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Natural selection: variation, selection and allele frequency questions
The three sources of genetic variation and which of them makes new alleles, environmental variation and why it is not inherited, selection pressure and a usable definition of fitness, stabilising, directional and disruptive selection with a worked example of each, and how selection on phenotype changes allele frequency in a gene pool.
6 original questions · 19 marks · the natural selection: variation, selection and allele frequency notes · Evolution and speciation
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A field of crops is sprayed with a new insecticide every year, and after ten years most of the insects in that field survive the spray. Explain how this change came about.
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- B1 genetic variation was already present in the population, produced by random mutation, so a few insects carried an allele giving resistance before the insecticide was ever used
- B1 the insecticide is the selection pressure: insects without the allele are killed, while those carrying it survive
- B1 the survivors reproduce and pass the allele to their offspring, so a greater proportion of the next generation carries it
- B1 repeated over many generations the frequency of the resistance allele in the gene pool rises, and that change in allele frequency is the evolution. The spray selected among variation already present rather than causing the mutation
Compare stabilising selection with disruptive selection, referring to which phenotypes are favoured and to what happens to the mean and to the variation.
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- B1 stabilising selection favours the mean phenotype and acts against both extremes, whereas disruptive selection acts against the mean and favours both extremes
- B1 under stabilising selection the mean is unchanged, and under disruptive selection the mean may not move either, so the mean alone does not distinguish them
- B1 stabilising selection reduces the variation, so the curve becomes taller and narrower, whereas disruptive selection increases it and the single peak splits into two with a trough between
- B1 stabilising selection is the usual mode in a stable environment, whereas disruptive selection needs two distinct resources with nothing in between, as for a finch feeding on hard seeds and on soft ones
A recessive allele causes a metabolic condition, and 1 person in every 2500 is born with it. Assuming the Hardy-Weinberg conditions hold, calculate the frequency of the recessive allele and calculate the percentage of the population who are heterozygous carriers.
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- M1 the homozygous recessive genotype has frequency q2, so q2 = 1 ÷ 2500 = 0.0004; the square root is taken of that genotype frequency, not of 2500
- A1 q = the square root of 0.0004 = 0.02
- M1 p = 1 − q = 0.98, and the frequency of heterozygotes is 2pq = 2 × 0.98 × 0.02
- A1 0.0392, so 3.92 per cent of the population are carriers
An allele giving resistance to a pesticide reaches a frequency of 0.6 in a field where the pesticide is used every season. Spraying then stops, and over the following years the frequency of the allele falls. Suggest why.
Name the only source of alleles that did not exist in a population before, and name two processes that produce new combinations of alleles that already exist.
State what is meant by the fitness of an individual, and state why survival on its own does not raise it.
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