← Unit 4: Heredity and continuity of life
Topic 3: Continuity of life on Earth
Explain natural selection as a mechanism of evolution including variation, selection pressure, differential survival and reproduction, fitness, and compare Darwinian and neo-Darwinian theories
A focused answer to the QCE Biology Unit 4 dot point on natural selection. Covers the four preconditions (variation, heritability, differential survival and reproduction), defines fitness as reproductive success, distinguishes Darwin's theory from the neo-Darwinian synthesis (Mendelian genetics, mutation, population genetics) with examples in peppered moths, bacteria and cane toads.
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What this dot point is asking
QCAA wants you to explain how natural selection produces evolution, define the technical terms (variation, selection pressure, fitness, differential survival and reproduction), and compare Darwin's original theory with the modern synthesis. Worked examples on real populations (peppered moths, antibiotic resistance, Galapagos finches, cane toads) are the standard format.
The answer
Evolution is descent with modification: change in the heritable characteristics of populations over generations. Natural selection is one of the main mechanisms (alongside mutation, genetic drift and gene flow). Charles Darwin and Alfred Russel Wallace proposed natural selection independently in 1858, and Darwin's On the Origin of Species (1859) laid out the case.
The four preconditions for natural selection
Natural selection acts whenever all four of the following are true.
- Variation. Individuals in a population differ in their traits (size, colour, behaviour, biochemistry).
- Heritability. Some of that variation is genetic and so is passed from parents to offspring.
- Differential survival. Some traits help individuals survive in the current environment better than others. Survival depends on the environment, not on the merit of the trait in isolation.
- Differential reproduction. Survivors that go on to reproduce contribute more offspring (and therefore more copies of their alleles) to the next generation.
Over many generations the frequency of beneficial alleles rises and the frequency of harmful alleles falls. Evolution is the resulting change in allele frequencies.
Key terms
- Selection pressure. A factor in the environment that affects survival or reproduction (predators, food availability, climate, parasites, antibiotics, sexual partners). Pressures can change over time.
- Fitness. Reproductive success relative to other individuals in the population. Fitness is not strength or speed; it is the number of viable offspring contributed to the next generation. A strong, fast antelope that dies before breeding has zero fitness.
- Adaptation. A heritable trait that increases an organism's fitness in its current environment. Adaptations include structural (camouflage), physiological (kidney function in desert mammals) and behavioural (migration timing) traits.
- Selection coefficient. The fractional reduction in fitness of one genotype compared with the fittest.
Types of selection
- Directional selection. One extreme of a trait is favoured. The mean shifts toward that extreme. Examples: peppered moths darkening during industrial pollution; cane toads in Australia evolving longer legs at the invasion front because long-legged individuals disperse fastest.
- Stabilising selection. Intermediate phenotypes are favoured and extremes are selected against. The mean stays the same but the variance shrinks. Example: human birth weight, where very small and very large babies have higher mortality.
- Disruptive selection. Both extremes are favoured over the intermediate. Can lead to bimodal distributions and, eventually, speciation. Example: African seedcracker finches with two distinct beak sizes specialised for hard and soft seeds.
- Sexual selection. Selection acts on traits that influence mating success (elaborate male displays, antlers, female choice).
Worked examples
Peppered moths in industrial Britain. Before industrial revolution: pale moths (peppered) camouflaged on lichen-covered tree bark; dark (melanic) form rare. As soot killed lichens and darkened bark, pale moths became visible to bird predators. By 1900, dark form was over 95 per cent in industrial areas. After clean air legislation, lichens recovered and the pale form recovered. Selection pressure: bird predation. Allele frequency tracked the environment.
Antibiotic resistance in bacteria. Mutations conferring resistance occur spontaneously at low frequency. Antibiotic application kills sensitive cells. Resistant survivors reproduce and pass the resistance alleles on. Resistance allele frequency rises rapidly because of fast bacterial generation times and large population sizes. MRSA, vancomycin-resistant enterococci and multidrug-resistant TB are major public health consequences.
Cane toads in northern Australia. Released in 1935 to control sugarcane pests, they have expanded across northern Australia. The invasion front is moving faster than the rear, because the toads with the longest legs disperse fastest, reach new ground first, and have higher reproductive success there. Leg length at the invasion front has increased by about a third in 70 years.
Galapagos ground finches. Beak depth in Geospiza fortis tracks rainfall. In dry years only large hard seeds remain; large-beaked birds survive and breed. In wet years small soft seeds dominate and beak depth declines. The Grants documented annual selection events over decades.
Darwinian versus neo-Darwinian theory
Darwin's original theory (1859).
- Populations vary.
- Variation is heritable (mechanism unknown to Darwin).
- More offspring are produced than can survive.
- Individuals with traits that suit them to the environment survive and reproduce more.
- Over many generations this produces new species.
Limitations Darwin faced.
- He did not know about Mendelian particulate inheritance (Mendel's work was published in 1866 but ignored until 1900).
- He had no theory for the source of new variation. He proposed pangenesis, which was wrong.
- Many contemporaries thought variation would be diluted by blending inheritance.
Neo-Darwinian modern synthesis (1930s to 1940s). Fisher, Haldane, Wright, Mayr, Dobzhansky, Simpson and others integrated:
- Mendelian genetics. Discrete alleles preserve variation across generations (no blending).
- Mutation. The ultimate source of new alleles.
- Population genetics. Allele frequencies change under selection, mutation, drift, gene flow and non-random mating. The Hardy to Weinberg equilibrium gives a null model.
- Speciation theory. Reproductive isolation produces new species (allopatric and sympatric speciation).
- Palaeontology and biogeography. Fossil and geographic patterns consistent with descent with modification.
The modern synthesis defines evolution as change in allele frequencies in populations over generations, with natural selection as one (very important) cause among several. Subsequent additions (molecular evolution, neutral theory, evo-devo, epigenetics) have extended but not replaced this framework.
Common traps
Defining fitness as physical strength. Fitness is reproductive success.
Saying organisms "evolve" in their lifetime. Individuals do not evolve; populations do, across generations.
Treating mutation as directed by the environment. Mutations are random with respect to need. The environment selects among the variants already present.
Confusing acclimation with adaptation. A single individual acclimatises to heat (physiology). A population adapts to heat through changes in allele frequency over generations.
Forgetting that natural selection is just one mechanism. Genetic drift (especially in small populations), gene flow and mutation also change allele frequencies.
In one sentence
Natural selection operates when heritable variation, a selection pressure and differential survival and reproduction combine so that fitter genotypes leave more offspring, raising the frequency of their alleles in the next generation; the original Darwinian theory described this mechanism without a model of inheritance, while the neo-Darwinian modern synthesis added Mendelian genetics, mutation as the source of new variation, and population genetics to define evolution as change in allele frequencies over generations.
Past exam questions, worked
Real questions from past QCAA papers on this dot point, with our answer explainer.
2024 QCAA6 marksExplain how natural selection acts on a population of bacteria exposed to an antibiotic to produce a resistant population. Refer to variation, selection pressure, differential survival, fitness and the role of mutation, and explain how this is an example of evolution in action.Show worked answer →
A 6-mark answer needs every named term and a clear mechanism.
Initial variation. A bacterial culture is genetically variable. Random mutations produce a small fraction of cells with alleles that confer resistance (a mutation in the antibiotic target gene, or production of an enzyme that breaks down the drug). The variation already exists; the antibiotic does not create it.
Selection pressure. Application of the antibiotic kills most cells. Survival now depends on the resistance genotype.
Differential survival. Sensitive cells die. Resistant cells survive.
Differential reproduction. Resistant survivors reproduce, passing the resistance allele to their descendants. Bacteria reproduce rapidly (one division every 20 to 30 minutes in optimal conditions), so the resistant allele increases in frequency very quickly.
Fitness. In the presence of the antibiotic, resistant cells have higher fitness (they contribute more offspring to the next generation). Without the antibiotic, resistance may carry a small fitness cost, which is why resistant strains can decline if the antibiotic is withdrawn (although they often persist).
Role of mutation. Mutation is the source of new resistance alleles. Horizontal gene transfer (conjugation, transformation, transduction) spreads existing resistance genes between species.
Why this is evolution. Evolution is a change in allele frequencies in a population over generations. The frequency of resistance alleles has risen from near zero to near 100 per cent. This is evolution observed in real time, and it is the basis for hospital and community concerns about MRSA, VRE and multidrug-resistant TB.
Markers reward variation, the pressure, the differential survival and reproduction, fitness defined as reproductive success, mutation as the source of new alleles and an explicit statement that allele frequency change equals evolution.
2023 QCAA4 marksCompare Darwin's original theory of natural selection with the neo-Darwinian modern synthesis. Identify two limitations of Darwin's theory and explain how the modern synthesis addressed each.Show worked answer →
A 4-mark answer needs the comparison and two limitations with their resolutions.
Darwin's theory (1859). Populations vary; variation is heritable; more offspring are produced than can survive; those with traits that suit them to the environment survive and reproduce more (natural selection); over many generations this produces new species. Darwin did not know the mechanism of heredity or the source of variation.
Neo-Darwinian modern synthesis (1930s to 40s). Integrated Mendelian genetics, population genetics (Fisher, Haldane, Wright) and palaeontology with natural selection. Evolution is now defined as change in allele frequencies in populations over generations, driven by selection, mutation, genetic drift, gene flow and non-random mating.
Limitation 1: source of variation. Darwin could not explain where variation came from or why it persisted. The synthesis added mutation as the ultimate source of variation, and meiosis (independent assortment, crossing over) plus random fertilisation as recombination mechanisms.
Limitation 2: mechanism of inheritance. Darwin's contemporary model of blending inheritance would have caused variation to disappear quickly. The synthesis added Mendelian particulate inheritance (discrete alleles that retain their identity through generations) and Hardy to Weinberg equilibrium showing that allele frequencies remain stable in the absence of selection.
Markers reward identifying Darwin's two main gaps (variation and inheritance), naming Mendelian genetics and mutation as the synthesis additions, and the population genetics frame.
Related dot points
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- Apply Mendel's laws of segregation and independent assortment to predict the outcomes of monohybrid and dihybrid crosses using Punnett squares, and explain the purpose of a test cross
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