Once we know allele frequencies can change, the next question is what pattern that change takes. This lesson explains how directional, stabilising and disruptive selection shape phenotype distributions and why reading bell curves matters for understanding real evolutionary outcomes.
Use the PDF for classwork, homework or revision. It includes key ideas, activities, questions, an extend task and success-criteria proof.
Make your first judgement before the graphs start guiding you.
1. If a selection pressure favours one extreme phenotype, what do you expect to happen to the population distribution over time?
2. If both extremes are favoured and the middle is selected against, what might that mean for future divergence or even speciation?
Write your starting answer now. We will revisit it once the three graph patterns are clear.
Write your initial answer in your book, then return later to compare it with your final explanation.
Core Content
Connect this concept to the broader biology framework. Understanding how systems interact is essential for HSC success.
One extreme is favoured and the whole distribution shifts
Directional selection occurs when one extreme phenotype has the highest fitness under the current selection pressure, so the average phenotype of the population shifts in that direction.
Classic examples include antibiotic resistance and industrial melanism in peppered moths. In polluted environments, darker moths were harder for predators to see against soot-darkened trees, so the frequency of the melanic form increased dramatically. In both examples, the graph does not simply get narrower. It moves sideways because one end of the distribution is being favoured.
The middle is favoured and variation narrows
Stabilising selection occurs when the intermediate phenotype has the highest fitness and both extremes are selected against.
Human birth weight is the classic example. Very low birth weight can increase risks linked to underdevelopment, while very high birth weight can increase birth complications. The intermediate range has the highest survival, so the distribution becomes narrower around the centre. The important visual clue is not a sideways shift. It is a tightening around the middle.
| Selection Type | Phenotypes Favoured | Effect on Graph | Example |
|---|---|---|---|
| Stabilising | Intermediate phenotype | Bell curve becomes narrower around the mean | Human birth weight, egg size in birds |
Both extremes are favoured and the middle loses out
Disruptive selection occurs when both extreme phenotypes have higher fitness than the intermediate phenotype, producing a split distribution.
The classic oyster example shows how this can work. If predators ignore very small oysters and struggle to open very large oysters, mid-sized oysters may be eaten most often. Over time, the intermediate phenotype declines while both extremes become more common. This type of selection can increase variation and, if the two favoured extremes become reproductively isolated, it can contribute to speciation.
Activities
A population of birds lays eggs of different sizes. Birds with medium-sized eggs have the highest reproductive success because very small eggs contain too little yolk and very large eggs are more likely to break. Identify the type of selection and explain why.
Name the selection type and refer to which phenotypes are favoured or selected against.
Sketch the bell curve in your book first, then record your explanation here.
Explain why disruptive selection can be a precursor to speciation, but does not automatically guarantee that new species will form.
A strong answer should mention both divergence and reproductive isolation.
Draft the explanation in your book, then write your final version here.
The three selection types are easiest to remember when you focus on what happens to the distribution: shift, narrow, or split. Once you can read that graph change, the examples become much easier to classify.
If your original answer blurred the categories together, the key correction is this: ask which phenotypes are favoured, then match that pattern to the graph shape that follows.
Assessment
Answer first, then read the explanation
1. Which type of selection favours one extreme phenotype and shifts the distribution in one direction?
2. Which graph change best indicates stabilising selection?
3. Industrial melanism in peppered moths is a classic example of:
Industrial melanism in peppered moths is a classic instance of:
4. Which scenario best fits disruptive selection?
5. Why can disruptive selection be important in speciation?
1. Distinguish between directional, stabilising and disruptive selection using one example for each. (4 marks)
1 mark each for correct definition/example pairing, plus 1 mark for clear distinction
2. Explain why stabilising selection reduces variation in a population. (3 marks)
1 mark: intermediate favoured | 1 mark: extremes selected against | 1 mark: narrower distribution explanation
3. Explain how disruptive selection can be a precursor to speciation. (3 marks)
1 mark: both extremes favoured | 1 mark: divergence increases | 1 mark: reproductive isolation needed for speciation
Answers
SA1: Directional selection favours one extreme phenotype and shifts the distribution in one direction, such as darker peppered moths becoming more common in polluted environments. Stabilising selection favours the intermediate phenotype and reduces variation, such as human birth weight where the middle range has the highest survival. Disruptive selection favours both extremes and selects against the intermediate phenotype, such as oyster populations where very small and very large individuals survive better than mid-sized ones. The key distinction is which phenotypes are favoured and how the distribution changes.
SA2: Stabilising selection reduces variation because individuals with the intermediate phenotype have the highest fitness, while both extremes are selected against. As the extremes leave fewer offspring, those less-common extreme phenotypes decline. Over generations, the population distribution becomes narrower around the mean, so overall variation is reduced.
SA3: Disruptive selection can be a precursor to speciation because it favours both extreme phenotypes and reduces the success of the intermediate phenotype. This increases divergence within the population and can split the distribution into two groups. If those diverging groups also become reproductively isolated, they may eventually form separate species.
Say each answer aloud before moving to the next prompt