Natural Selection & Evolution (OCR Gateway GCSE Biology)

Zebras (Fig. 17.1) have evolved to live in hot grassland in Africa.

Fig. 17.1

Scientists have tried to find out why zebras have evolved stripes on their body.

One theory is that the stripes help to keep the zebra cooler than other colours. Scientists did an experiment to test this theory. They covered barrels of cold water with the skin of different animals. Then they measured the temperature of the water several hours later.

The results are shown in Fig. 17.2.

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Another theory says that the stripes make a zebra less likely to be bitten by insects.

To test this theory scientists made models of zebras and covered them with sticky tape.
One model was black. The other models had different widths of stripes.

Fig. 17.3 shows the number of insects that stuck to the tape.

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Scientists have been studying an area of Mexico that is covered with black rocks.

Most of the rocks in other areas are grey.

The black rocks were formed about 1000 years ago when a volcano erupted.

They found that most of the mice that lived on this rock were black in colour.

Hedgehogs are covered in small spines.

When they are frightened they often roll up into a ball and keep still.

Blue tits are small birds seen in gardens.

Many people in the UK feed birds nuts using a bird feeder, as shown in the diagram below.

Scientists measured the mean lengths of blue tits’ beaks in the UK.

They found that blue tit beaks in the UK are slightly longer than blue tit beaks in the rest of Europe.

Very few people in the rest of Europe put bird feeders out for their birds.

Use Charles Darwin’s theory of natural selection to explain why the blue tits in the UK have slightly longer beaks than those in the rest of Europe.