Stabilising Selection
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of directional selection. You can test your knowledge on this below.
What is directional selection?
Directional selection occurs when one extreme phenotype is favoured over others, causing a shift in allele frequencies in a population over generations.
Give an example of directional selection.
Antibiotic resistance in bacteria – bacteria with a mutation for resistance survive and reproduce, increasing the frequency of the resistance allele in the population.
How does directional selection affect the normal distribution curve?
The curve shifts towards one extreme, as individuals with the beneficial extreme trait have a higher survival and reproductive success.
Topic Explainer Video
Check out this @LauraDoesBiology video that explains stabilising selection or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Stabilising Selection
What is Stabilising Selection?
Stabilising selection is a type of natural selection where the average phenotype is favoured, and extreme traits are selected against. This reduces variation and occurs in an unchanging environment. This is different to directional selection where one extreme trait is selected for.
Example: Human Birth Weight
- Babies of average weight (3–4 kg) have the highest survival rates.
- Very small babies struggle to maintain body temperature and may not develop properly.
- Very large babies may face complications during birth.
- Over generations, the alleles for extreme birth weights decrease in frequency.
Graph of Stabilising Selection
- The normal distribution curve becomes narrower.
- The mean phenotype remains the same, but variation decreases.
- The range of phenotypes and the standard deviation is reduced.
- More individuals become well adapted for the environment.
How Stabilising Selection Works – Step by Step
- Genetic Variation: A population has individuals with a range of traits due to mutations.
- Selection Pressure: Extremes of a trait reduce survival and reproductive success.
- Survival of the Fittest: Individuals with the mean (average) phenotype have higher survival rates.
- Inheritance: These individuals reproduce, passing on their alleles for the average trait.
- Reduction in Variation: Over generations, extreme traits decrease in frequency, making the population more well adapted to the unchanging environment.
Differences Between Directional and Stabilising Selection
| Feature | Directional Selection | Stabilising Selection |
| Favoured Phenotype | One extreme. | The average. |
| Effect on Population | Shift in allele frequency towards one extreme. | Reduction in range and standard deviation, mean remains unchanged. |
| Graph Effect | Curve shifts left or right. | Curve narrows, reducing extremes. |
| Example | Antibiotic resistance in bacteria. | Human birth weight. |
Key Terms
- Stabilising Selection: A type of natural selection where the average phenotype is favoured, and extremes are selected against.
- Selection Pressure: An environmental factor that affects survival and reproduction.
- 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.
- Normal Distribution: A bell-shaped curve showing trait variation in a population.
- Standard deviation: The spread of data around the mean.
No answer provided.
Exam Tips
When answering questions on selection, always tailor your response to the example in the question if you are provided with one. For example:
- Describe what the selection pressure is if you are able.
- Explain why the advantageous trait increases survival chances if you can tell.
No answer provided.
Explain what is meant by stabilising selection and describe the circumstances under which it takes place. (5 marks)
- Occurs in an unchanging environment.
- Selection against extremes.
- Selection for the mean or the mean / median / mode is unaltered.
- Range / S.D is reduced.
- Increasing proportion of populations becomes well adapted to environment.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!