How does selection pressure reshape a population, and why does it sometimes favour the extremes and sometimes the average?
Explain natural selection as a mechanism of evolution and describe directional, stabilising and disruptive selection with reference to their effects on a population
A focused answer to the WACE Year 12 Human Biology Unit 4 dot point on natural selection. The four conditions for selection, fitness and selection pressure, and how directional, stabilising and disruptive selection each shift a population's distribution.
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What this dot point is asking
WACE wants you to explain the mechanism of natural selection precisely and then apply the three types to data or scenarios. This extends the population genetics topic by focusing on selection itself and the shapes it produces in a population's distribution.
The conditions for natural selection
Natural selection follows from four observations:
- Variation: individuals in a population differ in their traits, originally because of mutation and recombination.
- Overproduction: more offspring are produced than the environment can support, so resources are limited.
- Struggle for existence: competition, predation, disease and the environment mean not all survive (this is the selection pressure).
- Differential survival and reproduction: individuals with favourable variations are more likely to survive and reproduce, and because the favourable alleles are heritable, they are passed on. Over generations these alleles become more common in the gene pool.
Directional selection
Directional selection favours one extreme of a trait, so the population mean shifts in that direction over generations. It happens when the environment changes or a new selection pressure appears, making one extreme advantageous. The classic example is antibiotic resistance: when an antibiotic is used, the resistant extreme is favoured and resistance increases. On a graph, the whole distribution moves toward the favoured extreme.
Stabilising selection
Stabilising selection favours the average phenotype and selects against both extremes, so variation is reduced and the population becomes more uniform around the mean. Human birth weight is the standard example: very small and very large babies historically had lower survival, so intermediate birth weights were favoured. On a graph, the distribution becomes taller and narrower around the same mean.
Disruptive selection
Disruptive selection favours both extremes of a trait and selects against the intermediate, so the population may split into two groups. It can occur when two different niches favour different extremes. Over time, if the two groups stop interbreeding, disruptive selection can contribute to speciation. On a graph, the distribution develops two peaks with a dip in the middle.
How this maps to the exam
Expect questions that ask you to state the conditions for natural selection, apply selection to a scenario (often resistance, camouflage or a changing food source), or identify directional, stabilising or disruptive selection from a graph or description. The highest-value skill is reading a distribution and naming the selection type with a justification.
Exam-style practice questions
Practice questions written in the style of SCSA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WACE 20226 marksUsing the example of antibiotic resistance in bacteria, explain how natural selection leads to a change in the characteristics of a population over time.Show worked answer →
A 6 mark response needs the steps of natural selection applied to the example.
- Variation
- Within a bacterial population there is genetic variation; by chance a few bacteria carry an allele giving antibiotic resistance.
- Selection pressure
- When antibiotics are present, they act as a selection pressure: susceptible bacteria are killed but resistant bacteria survive.
- Differential survival and reproduction
- The surviving resistant bacteria reproduce and pass the resistance allele to their offspring.
- Change in allele frequency
- Over many generations the proportion of resistant bacteria increases, so the population becomes predominantly resistant. The characteristics of the population have changed, which is evolution.
Markers reward variation, the selection pressure, differential survival, and the increase in the favourable allele over generations.
WACE 20234 marksDistinguish between directional and stabilising selection, and give one example of each.Show worked answer →
A 4 mark distinguish answer needs the effect on the distribution plus an example each.
Directional selection. Favours one extreme of a range of phenotypes, so the population mean shifts in that direction over time. Example: bacteria becoming increasingly antibiotic resistant.
Stabilising selection. Favours the intermediate phenotype and selects against both extremes, so variation is reduced and the mean stays the same. Example: human birth mass, where very low and very high birth masses have higher mortality, favouring an intermediate mass.
Markers reward the shift-the-mean versus favour-the-middle distinction and a correct example for each.