Read Natural Selection Through Examples
Natural selection changes trait frequencies when heritable differences lead to different reproductive success. Read each example for the direction of selection, not just the organisms named.
Natural Selection is the central idea in this question pattern. Identify the condition in the stem before applying the method below.
Identify the shape of selection
Directional selection favours one end of a trait range, shifting the population distribution in one direction. Stabilising selection favours intermediate values and selects against extremes. Disruptive selection favours both extremes over intermediates. In a question, first ask whether one extreme, the middle, or both extremes have the higher survival or reproductive success.
Measure fitness by offspring
Darwinian fitness is relative reproductive success: how much an individual contributes to later generations. A longer lifespan alone does not mean greater fitness if it leaves fewer offspring. Natural selection acts on heritable variation; variants that improve survival or reproduction tend to become more common over generations.
Read selection experiments
Antibiotic-resistance experiments show that resistant variants can exist before exposure; the antibiotic selects among variants rather than instructing bacteria to mutate. Compare populations with and without the selecting condition to identify whether the condition favours a pre-existing heritable variant.
Separate natural from artificial selection
In natural selection, environmental conditions affect which heritable variants leave more offspring. In artificial selection, people choose which organisms reproduce—for example, selectively breeding plants for disease resistance. Directional selection may occur naturally or through human choice; classify it by the selecting agent and the change in trait distribution.
Connect selection examples to Darwin
A shift toward lower light preference is directional selection; highest success of intermediate-weight duck eggs is stabilising selection. Darwin's account involves heritable variation, struggle and differential reproduction with gradual change—not saltation, or sudden leaps. Generation time also affects how quickly changes can accumulate.
Recognise a directional shift
When selection repeatedly favours one phenotypic extreme, the population mean moves in that direction over generations. Selecting the most bristly flies shifts average bristle number upward: this is directional selection, even though the selecting agent is an experimenter. Choose D.
See disruptive selection in resource niches
When individuals at both ends of a trait range can exploit underused resources better than intermediates, selection favours both extremes. Here small- and large-billed birds can specialise on the available seed extremes, while intermediate bills compete poorly. That branching pattern is disruptive selection, not a compromise around the most abundant seed. Choose D.
OlympIQ exam lens
- ✓One extreme favoured → directional; intermediate favoured → stabilising; both extremes favoured → disruptive.
- ✓Fitness means reproductive contribution, not age or lifespan by itself.
- ✓Selection acts on heritable variation; the environment selects variants rather than producing a needed one on demand.
- ✓People choose breeders → artificial selection; predators, climate, antibiotics or other conditions → natural selection.
- ✓Darwinian change is gradual across generations, not saltation.
- ✓Repeated selection for one extreme shifts the population mean directionally.
- ✓Underused resources at both trait extremes can favour disruptive selection.
Do not confuse survival time with fitness or assume a changed environment creates the useful variation. Identify the inherited trait, who or what selects, and which individuals leave more offspring.
Ready to try the pattern?
100% coverage across the 10 linked questions.
Practise 10 linked questions