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  • Writer's pictureWilliam Malpass

The Peppered Moth

"Which brings me to Biston betularia, the peppered moth, a real celebrity among Order Lepidoptera, because its story provides one of the most easily observable and understood historical examples of natural selection." Stuart Gunderson, PhD, looks out at us from behind his lectern. "Who among you knows about the famous peppered moth? Hmm?" An awkward pause as a few of us halfheartedly raise our hands. "Ah, Mr. Matheson! Delightfully good news to see you've been keeping up with your reading for a change." Several relieved snickers from my classmates in Honors Biology 102. "Won't you explain for us the significance of the peppered moth?"

I cringe inside and begin. "Um, the peppered moth is kind of a classic example of natural selection. There are two phenotypes, a light-colored variety and a darker one. In pre-Industrial Revolution England, the light-colored variety was way more widespread, because its coloration helped it blend in against the light-colored bark of trees. But then as the Industrial Revolution really got underway, and the soot from factory chimneys started coating the trees in urban areas especially, the darker moths began to be more common."

"And why was the peppered moth's ability to camouflage itself against the bark so critical?" Dr. Gunderson asks.

The light and dark variants of the peppered moth.

Light and dark peppered moths use camouflage to evade hungry birds. Which moth in this image seems to understand the principle of natural selection better?

I clear my throat nervously. "Well, their camouflage helped them not get eaten by birds, right? When the tree trunks were covered with dark soot, light-colored moths stood out more and birds could find and catch them more easily. But the dark moths had become better camouflaged."

"So far so good, Andrew, but how does the story of the peppered moth end?" Dr. Gunderson calls me Andrew, which I think is a sign he's pleased.

"Um, England eventually changed the laws about how much pollution factories could produce, and the trees started to go back to their natural coloration. And the light-colored moths made a comeback, while the dark moths began to be less common. Birds could see the dark moths better under those conditions."

"Exactly right, and well done, Andrew." Dr. Gunderson smiles thinly. "The peppered moth is the quintessential example of observable natural selection." He pauses dramatically. "And a word of warning to those of you who aren't doing your reading—natural selection is coming for you on next week's midterm!"


(Image courtesy of Martinowksy, CC BY-SA 3.0, https://commons.wikimedia. org/w/index.php?curid=1855638)

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