Variation and evolution by natural selection

The phenotype of an organism can be affected by its genes, by its environment, or by both.

(a) Complete the table. Tick (✓) one box in each row. [2]

(b) Identical twins have exactly the same genes.

Two adult identical twins have different body masses.

Explain why. [2]

Mutations are a source of variation in a species.

(a) What is a mutation? [1]

(b) Which statement about mutations is correct? Tick (✓) one box. [1]

☐ Mutations only happen when an organism is exposed to radiation.
☐ Most mutations have no effect on the phenotype.
☐ Most mutations completely change the phenotype.
☐ Mutations only happen in old organisms.

(c) Very rarely, a mutation produces a new phenotype.

Under what circumstances could this new phenotype lead to a relatively rapid change in a species? [1]

A student investigated variation in the length of ivy leaves.

She picked 10 leaves from the shaded side of one ivy plant and 10 leaves from the sunny side of the same plant. She measured the length of each leaf.

(a) Calculate the mean length of the leaves from the shaded side. [2]

(b) Give the range of the lengths of the leaves from the sunny side. [1]

(c) All the leaves came from the same plant, so they have the same genes.

What does this tell you about the cause of the difference in leaf length between the two sides? [1]

(d) Suggest the environmental factor most likely to have caused the difference in leaf length. [1]

(e) Why did the student measure 10 leaves from each side rather than just 1 leaf? [1]

Rock pocket mice live in deserts. Most live on pale sandy rock, and most have pale fur.

In one area there is dark rock from old lava flows. Most of the mice living on the dark rock have dark fur. Owls hunt the mice by sight.

The statements describe how the population of mice on the dark rock changed.

A Mice with dark fur are better camouflaged on dark rock, so fewer are eaten by owls.
B A mutation produces an allele for dark fur in some mice.
C Over many generations, most mice in the population have dark fur.
D Dark mice survive to breed and pass on the allele for dark fur to their offspring.

(a) Write the letters A, B, C and D in the correct order. [2]

(b) Explain why this change in the mouse population is an example of evolution. [1]

(c) A student wrote:

“The mice turned dark so that they could hide from the owls.”

Explain why the student is wrong. [2]

Sticklebacks are small fish. Most sticklebacks living in the sea have many bony armour plates. A rare allele causes fish to have very few plates (low armour).

A group of sea sticklebacks entered a freshwater lake. In the lake, fish with low armour grow faster and reach breeding size sooner.

Scientists recorded the percentage of low-armour fish in the lake every 2 years. The graph shows the results.

(a) Describe how the percentage of low-armour fish changed over the 12 years. [2]

(b) Calculate the mean rate of increase in the percentage of low-armour fish between year 4 and year 8.

Give your answer in percentage points per year. [2]

(c) After 12 years, low-armour fish and full-armour fish in the lake still breed together. Their offspring are fertile.

Explain why scientists do not consider the low-armour fish to be a new species. [2]

(a) Complete the sentence. Choose answers from the box. [2]

Evolution is a change in the ____________ characteristics of a population over time through a process called natural ____________.

(b) All species of living things have evolved from simple life forms.

When did these simple life forms first develop? Tick (✓) one box. [1]

☐ About 3.5 thousand years ago
☐ About 3.5 million years ago
☐ About 3.5 billion years ago
☐ About 3.5 trillion years ago

(c) Natural selection can only act on variation between individuals.

What is the original source of all new genetic variation? [1]

Warfarin is a poison used to kill rats. It was first used widely in the 1950s.

A few years later, farmers found rats that were not killed by warfarin. These rats have an allele for warfarin resistance, which arose by mutation.

Resistant rats need much more vitamin K in their diet than non-resistant rats. Rats that do not get enough vitamin K are less healthy.

(a) The resistance allele was very rare before warfarin was used.

Suggest why. [2]

(b) Explain why the percentage of resistant rats increased rapidly on farms where warfarin was used. [3]

(c) Predict what would happen to the percentage of resistant rats on a farm if warfarin stopped being used. Give a reason for your answer. [1]

Ground finches are birds that live on a small island. They eat seeds. Beak depth in these finches is mainly controlled by genes.

One year there was a severe drought. Plants that produce small, soft seeds died. Plants that produce large, hard seeds survived. Only finches with deep beaks can crack large, hard seeds.

The table shows information about the finch population.

Explain how natural selection caused the change in mean beak depth.

Use information from the table in your answer. [6]

Peppered moths can be pale or dark. They rest on tree bark in the day. Birds eat the moths.

Scientists released 250 pale moths and 250 dark moths into a wood where the tree bark had been blackened by soot. The next day they caught as many of the moths as they could.

(a) Calculate the percentage of the released pale moths that were recaptured and the percentage of the released dark moths that were recaptured. [2]

(b) Explain how these results support the theory of evolution by natural selection. [2]

(c) Suggest one reason why not all of the dark moths were recaptured. [1]

(a) Give the term used for the differences in characteristics between individuals of the same species. [1]

(b) Most species show extensive genetic variation. Cheetahs are unusual because almost all cheetahs have very similar alleles.

Scientists are worried that a new disease could kill all of the cheetahs in the world, but would be unlikely to kill all the individuals of a species that has extensive genetic variation.

Explain why. [3]