Question
Describe the three types of natural selection — stabilising, directional, and disruptive. Include examples of each and explain how each affects the distribution of traits in a population.
Solution — Step by Step
Natural selection acts on phenotypic variation — the range of traits visible in a population. If we plot a trait (like body size, or beak length) on a graph against frequency, we typically get a bell curve (normal distribution).
Natural selection can change this bell curve in three ways, depending on which end of the trait spectrum is favoured.
Definition: Individuals with the average (intermediate) value of a trait have the highest fitness. Both extremes are selected against.
Effect on bell curve: The curve becomes narrower and taller — the mean stays the same, but variance decreases. Extreme variants are eliminated.
Example 1: Human birth weight. Babies with very low or very high birth weight have higher mortality (premature birth problems vs difficult delivery). Babies of average weight (about 3.5 kg) have the highest survival rate. Natural selection favours the middle.
Example 2: Shell thickness in molluscs. Too thin = crushed by predators; too thick = too heavy to move and find food. Intermediate thickness is favoured.
When it occurs: When the current environment is stable and the existing average trait is well-adapted. No environmental change, no pressure to shift the mean.
Definition: Individuals at one end of the trait distribution have higher fitness. The bell curve shifts in that direction over generations.
Effect on bell curve: The entire distribution shifts toward one extreme. The mean changes.
Example 1: Antibiotic resistance. When antibiotics are used, bacteria with slight resistance have higher fitness. Over time, the entire bacterial population becomes more resistant — the distribution shifts toward the resistant end.
Example 2: The classic case of Biston betularia (peppered moth) during the Industrial Revolution. Before industrialisation, light-coloured moths were camouflaged on light-coloured tree bark. After soot darkened the bark, dark-coloured moths were better camouflaged. Selection favoured one extreme (dark moths), shifting the population toward darker colouration.
Example 3: Larger beak size in medium ground finches (Geospiza fortis) on Daphne Major island during drought — only large, hard seeds available, so birds with larger beaks survived better.
When it occurs: When the environment changes in one direction, making one extreme consistently more advantageous.
Definition: Individuals at both extremes of the trait distribution have higher fitness than those in the middle. The intermediate trait is at a disadvantage.
Effect on bell curve: The single peak splits into two peaks — a bimodal distribution. Over time, this can lead to two distinct subgroups and eventually speciation.
Example 1: Beak size in the African black-bellied seedcracker (Pyrenestes ostrinus). This bird eats seeds that are either very hard or very soft. Birds with intermediate beak sizes are inefficient at both. Large-beaked birds efficiently crack hard seeds; small-beaked birds efficiently eat soft seeds. Birds with medium beaks are outcompeted by both.
Example 2: Oysters in areas with both fine sand and rocky substrate. Medium-sized oysters attach poorly to rocks and sink in sand; extremes are favoured.
When it occurs: When the environment has two distinct niches that reward different extreme traits, and intermediate forms cannot compete effectively in either niche.
| Type | What is favoured | Effect on mean | Effect on variance | Outcome |
|---|---|---|---|---|
| Stabilising | Average/intermediate | No change | Decreases | Population becomes more uniform |
| Directional | One extreme | Shifts | May decrease | Population shifts toward new environment |
| Disruptive | Both extremes | May be stable | Increases (bimodal) | Population may split into two groups |
Why This Works
All three types are the same mechanism — differential reproductive success based on phenotype — applied to different environmental conditions. The environment determines which phenotype is “fit,” and that shapes which end of the distribution survives and reproduces.
Disruptive selection is particularly interesting because it can drive speciation: if the two extreme subgroups stop interbreeding (perhaps because they look different or occupy different habitats), they can eventually become separate species.
Alternative Method
Think of it graphically:
- Stabilising: arrow pointing inward from both sides of the bell curve (squeezing it)
- Directional: one arrow from one side (pushing the curve to one side)
- Disruptive: arrows pointing outward from both sides (pulling the curve apart)
Common Mistake
Students often confuse stabilising selection with “no selection.” Stabilising selection is still strong selection — it actively removes extreme variants from the population every generation. The reason the population “looks the same” is precisely because selection is working hard to maintain the intermediate. Absence of change does not mean absence of selection pressure.