moth story) was crucial because it provided the raw material that natural selection could act on, both during and after the Industrial Revolution.

During the Industrial Revolution

When factories in and around London and other industrial cities burned coal, soot killed pale lichens and darkened tree trunks and walls.

That environmental change flipped which moths were safest from predators.

  • There were two main variants of peppered moths: a light ā€œsalt-and-pepperā€ form and a dark (melanic) form.
  • Before heavy pollution, light moths were better camouflaged on lichen-covered, pale bark, so birds more often spotted and ate dark moths.
  • As soot darkened the trees, light moths suddenly stood out and were eaten more, while dark moths became well-camouflaged and survived to reproduce.
  • Because the dark color was caused by a specific mutation (a change associated with a ā€œjumping geneā€), once it existed in the gene pool, it could rapidly spread when conditions favored it.

So the significance of genetic variation during the Industrial Revolution is:

  • It allowed some moths (the dark type) to have a survival advantage when the environment changed.
  • Natural selection increased the frequency of the dark allele, turning a once-rare type into the dominant form in polluted areas (up to about 98% dark in places like Manchester by the late 1800s).

In other words, without that preā€‘existing variation, the whole population might have declined because no individuals would have matched the new, soot- dark background.

After pollution controls and cleanup

Later, as air-quality laws and cleaner technologies reduced soot, tree bark and walls became lighter again.

This reversed the selective pressures.

  • Light-colored moths once again blended in better with the cleaner, paler environment.
  • Dark moths now stood out, so birds ate proportionally more of them, and the frequency of the dark form fell over time.
  • The same genetic variation (light vs. dark alleles) allowed the population to ā€œtrackā€ the new environmental change in the opposite direction.

So the significance of genetic variation after the Industrial Revolution is:

  • It allowed the population to re-adapt when conditions improved, with the light form becoming common again as pollution decreased.
  • It demonstrates that natural selection is not a one-time event; as environments keep changing, standing genetic variation enables populations to shift back and forth.

Why this example matters in biology

The London/Manchester peppered moth case is often used in textbooks because it is a clear, real-world demonstration of evolution by natural selection in just a few generations.

Key takeaways about genetic variation:

  1. Variation is essential
    Without different heritable traits (light vs. dark), selection has nothing to favor or eliminate.
  1. Environment decides whatā€™s ā€œfitā€
    The same trait (dark wings) can be harmful in a clean environment but helpful in a sooty one.
  1. Allele frequencies can shift quickly
    The dark form went from rare to dominant in many industrial regions within decades, then declined again when pollution dropped.
  1. Clear genetic basis
    Modern genetic work has identified a specific mutation associated with the dark form, supporting the link between DNA change, trait change, and natural selection.

Put simply: genetic variation gave the moth population ā€œoptions,ā€ and as Londonā€™s environment shifted from clean to polluted and back toward clean, natural selection reshuffled which option was most common.

TL;DR:
Genetic variation in Londonā€™s peppered moths meant some individuals were light and some dark. During the Industrial Revolution, dark moths were better camouflaged on soot-darkened trees, survived more, and became common. When pollution later decreased, light moths were favored again. This variation made it possible for the population to adapt to rapid environmental change and is a classic, well-documented example of evolution by natural selection.

Information gathered from public forums or data available on the internet and portrayed here.