How does meiosis produce genetic variation?

Independent assortment of chromosomes and crossing over produce new combinations of existing alleles.

How does random fertilisation produce variation?

Any male gamete can fuse with any female gamete, giving many different combinations of alleles.

Name three selection pressures.

Predation, disease and competition.

Define natural selection.

Differential survival and reproduction of individuals due to differences in phenotype.

What is a selective advantage?

A phenotype that makes an individual more likely to survive and reproduce, so it passes on its alleles.

A change in allele frequencies in a population over time.

What does stabilising selection do?

Favours the intermediate phenotype, reducing variation; the mean stays the same.

What does directional selection do?

Favours one extreme, so the mean shifts towards it.

What does disruptive selection do?

Favours both extremes and acts against intermediates, giving two peaks.

Give an example of stabilising selection.

Human birth weight: babies of intermediate weight have the highest survival.

Give an example of directional selection.

Antibiotic resistance in bacteria.

How do alleles for an advantageous phenotype change in frequency?

They are passed on by more offspring so their frequency in the gene pool increases.

Selection and changing allele frequenciesAQA A-Level Biology: Flashcards

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What is the primary source of genetic variation?

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What is the primary source of genetic variation?: Mutation, which creates new alleles.
How does meiosis produce genetic variation?: Independent assortment of chromosomes and crossing over produce new combinations of existing alleles.
How does random fertilisation produce variation?: Any male gamete can fuse with any female gamete, giving many different combinations of alleles.
Name three selection pressures.: Predation, disease and competition.
Define natural selection.: Differential survival and reproduction of individuals due to differences in phenotype.
What is a selective advantage?: A phenotype that makes an individual more likely to survive and reproduce, so it passes on its alleles.
Define evolution.: A change in allele frequencies in a population over time.
What does stabilising selection do?: Favours the intermediate phenotype, reducing variation; the mean stays the same.
What does directional selection do?: Favours one extreme, so the mean shifts towards it.
What does disruptive selection do?: Favours both extremes and acts against intermediates, giving two peaks.
Give an example of stabilising selection.: Human birth weight: babies of intermediate weight have the highest survival.
Give an example of directional selection.: Antibiotic resistance in bacteria.
How do alleles for an advantageous phenotype change in frequency?: They are passed on by more offspring so their frequency in the gene pool increases.

Exam questions on Selection and changing allele frequencies

On a hospital ward, a population of bacteria is exposed to an antibiotic. Before treatment, a very small proportion of the bacteria carried an allele that makes them resistant to the antibiotic.
Explain how the frequency of the resistance allele in the bacterial population increases after the antibiotic is used.2 marks
In human populations, babies with a birth weight close to the population mean of about 3.3 kg have the highest rate of survival. Babies with a very low or very high birth weight have a higher death rate.
Explain the effect of this type of selection on the frequencies of the alleles affecting birth weight.2 marks
On a small island, seed-eating birds feed on seeds of different sizes. Beak depth varies widely between individual birds and is partly inherited. During a severe drought, the plants producing small, soft seeds died, leaving mainly large, hard seeds. After the drought, the mean beak depth of the next generation of birds was greater than before.
Explain how the drought led to an increase in the mean beak depth of the birds in the next generation.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).