The peppered moth (Biston betularia) provides a classic example of natural selection in action. Here's how it played out:
1. The Original Population:
* Before the Industrial Revolution, peppered moths were predominantly light-colored with black speckles, blending well with the lichen-covered bark of trees.
* A rare, dark-colored (melanic) variant of the moth also existed, but it was much less common.
2. The Environmental Change:
* The Industrial Revolution brought about significant air pollution, which darkened the bark of trees by killing the lichen and coating them with soot.
3. The Advantage:
* Against the dark bark, the light-colored moths became highly visible to predatory birds.
* The darker moths, however, camouflaged better against the soot-covered trees.
4. Natural Selection:
* Birds preferentially preyed on the more visible light-colored moths, leading to a decrease in their population.
* The darker moths, better camouflaged, were more likely to survive and reproduce, passing on their genes for dark coloration.
5. The Result:
* Over time, the frequency of the melanic form of the peppered moth increased dramatically in polluted areas, while the light-colored moths became less common.
* This shift in population composition clearly demonstrated how natural selection acted on the moth's coloration, favoring the variant that was best adapted to the changed environment.
6. The Comeback:
* When pollution levels decreased, the light-colored moths started to reappear, highlighting the ongoing interplay between environmental factors and evolution.
This example showcases the core principles of natural selection:
* Variation: Within a population, there are variations in traits, like moth coloration.
* Heritability: These variations are passed on from parents to offspring.
* Differential Survival and Reproduction: Individuals with advantageous traits, like the dark moths in a polluted environment, are more likely to survive and reproduce, passing on those traits.
This leads to a shift in population characteristics over generations, demonstrating how natural selection drives evolution.
