From the smallest bacteria living in volcanic vents to the tallest mountain ash eucalypt, life on Earth is staggeringly diverse. Yet beneath this diversity lies a unifying principle: all living things are connected through evolutionary history.
Scientists estimate there are around 8.7 million species on Earth, but we have only described about 1.2 million of them.
Write down your answers to these questions before reading on:
Earth hosts an estimated 8.7 million species, from bacteria surviving in volcanic vents to kangaroos bounding across the outback. This variety is not random decoration — it is the product of billions of years of evolutionary change.
Biodiversity exists at three levels:
Scientists classify organisms to organise this diversity. Without classification, biology would be an impossible list of millions of unrelated facts. Classification groups organisms by shared features, which often reflects shared evolutionary history.
Australia is one of 17 megadiverse nations on Earth. We harbour more than one million native species, and roughly 80% of our mammals, reptiles and flowering plants are found nowhere else — they are endemic. Our unique biodiversity is largely a legacy of Gondwana: when Australia separated from other continents around 50 million years ago, its flora and fauna evolved in isolation. This is why we have kangaroos instead of deer, eucalyptus instead of oak, and platypuses instead of beavers.
To manage the staggering variety of life, scientists classify organisms into hierarchical groups based on shared characteristics and evolutionary relationships.
The modern system of biological classification traces back to Swedish botanist Carl Linnaeus (1707–1778). He introduced a hierarchical structure and the system of binomial nomenclature — giving every species a two-part Latin name. The hierarchy runs from broad to specific:
Long before Linnaeus, Aboriginal and Torres Strait Islander Peoples developed sophisticated systems for classifying organisms. These systems are not "primitive" versions of Western science — they are different but equally rigorous ways of understanding the living world, shaped by tens of thousands of years of observation and relationship with Country.
Indigenous classification often groups organisms by:
For example, many Aboriginal nations group plants by the season in which they flower or produce edible parts, rather than by flower structure alone. This creates a classification system that is deeply practical and embedded in place.
If classification organises the diversity of life, evolution explains why that diversity exists. Evolution is the idea that all living things share common ancestors and have changed over time.
In the 19th century, Charles Darwin and Alfred Russel Wallace independently proposed that natural selection is the main mechanism driving evolutionary change. Their insight was simple but profound: individuals in a population vary, and those with traits better suited to their environment tend to survive and reproduce more. Over many generations, populations change.
Evolution is called the unifying theory of biology because it connects every sub-discipline:
The emergence of COVID-19 variants (Alpha, Delta, Omicron) was evolution happening before our eyes. The SARS-CoV-2 virus replicated billions of times inside human hosts. Random mutations occurred during replication. Variants with mutations that helped them spread faster outcompeted slower-spreading variants. This is natural selection operating on a virus — a powerful reminder that evolution is not just ancient history.
Wrong: "Evolution is just a theory, so scientists are not sure it is true."
Right: In science, a theory is a well-tested, evidence-based explanation — not a guess. Evolution is supported by fossils, DNA, anatomy, embryology and biogeography. It is one of the most strongly supported theories in all of science.
Wrong: "Evolution happens to individual organisms during their lifetime."
Right: Evolution happens to populations over generations, not to individuals. An organism is born with its genes; it does not evolve during its life.
1 Red kangaroo (Macropus rufus)
2 Golden wattle (Acacia pycnantha)
3 Bread mould (Rhizopus sp.)
4 Escherichia coli (gut bacteria)
5 Paramecium (a single-celled protist)
1 Choose an Australian organism (e.g., kangaroo, eucalyptus, emu, barramundi). Write its Western scientific name and classification.
2 Describe how an Aboriginal or Torres Strait Islander group might classify or categorise this same organism. What criteria do they use?
3 Explain why both classification systems are valuable. What does each system reveal that the other might not?
1. Which statement best defines biodiversity?
2. In the Linnaean classification system, which is the correct order from broadest to most specific?
3. Which statement about evolution is correct?
4. Aboriginal and Torres Strait Islander classification systems often group organisms by...
5. Why is evolution called the unifying theory of biology?
6. Define biodiversity and explain why Australia is called a megadiverse country. 3 MARKS
7. Describe one way Aboriginal and Torres Strait Islander Peoples classify organisms, and explain how this reflects deep ecological knowledge. 4 MARKS
8. Explain why evolution is described as the 'unifying theory' of biology. Refer to at least two branches of biology in your answer. 5 MARKS
Go back to your Think First responses at the top of the lesson.
1. Red kangaroo: Animalia — multicellular, heterotrophic, motile, lacks cell walls.
2. Golden wattle: Plantae — multicellular, autotrophic (photosynthetic), has cell walls containing cellulose.
3. Bread mould: Fungi — heterotrophic, absorbs nutrients, has chitin cell walls, reproduces by spores.
4. E. coli: Bacteria (Monera) — prokaryotic, single-celled, no nucleus, unicellular.
5. Paramecium: Protista — eukaryotic, mostly unicellular, does not fit neatly into plant, animal or fungi kingdoms.
3. Both systems are valuable: Western taxonomy reveals evolutionary relationships and shared ancestry [1 mark]. Indigenous classification reveals ecological relationships, seasonal cycles and practical uses that Western taxonomy may overlook [1 mark]. Both are evidence-based systems accumulated over long periods — Western science over centuries, Indigenous knowledge over tens of thousands of years [1 mark]. Together they give a richer, more complete understanding of biodiversity [1 mark].
1. C — Biodiversity is defined as the variety of life at genetic, species and ecosystem levels. Options A, B and D are too narrow.
2. A — Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species is the correct hierarchy. Option B reverses it. Option C swaps Domain and Kingdom. Option D swaps Phylum and Class.
3. C — Evolution is a well-supported scientific theory. Option A confuses "theory" with "guess." Option B describes Lamarckism or individual change, which is wrong. Option D is false — evolution applies to all life.
4. B — Indigenous classification uses habitat, season, use and relationships. Option A describes Western taxonomy. Option C is disrespectful and false. Option D is too narrow.
5. B — Evolution unifies genetics, ecology, palaeontology and anatomy. Option A is about consensus, not explanatory power. Option C is historically false. Option D confuses evolution with geology/cosmology.
Q6 (3 marks): Biodiversity is the variety of life on Earth at three levels: genetic, species and ecosystem [1 mark]. Australia is called megadiverse because it harbours more than one million native species and is home to approximately 80% endemic species — found nowhere else on Earth [1 mark]. This high endemism is a result of Australia's long isolation from other continents since the breakup of Gondwana, allowing unique evolutionary pathways [1 mark].
Q7 (4 marks): Aboriginal and Torres Strait Islander Peoples often classify organisms by ecological relationships and seasonal behaviour [1 mark]. For example, plants may be grouped by the season in which they flower or produce edible fruit, or by which animals depend on them [1 mark]. This reflects deep ecological knowledge because it is based on careful observation of species interactions over tens of thousands of years [1 mark]. It is not inferior to Western taxonomy — it is complementary, revealing practical and relational knowledge that Linnaean classification does not capture [1 mark].
Q8 (5 marks): Evolution is called the unifying theory because it provides a single explanatory framework that connects all branches of biology [1 mark]. In genetics, evolution explains why DNA is conserved across species and why mutations create the variation that natural selection acts upon [1 mark]. In palaeontology, evolution explains why simpler fossils appear in older rocks and why transitional fossils like Tiktaalik show intermediate forms [1 mark]. In ecology, evolution explains why species are adapted to their environments and why Australia has such unique fauna [1 mark]. Because evolution makes testable predictions across multiple independent fields, it unifies biology into a coherent scientific framework [1 mark].
Test your knowledge of biodiversity, classification and evolutionary thinking in this fast-paced quiz battle. Correct answers power your attacks!
Climb platforms using your knowledge of biodiversity, classification and evolution. Pool: Lesson 11.
Tick when you have finished all activities and checked your answers.