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Cambridge IGCSE0610

Variation

Biology 0610 Chapter Notes

What this chapter covers

VariationAdaptive featuresSelection
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In this chapter

  1. 1What variation is
  2. 2Continuous variation
  3. 3Discontinuous variation
  4. 4Genes and the environment
  5. 5Describing examples from data
  6. 6Mutation
  7. 7Gene mutation
  8. 8Sources of genetic variation
  9. 9What an adaptive feature is
  10. 10Reading a species from an image
  11. 11Hydrophytes
  12. 12Xerophytes
  13. 13Natural selection
  14. 14Selective breeding
  15. 15Improving crops and animals over many generations
  16. 16Adaptation
  17. 17Antibiotic resistance in bacteria
  18. 18Natural and artificial selection compared

1. What variation is

Variation is the differences between individuals of the same species.

The individuals compared must belong to the same species.

Worked example 12 marks

All the cattle in a herd belong to one species. Some of the cattle are heavier than others.

State the term for differences like these and describe what the term means.

  1. 1

    the term is variation [1]

  2. 2

    it means differences between individuals of the same species [1]

2. Continuous variation

Continuous variation results in a range of phenotypes between two extremes. Examples include body length and body mass.

The phenotypes between the two extremes are intermediates. Height in humans is another example.

Examiner insight

“Few candidates understood that continuous variation is variation that results in a range of phenotypes between two extremes.” “Although many candidates appreciated the definition of continuous variation, some believed that continuous variation results in a range of phenotypes with no intermediates.”

Worked example 11 mark

What is continuous variation?

A a limited number of phenotypes with no intermediates

B a limited number of phenotypes between two extremes

C a range of phenotypes with no intermediates

D a range of phenotypes between two extremes

  1. 1

    a limited number and no intermediates describe discontinuous variation

  2. 2

    D: a range of phenotypes between two extremes [1]

3. Discontinuous variation

Discontinuous variation results in a limited number of phenotypes with no intermediates. Examples include ABO blood groups, seed shape in peas and seed colour in peas.

There are four blood groups in the ABO system: A, B, AB and O. No person is in between two of them.

Examiner insight

“Many candidates did not read the question carefully and gave examples of continuous variation instead of discontinuous variation.” “A number of candidates incorrectly believed that seed colour in peas is an example of continuous variation.”

Worked example 11 mark

Four characteristics are listed.

ABO blood group in humans

body mass in humans

seed shape in peas

seed colour in peas

How many of these characteristics show continuous variation?

A one

B two

C three

D four

  1. 1

    ABO blood group, seed shape and seed colour show discontinuous variation

  2. 2

    only body mass shows continuous variation, so A: one [1]

4. Genes and the environment

Discontinuous variation is usually caused by genes only.

Continuous variation is caused by both genes and the environment. Height shows continuous variation, so the environment affects it as well as genes.

Examiner insight

“The shape of seeds is an example of discontinuous variation and so is caused by genes only.” “Many candidates could correctly describe the variation in height and the reasons for this variation, some did not think that the environment affects height.”

Worked example 11 mark

Which statement applies to the shape of pea seeds?

A The seed shape is caused by the environment only.

B The seed shape is caused by genes only.

C The seed shape phenotype has intermediates.

D The seed shape shows continuous variation.

  1. 1

    seed shape in peas shows discontinuous variation, so C and D are wrong

  2. 2

    B: discontinuous variation is caused by genes only [1]

5. Describing examples from data

Data for discontinuous variation are in discrete categories with no intermediate values. A bar chart is the way to present them.

Data for continuous variation show a range with intermediates. A histogram of body length describes continuous variation.

To name the type of variation from a graph, look for a range and for intermediates.

Examiner insight

“Many candidates did not appreciate that the bar chart does not contain a range of characteristics or any intermediates and therefore shows discontinuous variation.” “A bar graph or bar chart is the appropriate way to present data about discontinuous variation. Many candidates thought a line graph was the correct response.”

Worked example 11 mark

The graph shows the percentages of the global population with the four different blood groups in the ABO system.

What can be concluded from the graph?

A ABO blood groups show the same variation as body mass.

B ABO blood groups are an example of discontinuous variation.

C The variation in ABO blood groups results in a range of phenotypes between two extremes.

D Variation in ABO blood groups is caused by genes and the environment.

  1. 1

    the graph has four separate categories, with no range and no intermediates

  2. 2

    B: ABO blood groups are an example of discontinuous variation [1]

6. Mutation

Mutation is genetic change. It is the way in which new alleles are formed.

Ionising radiation and some chemicals increase the rate of mutation.

Examiner insight

“Many candidates gave radiation as an answer but failed to say that it was ionising radiation.” “Most candidates knew that radiation could cause mutations, but fewer identified chemicals or mutagens.”

Worked example 13 marks

(a) Describe mutation.

(b) State two factors that increase the rate of mutation.

  1. 1

    (a) mutation is genetic change [1]

  2. 2

    (b) ionising radiation [1]

  3. 3

    some chemicals [1]

7. Gene mutation

Gene mutation is a random change in the base sequence of DNA.

Writing only that the DNA was changed is not enough.

Examiner insight

“Some candidates wrote that a gene mutation is ‘a change in the genetic code’, which is not correct.” “Some responses just stated that the ‘DNA was changed’ which was insufficient.”

Worked example 12 marks

Describe gene mutation.

  1. 1

    a random change [1]

  2. 2

    in the base sequence of DNA [1]

8. Sources of genetic variation

Mutation, meiosis, random mating and random fertilisation are sources of genetic variation in populations.

Mutation is the one that forms new alleles. Sexual reproduction leads to different combinations of alleles, not new ones.

Sexual reproduction and natural selection are not accepted as sources.

Examiner insight

“Some candidates did not read the question and incorrectly referred to causes of mutation, such as radiation.” “Many responses referred to sexual reproduction or natural selection as being responsible for new alleles; these processes lead to different combinations of alleles rather than the generation of new ones.”

Worked example 11 mark

Which row shows the factors that contribute to variation in a population of animals?

Key: yes = contributes towards variation, no = does not contribute towards variation

gamete formationrandom change in DNA base sequencerandom fertilisationrandom mating
Ayesyesyesyes
Byesnoyesyes
Cnoyesyesno
Dnoyesnoyes
  1. 1

    gametes are formed by meiosis

  2. 2

    a random change in the DNA base sequence is gene mutation

  3. 3

    all four are sources of variation, so A [1]

Recap

  • Variation = differences between individuals of the same species.
  • Continuous: a range of phenotypes between two extremes (body length, body mass).
  • Discontinuous: a limited number of phenotypes with no intermediates (ABO blood groups, seed shape and seed colour in peas).
  • Discontinuous is usually caused by genes only. Continuous is caused by both genes and the environment.
  • Discrete categories on a bar chart: discontinuous. A range with intermediates: continuous.
  • Mutation = genetic change. It is the way new alleles are formed. Ionising radiation and some chemicals increase its rate.
  • Extended: gene mutation = random change in the base sequence of DNA.
  • Extended: sources of genetic variation are mutation, meiosis, random mating and random fertilisation.

9. What an adaptive feature is

An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment.

Inherited means the feature is genetically determined. It is not caused by the environment.

Both survive and reproduce are needed. The feature helps in the organism's own environment, not in all environments.

Examiner insight

“The most well-known term was survive, but many of these candidates did not recall the idea that they must also be able to reproduce and hence did not gain credit.” “A minority of candidates thought that longer filaments would attract more insects to this flower. This was a wind-pollinated flower so longer filaments to attract more insects would not be an advantage.”

Worked example 11 mark

Different organisms have different adaptive features. For example, the anthers of wind-pollinated flowering plants have long filaments. What is true of this adaptive feature?

A Filament length is not affected by genes · B It gives the plant an advantage in all environments · C Longer filaments attract more insects · D It makes the plant more likely to reproduce

  1. 1

    D: the adaptive feature makes the plant more likely to reproduce. [1]

10. Reading a species from an image

A question may give a photograph or a description of an organism. Name a feature that can be seen in it. Then link the feature to how it helps the organism survive or reproduce.

A feature that cannot be seen in the image does not answer the question. Scent and nectaries are not visible in a photograph.

In a photograph of an insect-pollinated flower, look for large petals, and the anthers and stigma inside the flower. In a wind-pollinated flower, look for anthers hanging outside the flower and feathery stigmas.

Examiner insight

“The most frequent error was to select features that were not visible in the diagrams.” “Many were unable to supply a second feature that was visible in the photograph.”

Worked example 12 marks

A photograph of a scented flower has large petals, with the anthers and stigma inside the flower. State two features, visible in the photograph, that adapt the flower for insect pollination.

  1. 1

    large petals [1]

  2. 2

    anthers and stigma inside the flower [1]

11. Hydrophytes

A hydrophyte is a plant adapted to live in water. 'Aquatic plant' is not the term.

A floating leaf has large air spaces in the spongy mesophyll. They reduce density and increase buoyancy, so the leaf floats. The air spaces also let gases diffuse.

A hydrophyte has no waxy cuticle, or a very thin one, because it lives in water and does not need to prevent water loss. A thin cuticle lets more light into the palisade mesophyll. Stomata are on the upper epidermis of a floating leaf, for gas exchange with the air. A submerged plant has no stomata: gases diffuse between the water and the leaf cells.

Examiner insight

“Some candidates incorrectly believed that in hydrophytes stomata are sunk in pits in the epidermis.” “In addition, some candidates repeated the information in the stem of the question and referred to the leaf floating rather than referring to a reduction in density or an increase in buoyancy.”

Worked example 11 mark

Which feature is found in a hydrophytic plant?

A Leaves are curled with stomata on the inside · B Leaves are reduced to spines · C Stomata are sunk in pits in the epidermis · D There is no waxy cuticle on the leaves

  1. 1

    D: a hydrophyte has no waxy cuticle on the leaves. [1]

12. Xerophytes

A xerophyte is a plant adapted to a dry environment. The environment is not always hot or tropical.

The leaf features reduce water loss by transpiration. A thick cuticle and hairs are typical. Some xerophytes have leaves that are rolled up and covered with hairs, and hairs may surround the stomata.

Do not give hydrophyte features. A rolled leaf does not make a large air space.

Examiner insight

“A common error was to give adaptations of hydrophytes instead of xerophytes.” “Some candidates incorrectly believed that having a thin waxy cuticle is an adaptation for living in a hot, dry desert.”

Worked example 12 marks

A photomicrograph of a marram grass leaf has a rolled leaf, hairs on its inner surface, a thick cuticle and vascular bundles. State two features that are evidence that marram grass is a xerophyte.

  1. 1

    a thick cuticle [1]

  2. 2

    hairs on the leaf (or the leaf is rolled up) [1]

Recap

  • An adaptive feature is an inherited feature that helps an organism to survive and reproduce in its environment.
  • From an image, give a feature you can see and link it to survival or reproduction. Scent and nectaries cannot be seen.
  • Extended: a hydrophyte lives in water. Large air spaces reduce density and increase buoyancy. It has no waxy cuticle, or a very thin one.
  • Extended: a submerged hydrophyte has no stomata. Gases diffuse between the water and its leaf cells.
  • Extended: a xerophyte lives in a dry environment, not always a hot one. A thick cuticle, hairs and rolled leaves reduce water loss by transpiration.

13. Natural selection

Natural selection acts on a population in which there is genetic variation. Organisms produce many offspring, but resources are limited. So there is a struggle for survival, including competition for resources.

Individuals that are better adapted to the environment have a greater chance of reproduction than others. These individuals pass on their alleles to the next generation.

The order is: variation, the struggle for survival, survival and reproduction of the better adapted, then alleles passed on.

Examiner insight

“The sequence starts with mutation giving rise to variation in the population. The struggle for survival means that only better adapted organisms survive to pass on their alleles.” “The most well-known term was survive, but many of these candidates did not recall the idea that they must also be able to reproduce and hence did not gain credit.”

Worked example 11 mark

Some of the stages involved in natural selection are listed.

1 The better adapted organisms survive to reproduce.

2 The surviving organisms pass on their alleles to their offspring.

3 There is a struggle for organisms to survive.

4 There is genetic variation within organisms in a population.

What is the order of the stages in natural selection?

A 2, 1, 3, 4 · B 1, 2, 4, 3 · C 4, 3, 1, 2 · D 4, 3, 2, 1

  1. 1

    C: variation (4), struggle to survive (3), better adapted survive to reproduce (1), alleles passed on (2). [1]

14. Selective breeding

Selective breeding is the selection by humans of individuals with desirable features.

These individuals are crossed to produce the next generation. The offspring showing the desirable features are then selected.

Humans choose individuals by their features, not by their genes or alleles. The choice is made by humans, not by a struggle for survival.

Examiner insight

“Some candidates described choosing genes or alleles rather than choosing phenotypic features.” “While many candidates appreciated that selective breeding involves parents that possess desirable features, some candidates incorrectly believed that it involves a struggle for survival.”

Worked example 11 mark

Farmers have bred Holstein-Friesian cattle to produce more milk than older breeds of cattle.

Which process was used to produce these cattle?

A adaptation · B genetic engineering · C natural selection · D selective breeding

  1. 1

    D: selective breeding [1]

15. Improving crops and animals over many generations

Selective breeding by artificial selection improves crop plants and domesticated animals, such as cattle bred to produce more milk, over many generations.

For a crop plant, humans select the plants with the largest yield. Pollen from these plants is rubbed onto the stigma of other plants of the same species. The offspring are observed. Those with the highest yields are selected and bred together. This is repeated until all the plants produced have a high yield.

Examiner insight

“Many responses dealt only with the choice of parents, without progressing to the choices to be made in the second and subsequent generations of breeding programmes.” “Fewer went on to suggest selecting offspring or repeating over many generations.”

Worked example 14 marks

Crop plants have been bred to increase yield. Complete the sentences by writing words or phrases in the spaces provided to describe this process.

Humans select crop plants that have the largest yield.

The pollen from these plants is taken and rubbed onto the ........... of other plants of the same species.

The ........... of these plants are then observed and the ones with the highest yields are then selected and bred together.

This is repeated for many ..........., until all the plants produced have a high yield.

This process is called ...........

  1. 1

    stigma [1]

  2. 2

    offspring [1]

  3. 3

    generations [1]

  4. 4

    artificial selection / selective breeding [1]

16. Adaptation

Adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations.

It describes a population over many generations. It is not one organism changing during its own life.

A population with little genetic variation adapts slowly to a change in the environment.

Examiner insight

“Some candidates were able to correctly identify that less genetic variety in the bees would mean that adaptation to environmental change would be slow.” “Some candidates referred to individual sharks developing good eyesight rather than this happening over many generations.”

Worked example 13 marks

Hares living in a snowy place have, over many generations, become more suited to their environment.

Describe what is meant by adaptation, using the hares as an example.

  1. 1

    adaptation is the process resulting from natural selection [1]

  2. 2

    by which a population (of hares) becomes more suited to its environment [1]

  3. 3

    over many generations [1]

17. Antibiotic resistance in bacteria

The development of strains of antibiotic resistant bacteria is an example of natural selection. There is variation in a population of bacteria. A mutation gives some bacteria an allele for resistance.

When an antibiotic is used, bacteria that are not resistant are killed. The resistant bacteria survive and reproduce. They pass on the allele for resistance to their offspring.

Over many generations the population becomes mostly resistant. Using antibiotics only when necessary decreases the natural selection of antibiotic resistant bacteria.

Examiner insight

“A common error was to use the term immunity instead of resistance. Another frequent error was the idea that bacteria mutated for a purpose, in this case to acquire antibiotic resistance.” “A significant proportion of candidates incorrectly believed that antibiotics kill resistant bacteria.”

Worked example 11 mark

How can bacterial resistance to antibiotics be reduced?

A using antibiotics frequently, which encourages the natural selection of antibiotic-resistant bacteria

B using antibiotics frequently, which decreases the natural selection of antibiotic-resistant bacteria

C using antibiotics only when necessary, which encourages the natural selection of antibiotic-resistant bacteria

D using antibiotics only when necessary, which decreases the natural selection of antibiotic-resistant bacteria

  1. 1

    D: using antibiotics only when necessary, which decreases the natural selection of antibiotic-resistant bacteria [1]

18. Natural and artificial selection compared

In natural selection, the environment decides which individuals survive and reproduce. There is a struggle for survival, with competition for resources.

In artificial selection, humans decide which individuals reproduce. They choose individuals with desirable features.

Both need genetic variation. In both, only certain individuals reproduce, and their alleles pass to the offspring. Both involve reproduction between individuals of the same species.

Examiner insight

“Some incorrectly opted for selective breeding, forgetting that this involves human intervention.” “Few stated clearly that environmental factors are responsible for natural selection and very few referred to the role of competition in the process.”

Worked example 11 mark

Which features of natural selection are also involved in selective breeding by artificial selection?

1 competition between individuals for environmental resources

2 presence of genetic variation

3 reproduction between individuals of the same species

A 1, 2 and 3 · B 1 and 2 only · C 1 and 3 only · D 2 and 3 only

  1. 1

    D: 2 and 3 only. Genetic variation and reproduction within a species are shared. Competition for resources belongs to natural selection. [1]

Recap

  • Natural selection: genetic variation, many offspring, struggle for survival, better adapted individuals reproduce, alleles passed on.
  • Selective breeding: humans select individuals with desirable features, cross them, then select the offspring showing the features.
  • Artificial selection is repeated over many generations to improve crop plants and domesticated animals.
  • Extended: adaptation is the process, resulting from natural selection, by which populations become more suited to their environment over many generations.
  • Extended: antibiotic resistance is natural selection. A mutation gives resistance, the non-resistant bacteria are killed, the resistant bacteria survive, reproduce and pass on the allele.
  • Extended: the environment drives natural selection and humans drive artificial selection. Both need genetic variation.

End-of-chapter exercise

Test yourself on the whole chapter. Work through these before moving on.

  1. Describe what is meant by variation.1 mark
  2. A student measures the body length of 100 beetles of one species. The lengths run from 8 mm to 14 mm, with many lengths in between. Name the type of variation and state what this type of variation results in.2 marks
  3. The seeds of pea plants are either yellow or green. Name the type of variation and state what this type of variation results in.2 marks
  4. State whether each characteristic shows continuous or discontinuous variation: body mass, ABO blood group, seed shape in peas, body length.4 marks
  5. Body mass in humans shows continuous variation. (a) State the two causes of continuous variation. (b) State the usual cause of discontinuous variation.3 marks
  6. The graph shows the lengths of 50 leaves from plants of one species. (a) State the type of variation shown by leaf length. (b) Explain how the graph shows this type of variation.3 marks
  7. (a) Describe mutation. (b) State how new alleles are formed. (c) State two factors that increase the rate of mutation.4 marks
  8. A scientist studies 100 snails of one species. Every shell is yellow, pink or brown. The table shows the results. Shell width runs from 16 mm to 24 mm, with many widths in between. (a) State the type of variation in shell colour and the type of variation in shell width. Give a reason for each from the information. (b) A student says that giving the snails more food could change shell colour but could not change shell width. Explain why the student is likely to be wrong about both. | Shell colour | yellow | pink | brown | | --- | --- | --- | --- | | Number of snails | 52 | 31 | 17 |6 marks
  9. Barley seeds are soaked in a chemical that increases the rate of mutation. One plant grown from these seeds has an allele found in no other barley plant. A student explains this: “The chemical affected the seeds and changed the genetic code. The new allele was then made by sexual reproduction.” The explanation has three faults. For each fault, write what the student should have said.5 marks
  10. Insects of one species live on two islands, P and Q. The soil on island P contains a chemical that increases the rate of mutation. Island Q has none of this chemical. (a) State on which island new alleles are likely to be formed at the higher rate. Explain your answer. (b) The insects on island Q still show genetic variation. A student gives the sources of this variation as “ionising radiation, mitosis and sexual reproduction”. None of the three earns a mark. Explain why ionising radiation is not a source of genetic variation. Then state three sources the student could have given.7 marks
  11. State what is meant by an adaptive feature.2 marks
  12. A student writes: 'An adaptive feature is caused by the environment. It helps an organism to survive.' Correct both sentences.2 marks
  13. Wind-pollinated flowers have anthers on long filaments. Explain why this is an adaptive feature.2 marks
  14. A photograph of a shark has its teeth in view. A candidate is asked for a feature, visible in the photograph, that adapts the shark for feeding. The candidate writes 'it is a fast swimmer'. Explain why this gains no credit, and give an answer that would.2 marks
  15. A photograph of a grass flower has small petals, anthers hanging outside the flower and feathery stigmas. A candidate writes: 'It has no scent and it has feathery anthers.' State two features, visible in the photograph, that adapt the flower for wind pollination.2 marks
  16. Give the term for a plant adapted to live in water. State one feature of the leaves of such a plant.2 marks
  17. The leaves of a water lily float on the surface of a pond. Explain how large air spaces in the leaf help it to float.2 marks
  18. A pondweed lives completely under water and has no stomata. Explain how oxygen and carbon dioxide move into and out of its leaves.2 marks
  19. Leaves from three plants at a lake are examined. The table gives data for each leaf. Leaf X floats on the surface of the water. One leaf is from a pondweed that lives completely under water. One leaf is from a tree on the bank. (a) Identify the pondweed leaf. Explain how carbon dioxide reaches its cells. (b) Explain why leaf X has its stomata on the upper epidermis. (c) Explain why a very thin cuticle is an adaptation for leaf X. | Leaf | Stomata per mm², upper epidermis | Stomata per mm², lower epidermis | Waxy cuticle | | --- | --- | --- | --- | | X | 460 | 0 | very thin | | Y | 0 | 0 | none | | Z | 0 | 150 | present |6 marks
  20. Plant R grows on a cold, dry, windy mountainside. Its leaves are rolled up, with hairs inside the roll, and have a thick cuticle. A student makes three statements. Explain what is wrong with each one. (a) 'R cannot be a xerophyte because its environment is cold.' (b) 'The space inside the rolled leaf is a large air space, so the leaf is buoyant.' (c) 'The dry wind made the plant grow a thick cuticle, so the thick cuticle is an adaptive feature.'6 marks
  21. Complete the sentence. In natural selection, individuals that are better adapted to the environment have a greater chance of ........... than others.1 mark
  22. A pair of rabbits has many offspring each year, but the number of rabbits in a field stays about the same. Use ideas from natural selection to explain why.3 marks
  23. A farmer uses selective breeding on her sheep. Describe the three things humans do to produce one new generation.3 marks
  24. A student writes: ‘In selective breeding, the animals that win a struggle for survival are chosen.’ Correct the student.2 marks
  25. A seed company grows wheat. A few plants have a much higher yield than the others, and the company breeds these together. Describe what the company must do next so that all its wheat plants have a high yield.4 marks
  26. Describe what is meant by adaptation.2 marks
  27. A student says: ‘The antibiotic caused the bacteria to mutate so that they became immune.’ Identify the two mistakes, and state what the antibiotic actually does.3 marks
  28. Over many generations, a population of island birds has come to have thicker beaks, which crack the hard seeds that are the only food there. A student calls this selective breeding. Explain why this is wrong, and give one feature that natural selection and selective breeding share.3 marks
  29. A fish farmer breeds only the heaviest salmon in every generation. In ten generations the mean mass of the farmed salmon rises from 2.5 kg to 3.6 kg. In a nearby river the water has become warmer. Wild salmon that tolerate warm water are the most likely to live long enough to breed. In the same time they rise from 10% to 70% of the wild population. (a) Calculate the percentage increase in the mean mass of the farmed salmon. [2] (b) Name the type of selection acting on each population. Give evidence for each. [2] (c) The farmer says: ‘My salmon have adapted to the farm.’ Explain why adaptation describes the wild salmon but not the farmed salmon. [2]6 marks
  30. A scientist puts bacteria from one population into two flasks, X and Y. On day 0, 2% of the bacteria in each flask are resistant to an antibiotic. The antibiotic is then added to flask X only. The bacteria reproduce many times each day. On day 6, 98% of the bacteria in flask X are resistant, and 2% in flask Y. (a) A student says the antibiotic made the bacteria mutate. Use day 0 to explain why this is wrong. [2] (b) Explain the change in flask X from day 0 to day 6. [3] (c) Explain why the percentage in flask Y does not change. [1]6 marks

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