Variation & Evolution (AQA GCSE Biology)

Our species, Homo sapiens, like all species, shows variation.

Explain what is meant by the term ‘variation’.

DNA replication is an essential part of the cell cycle in cells preparing to divide by mitosis.

Sometimes DNA is not replicated correctly.

Suggest how mistakes during DNA replication could impact variation within a species.

When a cell replicates its DNA during the cell cycle, its DNA replicates itself. Under normal conditions around 150 copying errors are made for each cell division.

There are 3 billion (3 000 000 000 or 3 × 10⁹) base pairs of DNA in every human cell. 

Calculate the percentage error rate for each human cell division.

Give your answer in standard form OR with regular numbers. 

The Bering Strait is a narrow passage of the Pacific Ocean that separates Asia from North America.

At its narrowest, the strait is 82 km across.

It is generally accepted that when sea levels were lower in the past, a ‘land-bridge’ in the strait was exposed, allowing animals to migrate from Asia to North America, and vice versa. When sea levels rose, the land-bridge disappeared.

Explain how the emergence and disappearance of a land-bridge could have fuelled speciation.

Even though nylon wasn't invented until the 1940s, bacteria were soon isolated that could degrade nylon.

The bacterium Paenarthrobacter ureafaciens has special enzymes that allow it to metabolise nylon and use it as a sole energy source.

This is a very simple example of natural selection.

Describe how organisms can evolve through the process of natural selection.

Humans have been selectively breeding animals and plants for many thousands of years.

For example, dogs can be selectively bred so that they produce fur which does not cause allergies in people.

Suggest two other reasons why dogs might be selectively bred.

Dalmatians are a popular breed of dog but due to extensive inbreeding, they are prone to genetic defects.

All pure-bred Dalmatians produce higher-than-normal levels of uric acid due to a gene mutation.

This can lead to the formation of kidney and bladder stones which have to be surgically removed.

Another breed of dog called Pointer dogs do not possess this faulty gene and have normal levels of uric acid.

Cross-breed Dalmatians, which are about 99% Dalmatian and 1% pointer, look and behave like Dalmatians but crucially do not have the gene mutation which leads to high levels of uric acid. 

Explain how selective breeding was used to produce cross-breed Dalmatians that do not suffer from kidney and bladder stones.

The gene mutation in the pure-breed Dalmatian changes one amino acid in a protein involved in regulating levels of uric acid.

Explain how a change in one amino acid in the protein could stop it from functioning normally.

A manufacturer wants to develop an extremely strong thread which can be used in their products.

They would like to produce a thread which has the same same properties as spider’s silk, a protein fibre made by spiders which has a high tensile strength.

Explain in basic terms how scientists can use bacteria to produce the spider silk protein.

The hormone thyroxine is produced by the thyroid gland and plays an important role in regulating the body’s metabolism, as well as controlling growth and development.

Hypothyroidism is a medical condition where an individual does not produce enough thyroxine.

The gene for thyroxine can be isolated from a skin cell. Explain why the gene does not have to be taken from cells from the thyroid gland.

Suggest why bacteria are suitable organisms to use in genetic engineering.

Higher Tier Only

Explain the role of enzymes and vectors in genetic engineering.

Selective breeding is particularly useful to farmers who want to produce animals with particular characteristics.

Cow X is a cow in a farmer’s herd that has a higher muscle density than normal when fed the same diet.

The farmer wants to produce more cows like Cow X.

Suggest why the farmer wants to do this.

The farmer wants to breed Cow X with a bull (male) that has produced larger calves than normal.

The farmer hopes to have multiple genetically identical offspring.

Describe how this could be achieved.

Potentially, scientists could use cloning techniques to increase the populations of endangered animals and prevent them from becoming extinct.

Evaluate the use of cloning versus conservation to prevent endangered animals from becoming extinct.

Figure 1 shows two species of snakes, the coral snake and scarlet kingsnake. 

Figure 1

The two snake species share similarities in their appearance with stripes of red, black and yellow, although they are not closely related.

How could scientists determine whether the two snakes were the same species or not?

The coral snake is venomous and can kills its potential predators with a bite, while the scarlet kingsnake is not venomous.

The scarlet kingsnake mimics the colours of the Eastern coral snake. 

Describe how the scarlet kingsnake would have evolved to mimic the appearance of the Eastern coral snake. 

A group of scientists carried out the following experiment:

They created plasticine models of three species of snake. The snake species were:

• The Eastern coral snake

  • The scarlet kingsnake

  • The Eastern indigo snake, which is a dark blue non-venomous species of snake native to the same region as the other two in South-Eastern USA.

All three species can experience predation from birds of prey in the area, which attack by swooping down and pecking at the snakes' bodies. 

The scientists placed 20 models of each species in the natural 'native' habitat of all three species together, and the same number of models in a different area where the Eastern coral snakes do not normally live (their 'non-native' habitat) but the birds of prey that hunt snakes do exist.

They left the models for 24 hours and went back and counted the number of bird pecks on the snake models, which they could determine by the imprints in the soft plasticine. The number of pecks represent the number of predation attempts on the snakes. They added up the number of pecks in each of the 20 models for each species and calculated an average number of pecks per snake model. 

The results are shown in Table 1.

Table 1

The scientists concluded:

"Mimicry is large advantage for non-venomous snake species"

Evaluate this conclusion, using evidence from Table 1. 

The same scientists wanted to investigate whether mimicry or camouflage was a more effective strategy for preventing predation in non-venomous snakes. 

Suggest how the scientists could alter the experiment to investigate this question. 

Cepaea nemoralis is a species of snail that shows a large amount of variation in their shell colours and patterns, as shown in Figure 2. 

Figure 2

Explain why it is important that the shells of the snail are so varied.

Some students wanted to plan an investigation into the effectiveness of each shell colour to camouflage the snails from predators. 

This is the method they used:

• They collected 40 shells from dead Cepaea nemoralis snails, 20 brown shells and 20 yellow shells

  • They went to three different locations to place the shells: a sand dune, a forest floor, and a grass meadow

  • One student scattered the shells around in a 2m² area so that both colours were evenly distributed around the area

  • The other student was then given 10 seconds to collect as many of the shells as possible

  • At the end they counted how many of each shell colour the second student had found in the time

They recorded their results in Table 2.

Table 2

What is the percentage of yellow shells that were collected in total across all three areas? 

What conclusion can be made from the students findings?

Use evidence from Table 2 to support your conclusion. 

The conclusion from this study may not be valid. 

Give two reasons why the conclusion you made may not be valid. 

There are a number of wild species of bananas that contain large seeds in the fruits, as shown in Figure 3.

Figure 3

The varieties of bananas that we consume do not have large seeds, which improves the experience of eating them.

Banana plants cannot reproduce asexually. 

Seeds play an important role in sexual reproduction in plants and therefore it is not possible for any species to naturally evolve to no longer produce seeds. 

Explain how it is possible for humans to develop varieties of bananas without seeds.

The bananas without seeds cannot reproduce sexually. 

Banana farmers on plantations have to produce a large number of plants. 

Describe two methods the farmer could use to grow a large number of banana plants without the use of seeds. 

There is currently an economically concerning disease called Black Sigtoka that affects banana plants. 

It is a fungal pathogen that causes black streaks to appear on the leaves of the banana plants. 

Explain why this reduces the yield of the banana plants. 

One of the reasons why the spread of Black Sigatoka is so concerning is because a huge number of banana plants are susceptible to the pathogen. 

Using your answer to part (b), suggest why so many banana plants are susceptible to Black Sigatoka. 

One suggested solution to this issue is to breed new banana varieties from wild banana plants that are resistant to the pathogen. 

An alternative solution is to produce genetically engineered banana plants that contain DNA that makes them resistant to the pathogen. 

Compare both solutions to the issue. 

Higher Tier Only

Hepatitis B is a viral infection that can be deadly in some cases. 

There is a vaccine to prevent infection with Hepatitis B that is produced from genetically modified (GM) yeast cells.

The process of producing these GM yeast cells is outlined in Figure 4.

Figure 4

Why was the gene for producing the Hepatitis B antigen selected to be used to produce the vaccine? 

Higher Tier Only

Figure 4 shows that the virus DNA and the bacteria plasmid were both cut using the same enzyme, labelled Enzyme 1. 

Enzyme 1 cuts the DNA so that there is a sequence of exposed bases on the DNA strand. These exposed bases are called 'sticky ends' and are free to bind to another DNA strand with the correct complementary bases exposed. 

Figure 5 shows how this works. 

Figure 5

There are lots of different types of enzymes that cut DNA, and they all cut the DNA at different base sequences. 

Using Figure 5, suggest why both pieces of DNA in Figure 4 should be cut using the same enzyme.

Higher Tier Only

The yeast takes the genetically modified DNA in the form of the plasmid and can read the DNA to produce the virus protein. 

Why is it possible for the yeast to be able to produce a virus protein by reading the virus DNA? 

Higher Tier Only

Once the yeast has been genetically modified it is placed in a machine called a fermenter. 

What is the purpose of the fermenter in this process?

The Lady's slipper orchid (Cypripedium calceolus) is extremely rare in the UK.  

Conservationists are keen to reintroduce the species into the wild and are using a variety of breeders from other countries to provide plants. 

Another method to help with the reintroduction is to produce clones of the orchid to be planted in the wild.

Figure 6 shows the method used to produce these clones. 

Figure 6

Give a reason for each of the following steps shown in Figure 6.

• Several groups of cells are scraped off the leaf

  • Nutrients are added to the agar jelly

  • Hormones are added to the agar jelly

  • The plant cells are kept in sterile conditions

  • The plant cells are kept at 20 °C

Some people argue that using this method to clone plants is a good method for growing orchids to use as house plants, but not for reintroduction to the wild.

Suggest why cloning plants in this way may not be a suitable method for producing plants to be reintroduced into the wild.

Suggest two advantages of trying to preserve endangered species such as the Lady's slipper orchid.