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Selection and changing allele frequenciesAQA A-Level Biology: Revision notes

Section 1

Variation in a population

Individuals in a population show a wide range of variation in phenotype. The causes are genetic and environmental.

Genetic causes:

  • mutation is the primary source of genetic variation, because it creates new alleles
  • meiosis produces new combinations of existing alleles, through independent assortment of chromosomes and crossing over
  • random fertilisation of gametes means any male gamete can fuse with any female gamete, giving more new combinations

Environmental causes include diet, climate, light and disease, which affect how the genotype is expressed. Many features, such as height, are affected by both.

Key termsphenotypemutationrandom fertilisation
Common mistake

Meiosis and random fertilisation do not create new alleles. They shuffle existing ones. Only mutation creates new alleles.

Section 2

Natural selection

In most populations more offspring are produced than can survive. Predation, disease and competition for the means of survival (food, water, light, mates, space) mean that some individuals survive and reproduce more than others. This is differential survival and reproduction, and it is natural selection.

  1. Variation exists, caused by different alleles.
  2. A selection pressure acts, e.g. a predator or antibiotic.
  3. Individuals with a selective advantage are more likely to survive and reproduce.
  4. They pass on the favourable alleles to more offspring.
  5. The frequency of these alleles in the gene pool increases in the next generation.
Key termsnatural selectionselective advantageselection pressure
Common mistake

Individuals do not change their alleles to adapt. Selection changes the proportion of alleles in the population over generations.

Section 3

Types of selection

Natural selection can act on a continuous characteristic in three main ways.

  • Stabilising selection favours the intermediate phenotype and acts against both extremes. The mean stays the same and the range narrows. Example: human birth weight.
  • Directional selection favours one extreme phenotype. The mean shifts towards that extreme. Example: antibiotic resistance in bacteria.
  • Disruptive selection favours both extremes and acts against the intermediate. The distribution develops two peaks. Example: beetles of two colours camouflaged on different backgrounds.
Key termsstabilising selectiondirectional selectiondisruptive selection
Exam tip

Describe what happens to the mean and to the range of the characteristic. These identify the type of selection.

Section 4

Selection and allele frequencies

Selection changes the frequency of alleles in the gene pool. Alleles that give a selective advantage become more common; alleles that give a disadvantage become less common.

Evolution is a change in the allele frequencies in a population over time. Selection is one cause of this. A good answer always links the phenotype advantage to survival, to reproduction, to passing on alleles and finally to a change in allele frequency.

Key termsevolution
Exam tip

Use the sequence: variation, selection pressure, survive and reproduce, pass on alleles, allele frequency changes.

Section 5

Evolution and the diversity of species

Over a very long period, the same process repeated in many populations has produced the diversity of species we see.

  • Mutation keeps producing new variation.
  • Different environments exert different selection pressures.
  • Populations in different places change in different directions and diverge.
  • Small changes in allele frequencies accumulate over very many generations.
  • When populations become so different that they cannot interbreed to produce fertile offspring, they are separate species.
Key termsdiversity

That's the notes covered.

Carry on to the next subtopic.

Exam questions on Selection and changing allele frequencies

  1. 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
  2. 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
  3. 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).