Directional Selection
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of natural selection. You can test your knowledge on this below.
What is natural selection?
Natural selection is the process by which individuals with advantageous traits survive, reproduce, and pass on their beneficial alleles, leading to changes in allele frequency over generations.
What are the three types of natural selection?
The three types are directional selection, stabilising selection, and disruptive selection.
Why is genetic variation important for natural selection?
Genetic variation provides different traits within a population, allowing some individuals to survive better under selection pressures and pass on their advantageous alleles.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains directional selection or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Directional Selection
What is Directional Selection?
- Directional selection occurs when one extreme phenotype is favoured over others, leading to a shift in the population’s allele frequencies.
- It often happens in response to environmental changes or new selection pressures.
- Over generations, individuals with the favoured trait increase in frequency, while those with less advantageous traits are selected against.
Example: Antibiotic Resistance in Bacteria
- Some bacteria mutate, gaining an allele for antibiotic resistance.
- When exposed to antibiotics, non-resistant bacteria die, but resistant bacteria survive and reproduce.
- They pass on the resistance allele.
- Over time, the resistant allele increases in frequency, making the population more resistant.
Graph of Directional Selection
- The normal distribution curve shifts in the direction of the advantageous phenotype.
- The population's mean value for the trait moves towards one extreme.
- The other extreme is selected against.
Directional selection:
- One extreme trait is favoured
How Directional Selection Works – Step by Step
- Genetic Variation: A population has individuals with a range of traits due to mutations.
- Selection Pressure: A change in the environment favours one extreme phenotype (e.g., a new predator, climate change, or antibiotics).
- Survival of the Fittest: Individuals with the advantageous extreme trait survive, while others die.
- Inheritance: Surviving individuals pass on their beneficial alleles to offspring.
- Shift in Population: Over generations, the average phenotype shifts towards one extreme phenotype, and the population becomes better adapted to the environment.
Key Terms
- Directional Selection: A type of natural selection where one extreme phenotype is favoured.
- Selection Pressure: An environmental factor that influences survival (e.g., predators, climate, antibiotics).
- Allele Frequency: The proportion of an allele within a population’s gene pool.
- Genetic Variation: Differences in DNA between individuals due to mutations and meiosis.
- Adaptation: A trait that increases an organism’s survival and reproductive success.
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Exam Tips
When explaining directional selection, always describe:
- The selection pressure (e.g., environmental change).
- Which Phenotype is favoured (one extreme).
- The Impact on allele frequency over generations.
No answer provided.
In an investigation, the tolerance to copper ions of the grass Agrostis tenuis was determined. Samples were taken of plants growing in waste from a copper mine and from nearby areas just outside the mine. The mean copper tolerance of plants from the mine waste was found to be four times higher than that of plants in the surrounding area.
What type of selection is occurring here and explain how this type of natural selection could produce a copper-tolerant population in the mine waste. (5 marks)
- Directional selection (as one extreme phenotype is selected for).
- Genetic variation present in original population (due to mutations).
- Copper tolerant individuals more likely to survive.
- These reproduce and pass on alleles to next generation / offspring.
- Increase in frequency of copper tolerance allele.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!