nature animals · Ages 9-12

The Moth That Turned Pollution Into Camouflage

Peppered moth populations shifted toward darker coloring in soot-darkened places, showing natural selection acting on inherited variation as environments changed.

The whole story

1. At first light, Pip clung to a pale, lichen-covered branch above a soot-stained town. Her wings matched the lichen so closely that only her tiny feet showed against the bark.

2. Pip wanted to stay hidden until nightfall, when she would search for a mate and a place to lay eggs. She folded her wings and held still as factory smoke drifted over the rooftops.

3. Year by year, soot settled on the oak. The bark grew darker, the lichen thinned, and Pip's pale wings became an easy shape for a bird to notice.

4. Yet peppered moths were never all colored alike. Some inherited pale wings like Pip's, while others inherited darker wings like Ash's; the difference was present before the smoke changed the trees.

5. Ash rested on a blackened patch of bark, where charcoal wings broke up his outline. Pip rested on a nearby pale patch, but the oak offered fewer of those hiding places each season.

6. A small robin searched the trunk. Pip stayed pressed to the bark, but her pale outline stood out; Ash was almost impossible to pick out. Moths that birds missed were more likely to survive and leave eggs.

7. Pip could not choose darker wings, and Ash had not changed his own. Their wing colors came from inherited differences. When survivors left eggs, those differences could pass to the next generation.

8. As the factories darkened the trees, darker-winged moths became more common in that neighborhood. Pale moths did not vanish everywhere; on clean, lichen-covered bark, their wings still gave them cover.

9. Peppered moth populations shifted toward darker coloring in soot-darkened places, showing natural selection acting on inherited variation as environments changed.

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