Variation

Like the parents, but not the same

Look at a family photo. The children have the same basic design as their parents: two eyes, a nose, ten fingers. Yet each child is a little different. One has her father's curly hair, another his mother's dimples, and no two children are exactly alike.

Two things pass from one generation to the next:

• a common basic design, so a child of humans is a human;
• small changes in that design, so each child is a little different.

The passing on of features from parents to young is called heredity. The features themselves, such as the shape of the earlobe or the colour of a flower, are called traits. The differences between the members of a species are called variations.

Variation in asexual reproduction

When a cell reproduces, it copies its DNA. The copying is very accurate, but never perfect. So even in asexual reproduction, a daughter may differ from its parent by a small change.

Think of one bacterium that divides into two. Each of those divides again, and so on. The four cells of the second generation are very similar. Only the few changes made during copying tell them apart.

But each daughter keeps every change of its parent, and may add a new one of its own. Over many generations the changes build up, and the family of cells grows more varied.

Variation in sexual reproduction

In sexual reproduction, each young one gets DNA from two parents, mixed in a new way. Brothers and sisters get different mixes, so they differ from each other as well as from their parents.

So sexual reproduction makes far more variation than asexual reproduction. Copying changes are added on top of the mixing.

Why variation matters

Not every variation has the same chance of lasting. Picture bacteria in a pond. A few carry a variation that lets them stand heat. In a normal year this makes no difference.

Then a hot summer heats the pond. Most ordinary bacteria die, but the heat-resistant ones survive. They now make up much of the population, and they pass the variation on to their young.

So variation can help a species to survive when its surroundings change. A variation that helps is more likely to be passed on, because its owners survive to reproduce.

Set the share of resistant bacteria and the strength of the heat below. Watch how the survivors change.

Inherited traits you can see

Look at the ears of your classmates. In some, the lower part, the earlobe, hangs free. In others it is joined to the side of the head. This is an inherited trait: the form of the earlobe is passed on from parents.

Other inherited traits include the colour of the eyes and the shape of the hairline. A scar or a tan is not inherited, because it does not change the DNA.

Try it in Python

The program follows bacteria that divide every generation. Now and then a copy picks up a new small change, and every daughter keeps the changes of its parent. It counts how many different kinds of cells there are.

Practise with new numbers

Common mistakes

• "Asexual reproduction makes exact copies." DNA copying is never perfect, so small variations still appear.
• "Brothers and sisters get the same DNA." Each gets a different mix from the two parents.
• "A variation is made because the organism needs it." Variations arise by chance. Some just happen to help later.
• "Every feature of a person is inherited." A scar, a tan or big muscles from exercise are not passed on to the young.

Remember these

• Heredity passes on a common basic design and small changes in it.
• DNA copying is never perfect, so asexual reproduction makes a little variation.
• Sexual reproduction mixes the DNA of two parents and makes far more variation.
• Variations build up over generations. One that helps survival is more likely to be passed on.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Two cuttings are taken from one rose bush. Will the new bushes be exactly alike? Why?

2. Why do two sisters differ more from each other than two daughter cells of an Amoeba?

3. A child has free earlobes, like her father. Is this an inherited trait or one she has learnt?

4. A man builds strong arm muscles by lifting weights. Will his children be born with strong arms? Why?

5. In a hot spell, why might bacteria with a heat-resistant variation soon make up much of a pond's population?