Page 176 - Big Ideas Year 10 sciPAD 2025

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176 SCIPAD - BIG IDEAS - BOOK 2
THE BASIS OF HEREDITY
Natural Selection
All organisms are controlled by genes found in their DNA. Mutations can affect the way in which a
gene works, which in turn may cause a change in the organisms’ appearance or behaviour. The
new form of the gene may mean the organism is better suited to its environment, which means it
is better able to survive and reproduce. Those that are not well suited tend to die off. In this way
the environment ‘selects’ the best individuals. Scientists call this process ‘natural selection’.
Read the information in the green boxes and answer the questions that follow.
1. Which moth colour is the most favourable? 
2. Explain your answer to question 1. 


An Example of Natural Selection in Peppered Moths.
The Peppered Moth (Biston betularia) is common throughout England. It rests on trees covered in
a light-coloured lichen. Birds eat the moths, so those that fail to camouflage due to unfavourable
genetics will be easily spotted and eaten.
Scientists collected moths and recorded their wing colour over a long period of time. When they
first started recording the colour of moths, the population was composed mostly of light-coloured
forms that matched their environment. Both very light and dark-coloured forms were rare.
3. Which colour moths would now be the most favoured? 

4. Over the next few decades, would you expect light coloured moths to be as common as
they were before the Industrial Revolution? Justify your answer. 


During the Industrial Revolution, the countryside between London and Manchester was
blanketed with soot from newly built coal-burning factories. Many of the light-bodied lichens
died from sulfur dioxide emissions, and the trees became covered with soot.
5. Explain what would happen to the black Peppered Moth population? 



6. Which form of the Peppered Moth would you expect to be common in Britain today?


In 1956 the British Government passed the “Clean Air Act” that banned factories from burning
coal and producing lots of soot.
Number of moths
These moths are very light
coloured and have a higher
chance of being eaten. They
are less likely to survive and
pass on their genes.
These moths are too dark
and have a higher chance
of being eaten. They are less
likely to survive and pass on
their genes.
These moths are a similar colour
to the moss. They are well hidden
and are less likely to be eaten.
Colour of moths
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