VCE Vault

Year 10 Science · Biological Sciences · 18m read

Evolution and Natural Selection

Ever wonder why finches in the Galápagos Islands have different beak shapes, or why your DNA is so similar to a chimpanzee's? Evolution by natural selection explains how organisms change over time—organisms with traits better suited to their environment survive and pass those traits to offspring, gradually shaping new species. Understanding this isn't just fascinating biology; it's crucial for VCE exams and explains everything from disease resistance to conservation, helping you see how life adapts and thrives.

Learning objective: Explain evolution by natural selection and evaluate evidence for evolution.

By the end of this lesson you can:

  • I describe the four steps of natural selection.
  • I evaluate multiple lines of evidence for evolution.
  • I distinguish natural selection from other evolutionary mechanisms.

Natural selection is nature's talent show. Variation provides contestants; the environment is the judge; winners survive and pass on their traits.

Natural selection

CHARLES DARWIN (1859) — On the Origin of Species.

NATURAL SELECTION — the mechanism by which evolution happens.

FOUR-STEP PROCESS:
1. VARIATION — individuals within a species differ (due to genetic mutations, gene combinations).
2. OVERPRODUCTION — populations produce more offspring than can survive.
3. SELECTION — individuals with traits better suited to environment survive and reproduce.
4. INHERITANCE — favourable traits are passed to offspring.

Over MANY generations, the population's traits shift — this is EVOLUTION.

TYPES OF SELECTION:
• Natural (environmental pressures).
• Artificial (human selection — dog breeds).
• Sexual (mate preference — peacock feathers).

EVIDENCE FOR EVOLUTION:
1. FOSSIL RECORD — progression of species over millions of years.
2. COMPARATIVE ANATOMY — homologous structures (same bones in human arm, whale flipper, bat wing).
3. BIOGEOGRAPHY — similar species in similar habitats but distant locations.
4. EMBRYOLOGY — early embryos of different species look remarkably similar.
5. MOLECULAR EVIDENCE — DNA similarities. Closer species have more similar DNA.
6. DIRECT OBSERVATION — bacteria antibiotic resistance; peppered moths; Darwin's finches.

COMMON MISCONCEPTIONS:
• "Evolution has a goal" — FALSE; it's not directed.
• "Individuals evolve" — FALSE; populations evolve.
• "Humans evolved from monkeys" — FALSE; humans and monkeys share a common ancestor.

GENETIC DRIFT:
• Random changes in allele frequency (e.g., bottleneck events).

Darwin's key ideas

  • Variation exists: Small differences within populations.
  • Struggle for survival: Not all offspring survive.
  • Fit thrive: Best-suited pass on genes.

Quick reference

| Step | Description |
|---|---|
| Variation | Differences exist |
| Selection pressure | Predators, climate, disease |
| Survival | Best-suited survive |
| Reproduction | Pass on alleles |
| Evidence | Fossils, DNA, comparative anatomy |

Worked example (Medium)

Explain how the peppered moth illustrates natural selection during the Industrial Revolution.

  1. Before. Most peppered moths were light-coloured, camouflaged against lichen-covered trees.
  2. Change. Soot from factories darkened tree bark, killed lichens.
  3. Selection. Dark moths now camouflaged; light moths eaten by birds.
  4. Result. Proportion of dark moths rose dramatically within decades.

Answer: A clear example of NATURAL SELECTION shifting allele frequencies in response to environmental change.

Evolution Concepts

  • Hardy-Weinberg equilibrium: p + q = 1, \quad p^2 + 2pq + q^2 = 1 (allele frequencies $p$ and $q$)

Worked example (Hard)

Australian bilbies live in pale desert habitats. Explain how natural selection could increase the frequency of pale fur alleles over many generations.

  1. Variation. Random mutations create individuals with slightly different fur shades — from dark to pale.
  2. Selection pressure. In pale desert environments, pale-furred bilbies are better camouflaged from predators. Darker individuals are spotted more easily and eaten more often.
  3. Differential reproduction. Pale bilbies survive longer and reproduce more, passing pale fur alleles to more offspring.
  4. Generational change. Over many generations, the frequency of pale alleles increases until pale colouration predominates in the desert population.

Answer: Natural selection increases the frequency of pale alleles because pale fur confers a survival advantage in desert habitat.

Common mistakes

  • Saying organisms deliberately evolved or adapted Evolution is not directed or intentional. Mutations occur randomly; natural selection merely acts on the variation that exists. Fix: Natural selection acts on EXISTING random variation. Organisms do not choose to change. Favourable variants survive more and pass on their alleles.
  • Saying individuals evolve POPULATIONS evolve over many generations. An individual organism cannot change its genotype. Fix: Evolution = change in allele frequencies in a POPULATION over GENERATIONS.
  • Confusing homologous and analogous structures Homologous structures share common ancestry (human arm and whale flipper). Analogous structures serve the same function but evolved independently (fish fin and whale tail). Fix: Homologous = common ancestor evidence. Analogous = convergent evolution. Different lines of evolutionary evidence.

Practice questions

  1. Which of the following is NOT a necessary condition for natural selection?

    1. Variation within the population.
    2. Heritability of traits.
    3. All individuals surviving equally.
    4. Differential reproductive success.

    (1 marks)

  2. Describe THREE types of evidence for evolution.

    (2 marks)

  3. Bacteria evolve antibiotic resistance rapidly. Explain how, using the four steps of natural selection.

    (3 marks)

  4. What is genetic drift and when does it have the greatest effect on allele frequencies?

    (2 marks)

Going Deeper — Speciation

SPECIATION is the formation of new species through evolution.

ALLOPATRIC SPECIATION (most common): geographic separation prevents gene flow → natural selection and drift cause populations to diverge → reproductive isolation develops → new species.

SYMPATRIC SPECIATION: new species form within the same geographic area (common in plants via polyploidy).

AUSTRALIAN EXAMPLE: when Australia separated from Gondwana ~45 million years ago, marsupials evolved in isolation from placental mammals. This produced extraordinary marsupial diversity through allopatric speciation — kangaroos, wombats, Tasmanian devils, koalas.

CONVERGENT EVOLUTION: unrelated species independently evolve similar traits in similar environments. Example: the Australian marsupial mole and the placental mole — different ancestry, same burrowing lifestyle, similar body shape.

Summary

  • Natural selection: variation → overproduction → differential survival → inheritance → evolution.
  • POPULATIONS evolve over generations. Individuals do not evolve.
  • Evidence: fossil record, comparative anatomy (homologous structures), molecular (DNA), direct observation.
  • Genetic drift: random changes; greatest in small populations (bottleneck, founder effect).
  • Speciation: allopatric (geographic isolation) is the most common mechanism.

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