An imaginary story

A long time ago, some animals belonging to a species of herbivore lived on the grassy plains of Africa. They ate leaves from trees. As in all species, there was genetic variation among them. Some individuals had varieties of genes that gave them long necks, and some had varieties of genes that gave them shorter necks.

In some years, the rains did not come. The animals ran short of food. When all the lower leaves had been eaten, only the long-necked animals could get food.

Most of the animals with short necks died of starvation.

When the animals reproduced, they passed on their genes to their offspring. Because only the long-necked animals had survived, it was only their genes that passed to the next generation. Most of their offspring grew up to have long necks.

This imaginary story describes how giraffes might have come to have such long necks. We do not know if this is what happened, and we can never really know. But this theory does help to explain how almost every species is so well adapted to its environment.

You can summarise the theory like this:

1. In every species, there is variation among individual organisms.

2. Some of this variation is caused by differences in their genes.

3. Some individuals have features that make it more likely that they will survive than individuals that do not have these features.

4. The individuals with these advantageous features are therefore more likely to reproduce, and pass on the genes that produce the advantageous features to their offspring.

5. Over many generations, the genes that produce these advantageous features get a little bit more common, and the genes that are not so useful get a little bit less common.

This process is called natural selection. In the imaginary giraffe example, natural selection has caused genetic changes in the giraffes over time because some varieties of genes have become more common, and some have become less common.

Bacteria and antibiotics

The giraffe story is an imaginary one. We have no evidence that any of this ever happened. But we do have very strong evidence for natural selection in other organisms.

Antibiotics are medicinal drugs that we can take to cure diseases caused by bacteria. There are many different antibiotics. But doctors are finding that some antibiotics do not work anymore. Bacteria have become resistant to them.

This is what happened.

In a population of bacteria, not every one is alike. By chance, one may have a gene that makes it resistant to an antibiotic.

Antibiotic is added, which kills the bacteria that are not resistant.

The resistant one can now multiply and form a population of resistant bacteria just like itself.

Peppered moths

Here is another example of natural selection in action.

Peppered moths live in England. Most peppered moths have pale wings, but some have a gene that gives them dark wings.

Peppered moths fly at night, and spend the daylight hours resting on tree trunks. They are hunted and eaten by birds. The pale colour of peppered moths camouflages them perfectly against lichen-covered tree bark. (Lichen is a plant-like organism that grows on rocks and trees.)

Until the year 1849, almost all peppered moths were pale. Then more and more dark ones started to appear. By 1900, almost all peppered moths near some English cities were dark ones.

What was happening? During this time, the Industrial Revolution was taking place in England. Many factories burnt coal, which polluted the air with smoke. The smoke killed lichens and made the tree trunks dark.

Scientists have done experiments that show that birds can see light moths more easily on dark tree trunks than on lichen-covered ones. And they can see dark moths more easily on lichen-covered tree trunks than on dark ones.

Now we can explain why the dark variety of the moth became more common. When the tree trunks got darker, the pale moths were not camouflaged. They were more likely than the dark ones to be eaten by birds.

The dark moths were more likely to survive and reproduce than the pale moths. They passed on their genes for their dark colour to their offspring. Over several generations of moths, more and more of the offspring inherited these advantageous genes for dark colour. Over time, the dark colour became more and more common. Each generation, more dark moths were born and fewer pale moths.

Today, pollution in England is much less. Lichens grow on tree trunks again. Today, most peppered moths are the pale variety.

Extinction

The proportions of dark and pale peppered moths changed over time, when their environment changed. But imagine what might have happened to the peppered moths if the proportions of pale and dark moths in the population had not changed, when the tree trunks became darker. If the moths were still all pale, then most of them could have been eaten by birds. The population of moths might have become so small that the peppered moths all died out. The species might have become extinct.

Changes in the environment can cause a population to become extinct, if the species cannot change over time. Scientists are worried that climate change may cause a very large number of species to become extinct.