How does natural selection lead to adaptation over generations?
Explain natural selection and how it produces adaptation through differential survival and reproduction
Natural selection acts on heritable variation: individuals with favourable traits survive and reproduce more, so over generations advantageous alleles increase and populations become adapted.
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What this dot point is asking
You need to explain the conditions required for natural selection, set out the logical steps of the process, and explain how it leads to adaptation. You should be able to apply the mechanism to a worked example.
The conditions for natural selection
Natural selection, proposed by Charles Darwin and Alfred Russel Wallace, depends on four conditions:
- Variation - individuals in a population differ in their characteristics.
- Heritability - some of this variation is genetic and can be passed to offspring.
- Overproduction and competition - more offspring are produced than can survive, so individuals compete for limited resources (a "struggle for existence").
- Differential survival and reproduction - individuals with traits better suited to the environment are more likely to survive and reproduce.
The source of the original variation is mutation (and, in sexual species, the reshuffling of alleles), linking this dot point back to Topic 1.
The process step by step
- A population shows genetic variation in some trait.
- The environment exerts a selection pressure (such as a predator, climate, or limited food).
- Individuals whose traits give them an advantage are more likely to survive ("survival of the fittest").
- These survivors reproduce and pass on the advantageous alleles.
- Over many generations, the frequency of the advantageous alleles increases in the population, and the population becomes adapted to its environment.
What adaptation means
An adaptation is an inherited feature that improves an organism's chances of survival and reproduction in its environment. Adaptations can be structural (body features), physiological (internal functioning) or behavioural. Natural selection produces adaptation gradually, as favourable alleles accumulate over generations.
Divergent and convergent evolution
Two patterns of evolution recur in SACE questions. Divergent evolution occurs when one ancestral population splits and its descendants adapt to different environments, becoming increasingly different over time - this produces homologous structures, such as the pentadactyl limb shared by mammals with different functions. Australian marsupials radiating from a common ancestor into koalas, wombats and kangaroos is the classic local example. Convergent evolution occurs when unrelated lineages face similar selection pressures and independently evolve similar features - this produces analogous structures, such as the streamlined body shape of sharks (fish) and dolphins (mammals), or echolocation in bats and dolphins. The test is ancestry: homologous structures share an ancestor and an underlying plan; analogous structures share only a function.
Selection is not the only force
It is worth keeping natural selection in context. Selection is the only mechanism that consistently produces adaptation, because it is non-random with respect to fitness. However, allele frequencies in a population can also change through genetic drift (random chance, strongest in small populations), gene flow (migration of alleles between populations) and mutation (the original source of new alleles). Strong evolution answers acknowledge that observed change might be due to selection or to these other mechanisms, and use the data provided to decide which best explains the pattern.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SACE 20186 marksThe koala is a marsupial living across eastern and southern Australia. Give two examples of human activities that could create selection pressures on the koala population, and describe how these activities may increase the likelihood of the koala becoming extinct.Show worked answer →
A full answer gives two distinct human activities and explains the selection-pressure and extinction link for each.
Example 1, land clearing and urban development: removing eucalypt forest destroys koala habitat and food, so only koalas able to survive in fragmented habitat persist. Smaller, isolated populations have reduced numbers and gene flow, increasing extinction risk.
Example 2, introduced threats from human activity (for example dogs, vehicles on roads, or human-driven climate change increasing bushfires and heat stress): these act as strong selection pressures that kill many individuals. Heavy mortality reduces population size and genetic diversity.
Across both: as populations shrink, genetic diversity falls, the ability to adapt to further change decreases, and the chance of local extinction rises. Markers reward two clearly different activities, each linked to a selection pressure and then to extinction.
SACE 20191 marksBats and dolphins are genetically unrelated but independently evolved the ability to navigate using sound (echolocation). Name the type of evolution that resulted in both bats and dolphins having the ability to use sound to navigate.Show worked answer →
Convergent evolution. Unrelated species exposed to similar selection pressures (here, the need to navigate or hunt where vision is limited) independently evolve similar adaptations, in this case echolocation. The mark is for naming convergent evolution.
SACE 20196 marksCompare the type of evolution that resulted in many marsupials evolving from a common ancestor with the type of evolution that resulted in bats and dolphins using sonar to navigate.Show worked answer →
A full comparison names both types and explains them with the examples.
Marsupials from a common ancestor: this is divergent evolution. A single ancestral population diversified into many species (koalas, wombats, kangaroos) as different subpopulations adapted to different environments and selection pressures, so related species developed different features (homologous structures).
Bats and dolphins using sonar: this is convergent evolution. These two unrelated lineages independently evolved the same feature (echolocation) because they faced similar selection pressures, producing analogous structures despite no recent common ancestor.
Comparison: both are driven by natural selection acting on variation, but divergent evolution produces increasing difference from one common ancestor, whereas convergent evolution produces similarity in unrelated lineages. Markers reward correctly naming both types and contrasting common-ancestor divergence with independent convergence.