ACARA v9 CONTENT DESCRIPTION “use the theory of evolution by natural selection to explain past and present diversity and analyse the scientific evidence supporting the theory”
Builds on heredity: that traits are passed from parents to offspring through genes, and that populations carry variation. Here we use those ideas to explain how populations change over time, and we look at the scientific evidence that supports the theory of evolution by natural selection.
Variation is the starting point
The individuals in a population are not identical. They differ in heritable traits such as body size, colour or beak shape, and these differences are passed from parents to offspring. Natural selection does not create this variation. It acts on the range of inherited traits that is already present, which is why variation is where the story begins.
Variation in a population
A population is not uniform. Its individuals already differ in heritable traits such as body size. Change the spread to compare.
Real populations carry a range of inherited values for a trait. Natural selection does not create these differences; it acts on the variation that is already present.
Selection pressure
An environmental factor, such as a predator, a climate or the food available, can favour some variants over others. A variant that is better suited to its surroundings tends to survive longer and leave more offspring. Moths resting on bark are a clear case: on pale bark the pale form is camouflaged and the dark form is seen and eaten, while on dark bark the reverse holds. This difference in survival and reproduction between variants is the selection pressure. The organisms do not choose to change; the environment simply removes the variants that do poorly.
Selection pressure and survival
The environment favours some variants. Camouflaged moths survive to breed; the ones that stand out are eaten. Toggle the bark.
On pale bark the pale moths blend in and survive to reproduce, while the dark moths are highlighted and eaten. This difference in survival between variants is the selection pressure.
Change over generations
Because the favoured variant survives and reproduces more, a larger share of the next generation inherits its trait. Repeat this over many generations and the frequency of the favoured trait rises across the whole population. This gradual change in the make-up of a population is natural selection over time. It is important to be precise here: the population changes, not the individual. No single moth darkens during its own life; rather, darker moths become a larger fraction of each new generation.
Trait frequency over generations
The favoured variant breeds more, so its inherited trait grows more common. Step through the generations and watch the frequency climb.
Because the camouflaged moths survive and reproduce more often, a larger share of the next generation inherits the favoured colour. Selection adds up across generations.
Evidence: comparative anatomy and fossils
Several independent lines of evidence support the theory. Comparative anatomy is one. Homologous structures are body parts that share the same underlying plan across different species, even when they are used for different jobs. The human arm, the whale flipper and the bat wing all share one upper bone, two lower bones, and a set of wrist and finger bones. A shared plan in limbs used so differently points to descent from a common ancestor. The fossil record adds another line: sequences of fossils in successive rock layers show features changing gradually over very long spans of time, consistent with populations changing across many generations.
Evidence: homologous limb bones
The same bone plan appears in very different mammal limbs. Compare them and look at the matching bone groups.
The human arm is built for grasping and lifting, yet it keeps the same plan: one upper bone, two lower bones, then wrist and digits. The proportions differ, but the shared layout points to descent from a common ancestor.
Putting it together: finch beaks
Finch populations on islands with different seeds end up with beaks suited to the food available. Where seeds are large and hard, birds with deep strong beaks crack them best and leave more offspring, so the population average beak depth rises over generations. Where seeds are small and soft, slender beaks do better and the population shifts the other way. Researchers have measured these shifts in wild finch populations after droughts changed which seeds were available, which is direct evidence of natural selection in action. The same three ideas run through every example: variation exists, the environment selects, and the population changes over generations.
Worked example: finch beaks and food
Set the seeds on the island, then advance the generations and watch the population average beak shift toward the form that the food favours.
Large hard seeds favour deep strong beaks that can crack them. Those birds survive and breed more, so over generations the population average beak depth rises. Field measurements of finch populations after droughts match this kind of shift.
Why this matters
The theory of evolution by natural selection explains both the present diversity of living things and the patterns we find in the fossil record, using a small set of well-evidenced ideas. Comparative anatomy, the fossil record and measured shifts in wild populations all point the same way, which is why this theory is the central organising model of modern biology.
Quick self-check
1. Before natural selection can act, what must already be present in a population?
2. On dark, soot-stained bark, why do dark moths survive and reproduce more than pale moths?
3. Over many generations of selection on dark bark, what changes?
4. The human arm, whale flipper and bat wing share the same bone plan. What does this homology suggest?
5. On an island where only large hard seeds are available, how does the finch population change over generations?