Page 56 - Science Coursebook

3.7 Natural selection in action

The examples of how giraffes might have got their long necks, and how the coats of

mountain hares might change, are just theories. We don’t have any evidence that this
is actually what happens.

However, scientists have now found many examples of natural selection happening.
Here are two of them.

Bacteria and antibiotics
In Stage 7, you learnt that antibiotics are drugs that we can take to kill bacteria that
are causing disease in our bodies.

There are many different antibiotics. But doctors are finding that many antibiotics do

not work any more. Bacteria have become resistant to them.
This is what has happened.

In a population of bacteria, not every one Antibiotic is added, which The resistant one can now
is alike. By chance, one may have a gene kills the bacteria that are multiply and form a population of
that makes it resistant to an antibiotic. not resistant. resistant bacteria just like itself.

Questions
1 Explain what is meant when we say that bacteria have become ‘resistant’ to
an antibiotic.
A+I 2 Scientists say that we should try not to use antibiotics for mild illnesses.
We should save them for when people have serious illnesses. Suggest an
explanation for this advice.

Peppered moths
Peppered moths live in England. Most peppered
moths have pale wings, but some have dark wings.
Peppered moths rest on tree trunks. They are hunted
and eaten by birds. The pale colour of peppered

moths camouflages them perfectly against tree trunks
that have lichens growing on them. (Lichen is an
organism that grows on rocks and trees, especially
where the air is unpolluted.)
The dark and pale varieties of
the peppered moth.

54 3 Variation and inheritance

A+E SE