Page 57 - Science Coursebook

3.7 Natural selection in action

Until the year 1849, almost all peppered moths
were pale. Then more and more dark ones began to
appear. By 1900, almost all of the peppered moths
near some English cities were dark ones.
During this time, industry was developing rapidly in
England. Many factories burnt coal, which produced
smoke. The smoke polluted the air. It killed lichens,
and made tree trunks dark.
Scientists have done experiments which show that Can you spot the moth?
birds can see light moths more easily than dark moths,
when they are resting on dark tree trunks.

Now we can explain why the dark variety of the
moths became more common. As the tree trunks got

darker, the pale moths were not well camouflaged.
Many of them were killed and eaten by birds.
But the birds could not see the dark moths. The dark
moths were more likely to survive than the light ones.
The dark moths reproduced, and passed on the gene

for their dark colour to their offspring. This illustration was drawn in the middle
of the nineteenth century. It shows steel
Each generation, more dark moths were born and factories in the town of Sheffield, England.

fewer pale moths.
Today, pollution has greatly decreased in England.
Factories are not allowed to pollute the air. Tree
trunks are covered by pale-coloured lichens again.
Almost all peppered moths found today are pale.

Dark and pale varieties of the
peppered moth on a dark tree trunk.

Question

A+I 3 Use the ideas of natural selection to explain why most peppered moths in
England today are pale, not dark.

Summary
• When antibiotics are used, any bacteria that have a gene that
makes them resistant to the antibiotic survive and reproduce. Natural
selection causes populations of resistant bacteria to be produced.
• Natural selection caused dark varieties of the peppered moth to
become more common in England during the nineteenth century.

3 Variation and inheritance 55