Variation and evolution | OCR Gateway GCSE Biology Notes

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The key idea

Variation may be genetic, environmental or both.Natural selection changes populations when advantageous variants reproduce more successfully.

Natural selectionSelection changes allele frequency across generations.

Revision notes

The bit that matters

Learn the process in clean chunks. If a sentence explains a cause, make sure you can say the effect too.

Variation

Variation is the differences in characteristics between individuals of the same species.It can be caused by genes, by the environment, or by a combination of the two.Genetic variation arises through mutations, the random changes to genes, and through the mixing of alleles in sexual reproduction.Most mutations have no effect but some can change the phenotype.

Natural selection and evolution

Evolution is the gradual change in the inherited characteristics of a population over many generations through natural selection.Individuals with characteristics best suited to the environment are more likely to survive and reproduce.They pass on the alleles for these advantageous characteristics to their offspring, so over many generations the population becomes better adapted.

Selective breeding

Selective breeding, or artificial selection, is when humans breed plants and animals for particular desired characteristics, such as high crop yield or gentle temperament in dogs.Over many generations the chosen characteristic becomes more common.A disadvantage is reduced genetic variation, which can lead to inbreeding and a greater risk of disease or inherited defects.

Antibiotic resistance

Antibiotic resistant bacteria, such as MRSA, are a clear example of natural selection happening quickly.A chance mutation makes some bacteria resistant to an antibiotic.When the antibiotic is used the non resistant bacteria are killed but the resistant ones survive and reproduce, passing on the resistance allele.To slow this, doctors should not over prescribe antibiotics and patients should complete the full course.

Key terms

Definitions to learn

Variation

Differences in characteristics between individuals of the same species.

Mutation

A random change to the DNA or genes of an organism.

Natural selection

The survival and reproduction of individuals best suited to their environment.

Evolution

The change in inherited characteristics of a population over many generations.

Selective breeding

Breeding organisms chosen for desired characteristics over generations.

Antibiotic resistance

When bacteria are no longer killed by an antibiotic due to a mutation.

Worked example

Explain how antibiotic resistance becomes more common in bacteria.

Random mutation creates variation.

Antibiotics kill non-resistant bacteria.

Resistant bacteria survive and reproduce.

They pass resistance alleles to offspring.

Final answer

Natural selection increases the frequency of antibiotic resistance.

Exam habit

Show the four-step natural selection argument: variation exists → selection pressure → best-adapted survive and reproduce → advantageous alleles passed to offspring.Organisms do not mutate by choice — mutations occur randomly.

Watch out

Organisms do not mutate because they need to. Mutations occur randomly.

Examiner tips

How to score full marks

1When explaining natural selection, always include variation, survival of the best suited, reproduction and passing on of alleles.
2Use the word allele rather than gene when describing what is passed to offspring in selection.
3For antibiotic resistance, stress that the mutation occurs by chance first and the antibiotic then selects for it; the antibiotic does not cause the mutation.

Practice questions

Try these yourself

Open each answer only after you have explained the full biological process.

1Give one example of inherited variation and one environmental variation.

Mark scheme

1.Choose one mainly genetic and one caused by surroundings.

Examples: blood group is inherited; accent or a scar is environmental.

2Why does natural selection require variation?

Mark scheme

1.Think survival differences.

Individuals need different characteristics so some are better adapted and reproduce more successfully.

3How can fossils support the theory of evolution?

Mark scheme

1.Link sequence over time to changing organisms.

Fossils provide evidence that organisms have changed over long periods.

4Give two causes of variation between individuals of the same species.[2 marks]

Mark scheme

1.Think about genes and environment.

Any two from: genetic causes or differences in alleles (1); environmental causes (1); a combination of genes and the environment (1)

5What is a mutation?[1 mark]

Mark scheme

1.State that it is a change to genetic material.

A random change to the DNA or genes of an organism (1)

6State one disadvantage of selective breeding.[1 mark]

Mark scheme

1.Think about genetic variation.

It reduces genetic variation which can lead to inbreeding and a greater risk of inherited disease or defects (1)

7Explain how a population of dark coloured moths could become more common than light coloured moths in a polluted area through natural selection.[4 marks]

Mark scheme

1.Link camouflage to survival and reproduction.

There is variation so some moths are dark and some are light (1); on polluted dark surfaces dark moths are better camouflaged so are less likely to be eaten by predators (1); the surviving dark moths reproduce and pass on the alleles for dark colour (1); over many generations the proportion of dark moths increases (1)

8A patient takes a course of antibiotics but stops early once they feel better. Explain how this can lead to a population of antibiotic resistant bacteria.[4 marks]

Mark scheme

1.Explain the chance mutation.
2.Explain selection and reproduction.

A chance mutation makes some bacteria resistant to the antibiotic (1); the antibiotic kills the non resistant bacteria but the resistant ones survive (1); the surviving resistant bacteria reproduce and pass on the resistance allele (1); stopping early leaves resistant bacteria alive so the resistant population spreads (1)

9Distinguish between continuous and discontinuous variation and give one example of each.[4 marks]

Mark scheme

1.Define each type.
2.Give an appropriate example.

Continuous variation shows a range of values between two extremes with no distinct groups (1); example: height or weight (1); discontinuous variation shows distinct groups with no intermediate values (1); example: blood group or ability to roll tongue (1).

10Explain how the theory of evolution by natural selection explains the diversity of life on Earth over millions of years.[5 marks]

Mark scheme

1.Link variation and selection to gradual change.
2.Extend to speciation over long periods.

Random mutations create new variation in populations (1); individuals with advantageous characteristics survive and reproduce more successfully (1); their alleles increase in frequency over many generations (1); as populations become isolated in different environments they are subject to different selection pressures (1); over millions of years populations can change so much that they become unable to interbreed, forming new species (1).

11Evaluate selective breeding as a method of improving crop plants, compared to genetic engineering.[5 marks]

Mark scheme

1.Give advantages and disadvantages of selective breeding.
2.Compare to genetic engineering.

Selective breeding is a proven method that has produced high-yielding varieties over centuries without the need for direct genetic manipulation (1); however it is slow, taking many generations to develop a desired characteristic (1); it also reduces genetic diversity making crops more vulnerable to disease (1); genetic engineering can introduce a specific gene in one step and from any organism (1); but concerns exist about unpredictable effects of transferred genes on ecosystems and human health (1); selective breeding is generally more accepted ethically than genetic engineering (1).

12The fossil record shows gradual changes in the structure of the horse's foot over 50 million years, from a multi-toed foot to a single hoof. Explain how natural selection could account for this change.[5 marks]

Mark scheme

1.Identify a likely selection pressure.
2.Apply the four-step natural selection argument.

Variation existed in the foot structure of early horse-like animals (1); a mutation producing fewer, larger toes may have increased running speed on open grassland (1); individuals with fewer toes survived better, being able to escape predators or compete for food more effectively (1); these individuals reproduced and passed on alleles for reduced toe number (1); repeated over many generations this produced the single-hoofed structure seen in modern horses (1).

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