Explain how stabilising selection maintains phenotypic stability in a population over successive generations.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Model answer (4 marks)
Stabilising selection removes individuals with extreme phenotypes, favouring those near the mean. Consequently the alleles that produce the extremes are lost from the gene pool at a higher rate, so their frequencies fall over successive generations. Because the majority of offspring come from parents with intermediate traits, the mean (or modal) phenotype of the population remains essentially unchanged. For example, human birth weight shows stabilising selection: babies that are too light or too heavy have higher mortality, so the population retains an intermediate, optimal birth weight.
Examiner tips
- Use the term "stabilising selection" and explain it removes extremes
- Show that allele frequencies of extremes decrease
- State that the mean phenotype stays constant
- Give a clear example such as birth weight
Common mistakes
- Confusing stabilising with directional or disruptive selection
- Failing to mention the change in allele frequencies
- Giving an irrelevant example or not linking it to an optimal phenotype
Mark scheme (4 marks)
- Stabilising selection acts against individuals at both extremes of a phenotypic distribution / selects for intermediate phenotypes
- Alleles associated with extreme phenotypes are removed from the gene pool at a higher rate / their frequency decreases over generations
- The mean / modal phenotype remains relatively unchanged across generations because individuals near the mean contribute disproportionately to the next generation
- A named, relevant example illustrates how an optimal phenotype is maintained, e.g. human birth weight — very low or very high birth weights are associated with higher infant mortality, maintaining an intermediate optimal birth weight
Key terms in this question
Related
- All IB DP Biology Higher Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
More Stability and change questions
- Explain how a bottleneck event can alter allele frequencies in a population and …
- Explain how genetic drift can lead to rapid evolutionary change in small, isolat…
- Explain how a founder effect can influence the genetic diversity and future evol…
- Explain how the concept of punctuated equilibrium differs from phyletic graduali…