Natural selection and types of selection: WACE Year 12 Human Biology Unit 4
“Explain natural selection as a mechanism of evolution and describe directional, stabilising and disruptive selection with reference to their effects on a population”
Natural selection is the process by which individuals with favourable variations survive and reproduce more, so favourable alleles become more common over generations. It needs variation, overproduction, a struggle to survive and heritability. There are three patterns: directional selection favours one extreme and shifts the mean; stabilising selection favours the average and reduces variation; disruptive selection favours both extremes against the middle.
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.
In biology, fitness is an individual's relative ability to survive and reproduce in its environment, measured by the number of fertile offspring it leaves. It does not mean physically strong or healthy; an organism is fitter if it leaves more surviving, reproducing offspring than others in the same population.
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.
Directional selection shifts the mean toward one extreme. Stabilising selection narrows the distribution around the existing mean. Disruptive selection splits the population by favouring both extremes over the middle. When given a before-and-after distribution, name the type by how the shape changes, not just by which individuals survived.
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.
Reading a before-and-after graph
Suppose a trait such as beak depth is measured in a bird population before and after a drought. If after the drought the mean beak depth has moved toward larger beaks (because only birds with large beaks could crack the hard seeds that remained), this is directional selection: a new pressure favoured one extreme and shifted the mean. If instead the spread of beak depths had narrowed around the original mean, it would be stabilising selection. If two peaks had appeared at small and large beaks with few intermediates, it would be disruptive selection. A marker wants you to name the type and justify it by describing how the distribution changed and which phenotype was favoured under the selection pressure.
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.
Two errors recur. First, claiming the environment or the need for a trait creates it; selection acts on variation that already exists from mutation, it does not produce new traits on demand. Second, saying an individual evolves or adapts in its lifetime; selection changes allele frequencies in a population across generations. State the change at the population level, and describe selection as favouring existing variants, not creating them.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style6 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 2023-style4 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.