AQA · A-Level · Biology

    Course version: 7402 · Version from Sept 2016

    Directional selection, exemplified by antibiotic resistance in bacteria,…

    Full specification wording

    Genetic diversity and adaptation: Directional selection, exemplified by antibiotic resistance in bacteria, and stabilising selection, exemplified by human birth weights.

    The two types differ in which phenotypes are selected against.

    Checked against the AQA 7402 specification (opens in a new tab) on .

    • Point 8 of 11
    • 2 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    The two types differ in which phenotypes are selected against. Directional selection acts when the environment changes or a new pressure appears: individuals at one extreme of the range have the advantage, so the modal value shifts in that direction over generations and the whole distribution curve moves along the axis. Antibiotic resistance is the example. Before widespread antibiotic use, resistant bacteria were rare; the antibiotic kills the non-resistant, the resistant survive and reproduce, and the mean level of resistance in the population rises. Stabilising selection acts when the environment is stable: individuals at both extremes are selected against and those near the mean have the advantage, so the mean stays the same while the range narrows and the curve becomes taller and thinner.

    What earns the marks

    • Directional selection favouring individuals at one extreme, so the mean or mode shifts in that direction.
    • Stabilising selection favouring individuals near the mean and selecting against both extremes, reducing the range.
    • Stating that resistant bacteria survive the antibiotic and reproduce, so resistance increases in the population.
    • Stating that very low and very high birth masses give lower survival, so those alleles are passed on less often.
    • Interpreting distribution curves correctly, the peak shifted along the axis versus the peak higher and narrower.

    Common mistakes and how to put them right

    • Saying stabilising selection means nothing changes, when allele frequencies at the extremes do fall.

    • Describing directional selection as favouring the strongest rather than one extreme of a measured characteristic.

    • Drawing a curve that changes height but not position for directional selection.

    • Writing that antibiotics make bacteria resistant.

    • Giving birth mass as the example without stating which extremes are disadvantaged and why.

    Exam tips

    • Sketch both curves before and after selection, labelling the axes as number of individuals against the characteristic.
    • Name the favoured extreme in a directional example; that is where the application mark sits.
    • For stabilising selection say the range decreases as well as the mean staying the same.

    Try the lesson

    The start of the Genetic diversity and adaptation: Directional selection, exemplified by antibiotic resistance in bacteria, and stabilising selection, exemplified by human birth weights. lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which type of selection occurs when bacteria with resistance to an antibiotic survive and reproduce while non-resistant bacteria die?

    Listen

    The two types differ in which phenotypes are selected against. Directional selection acts when the environment changes or a new pressure appears: individuals at one extreme of the range have the advantage, so the modal value shifts in that direction over generations and the whole distribution curve moves along the axis. Antibiotic resistance is the example. Before widespread antibiotic use, resistant bacteria were rare; the antibiotic kills the non-resistant, the resistant survive and reproduce, and the mean level of resistance in the population rises. Stabilising selection acts when the environment is stable: individuals at both extremes are selected against and those near the mean have the advantage, so the mean stays the same while the range narrows and the curve becomes taller and thinner. A strong answer earns marks for these points. directional selection favouring individuals at one extreme, so the mean or mode shifts in that direction. stabilising selection favouring individuals near the mean and selecting against both extremes, reducing the range. stating that resistant bacteria survive the antibiotic and reproduce, so resistance increases in the population. stating that very low and very high birth masses give lower survival, so those alleles are passed on less often. Exam tip: Sketch both curves before and after selection, labelling the axes as number of individuals against the characteristic.

    See it

    A top-down flowchart. At the top, a node labelled Selection branches into two nodes: Directional selection and Stabilising selection. From Directional selection, an arrow leads to Environment changes or new pressure appears, then to Individuals at one extreme have the advantage, then to Mean or mode shifts in that direction, and finally to Antibiotic resistance in bacteria. From Stabilising selection, an arrow leads to Environment is stable, then to Individuals at both extremes selected against, then to Mean stays the same and range narrows, and finally to Human birth mass.
    It shows the two types of selection, the conditions under which each acts, which phenotypes are favoured, and the resulting change in the distribution.

    Key words

    Say what each word means before you turn the card.

    directional selection

    Try answering aloud or in your head before turning the card.

    stabilising selection

    Try answering aloud or in your head before turning the card.

    See every definition
    directional selection
    Individuals at one extreme of the range have the advantage, so the modal value shifts in that direction over generations and the whole distribution curve moves along the axis.
    stabilising selection
    Individuals at both extremes are selected against and those near the mean have the advantage, so the mean stays the same while the range narrows and the curve becomes taller and thinner.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A top-down flowchart. At the top, a node labelled Selection branches into two nodes: Directional selection and Stabilising selection. From Directional selection, an arrow leads to Environment changes or new pressure appears, then to Individuals at one extreme have the advantage, then to Mean or mode shifts in that direction, and finally to Antibiotic resistance in bacteria. From Stabilising selection, an arrow leads to Environment is stable, then to Individuals at both extremes selected against, then to Mean stays the same and range narrows, and finally to Human birth mass.

    • 1Box 1

    • 2Box 2

    • 3Box 3

    0 of 3 labelled

    There is more to this lesson

    The full lesson goes on to check your understanding and help you earn the marks. Create a free account to carry on, with your progress saved.

    Carry on with the full lesson

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Test your understanding of Genetic diversity and adaptation: Directional selection, exemplified by antibiotic resistance in bacteria, and stabilising selection, exemplified by human birth weights.

    Active recall beats re-reading. Take an instant quiz now to test what you’ve learned and lock it into long-term memory.

    Take topic quiz

    A week of Premium from your first lesson · Then the free plan continues · No card, no charge

    Start free

    Cookies on MasteryMind

    Essential cookies keep you signed in.

    Analytics and advertising cookies stay off unless you accept.

    Read our Privacy Policy.