Types of Natural Selection
In 1954, British geneticist Anthony Allison published data showing that the sickle-cell allele (HbS) reached 20–40% frequency in malaria-endemic regions of sub-Saharan Africa, compared with less than 1% in non-endemic regions. Heterozygous individuals carrying one HbS allele had 25% lower malaria mortality than normal homozygotes. This study revealed that a gene-level allele frequency difference was being maintained by a specific selection pressure — malaria — and that the allele frequency pattern across Africa precisely tracked the geography of malaria risk. Allele frequencies do not change randomly; they change in predictable directions depending on which phenotypes selection favours.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions — start at whatever level suits you.
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?
Know
- The three major types of natural selection.
- What each type does to a bell-curve distribution.
- Examples such as peppered moths, human birth weight and oyster size.
Understand
- Why the type of selection depends on which phenotypes are favoured.
- Why stabilising selection reduces variation while disruptive selection can increase it.
- How disruptive selection can contribute to speciation if isolation follows.
Can Do
- Interpret before-and-after bell curves for each selection type.
- Match a scenario to directional, stabilising or disruptive selection.
- Explain why a selection pattern leads to a specific distribution change.
Core Content
One extreme is favoured and the whole distribution shifts
In Anthony Allison's 1954 study, the sickle-cell allele (HbS) reached 20–40% frequency in malaria-endemic regions of sub-Saharan Africa — far higher than the less-than-1% frequency in non-endemic regions. The malaria parasite was killing individuals without HbS protection at higher rates, so heterozygotes (one HbS copy) survived better and left more offspring. Over generations, the HbS allele frequency shifted upward in malaria zones — the whole population distribution of allele frequencies moved toward the HbS end. This is directional selection: one phenotype (HbS heterozygote) had higher fitness under the current selection pressure, so the population average shifted 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, directional selection shifts the entire distribution sideways — the mean phenotype moves toward the favoured extreme over successive generations. In both examples, the graph does not simply get narrower. It moves sideways because one end of the distribution is being favoured.
Favours One Extreme
The left or right tail is selected for; the other is selected against.
Mean Shifts
The whole population average moves in one direction over generations.
Example
Peppered moths or increasing resistance under stronger drug pressure.
Directional selection changes the mean phenotype by favouring one end of the distribution.
Directional selection: one extreme phenotype is favoured → bell curve shifts sideways → mean moves toward that extreme over generations. Example: industrial melanism in peppered moths.
Pause — copy the highlighted summary into your book before the check below.
Which type of selection favours one extreme phenotype and shifts the distribution in one direction?
The middle is favoured and variation narrows
We just saw that directional selection shifts the whole distribution toward one extreme. That raises a question: what happens when there is no clear advantage at either end — when the middle works best? This card answers it → stabilising selection narrows the distribution by removing both extremes.
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. Stabilising selection favours the intermediate phenotype — both extremes are selected against, the bell curve becomes narrower around the mean, and overall variation decreases. 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 |
Stabilising selection reduces variation by favouring the intermediate phenotype.
Stabilising selection: intermediate phenotype has highest fitness → both extremes selected against → curve narrows around the mean. Example: human birth weight.
Pause — copy the highlighted summary into your book before the check below.
Which graph change best indicates stabilising selection?
Both extremes are favoured and the middle loses out
We just saw that stabilising selection narrows variation by favouring the middle. That raises a contrasting question: what if the middle is actually the least fit option? This card answers it → disruptive selection splits the distribution by favouring both extremes simultaneously.
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. Disruptive selection favours both extremes and selects against the intermediate — the bell curve can split into a bimodal distribution as the middle declines, increasing overall variation. This type of selection can increase variation and, if the two favoured extremes become reproductively isolated, it can contribute to speciation.
Favours Both Extremes
Intermediate phenotypes are selected against; both tails are favoured.
Two Peaks
The bell curve can split into a bimodal pattern as the middle declines.
Speciation Link
Extreme groups may diverge further if they become reproductively isolated.
Disruptive selection increases variation and can create a split phenotype distribution.
Disruptive selection: both extremes favoured, middle selected against → bimodal split → variation increases. Can contribute to speciation if the two groups become reproductively isolated. Example: oysters.
Pause — copy the highlighted summary into your book before the check below.
Which scenario best fits disruptive selection?
Activities
Name the Selection Type
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.
From Selection to Speciation
Explain why disruptive selection can be a precursor to speciation, but does not automatically guarantee that new species will form.
True or false: disruptive selection can contribute to speciation if the diverging extreme groups become reproductively isolated.
Directional Selection
- Favours one extreme phenotype.
- The distribution shifts sideways toward that extreme.
Stabilising Selection
- Favours the intermediate phenotype.
- The distribution narrows around the mean and variation decreases.
Disruptive Selection
- Favours both extremes and acts against the middle.
- The distribution can split into two peaks and variation increases.
Graph Clues
- Shift sideways = directional.
- Narrower middle peak = stabilising. Split peaks = disruptive.
A fresh set drawn from this lesson's question bank — feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence — that tells the system what to drill next.
UnderstandBand 3(4 marks) 1. Distinguish between directional, stabilising and disruptive selection using one example for each.
AnalyseBand 3–4(3 marks) 2. Explain why stabilising selection reduces variation in a population.
EvaluateBand 4–5(3 marks) 3. Explain how disruptive selection can be a precursor to speciation.
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Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Activity 1 — Name the Selection Type
This is stabilising selection because the intermediate phenotype (medium-sized eggs) has the highest reproductive success. Very small eggs are selected against because they contain insufficient yolk, and very large eggs are selected against because they are more likely to break. Both extremes have lower fitness, so the distribution narrows around the middle — the defining signature of stabilising selection.
Activity 2 — From Selection to Speciation
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 phenotype distribution into two groups. However, disruptive selection alone does not guarantee speciation. For new species to form, the diverging groups must also become reproductively isolated — either by geographic separation, behavioural differences or other mechanisms that prevent gene flow between them. Without reproductive isolation, interbreeding between the extremes can prevent the populations from diverging into separate species.
Short Answer Model Responses
SA1 (4 marks): Directional selection favours one extreme phenotype and shifts the distribution in one direction, such as darker peppered moths becoming more common in polluted environments [1]. Stabilising selection favours the intermediate phenotype and reduces variation, such as human birth weight where the middle range has the highest survival [1]. 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 [1]. The key distinction is which phenotypes are favoured and how the distribution changes [1].
SA2 (3 marks): Stabilising selection reduces variation because individuals with the intermediate phenotype have the highest fitness, while both extremes are selected against [1]. As the extremes leave fewer offspring, those less-common extreme phenotypes decline [1]. Over generations, the population distribution becomes narrower around the mean, so overall variation is reduced [1].
SA3 (3 marks): Disruptive selection can be a precursor to speciation because it favours both extreme phenotypes and reduces the success of the intermediate phenotype [1]. This increases divergence within the population and can split the distribution into two groups [1]. If those diverging groups also become reproductively isolated, they may eventually form separate species [1].
Directional = shift
One extreme is favoured. The bell curve shifts sideways in that direction.
Stabilising = narrow
The intermediate is favoured. Both extremes decline and variation decreases.
Disruptive = split
Both extremes are favoured. The middle declines and variation increases.
Most common exam trap
Confusing stabilising (narrows) with disruptive (splits). Read which phenotypes are favoured first, then match the graph shape.
Rapid-fire questions on directional, stabilising and disruptive selection. Beat the boss to bank a tier — gold (perfect + fast), silver (80%+), or bronze (cleared).
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.
Anthony Allison's 1954 study of the sickle-cell allele (HbS) in sub-Saharan Africa is a clear case of directional selection: in malaria-endemic regions, the HbS allele reached 20–40% frequency because heterozygotes had 25% lower malaria mortality than normal homozygotes, so the allele frequency distribution shifted systematically toward higher HbS as malaria acted as a selection pressure across generations. In non-endemic regions where malaria exerts no selection pressure, the allele dropped below 1% — the distribution shifted back toward zero. The same allele, two environments, two directions of selection: a textbook demonstration that the type of selection depends entirely on which phenotypes the current environment favours.