Evolution and speciation: WACE Year 12 Biology
“Explain natural selection and the mechanisms that drive evolution and lead to speciation”
Evolution is a change in the allele frequencies of a population over generations; natural selection drives it when heritable variation produces differences in survival and reproduction, and speciation occurs when reproductive isolation splits one population into groups that can no longer interbreed.
What this dot point is asking
SCSA wants you to treat evolution as a population-level process measured by changing allele frequencies, not as individuals "trying" to change. A strong answer links the source of variation to the agents of selection, and then shows how isolation turns one gene pool into two.
Variation and the gene pool
A population is a group of interbreeding individuals of the same species. The gene pool is the total collection of alleles in that population. Evolution is defined as a change in the relative frequencies of alleles in a gene pool from one generation to the next.
Variation is the raw material. It arises from:
- Mutation, the ultimate source of new alleles.
- Meiosis, which shuffles existing alleles through crossing over and independent assortment.
- Sexual reproduction, which combines alleles from two parents through random fertilisation.
Without heritable variation there is nothing for selection to act on.
Natural selection
Natural selection follows a clear logic, often summarised as variation, selection pressure, differential survival and reproduction, and inheritance.
- Individuals in a population vary in their heritable characteristics.
- More offspring are produced than the environment can support, so there is competition for resources (a struggle for existence).
- A selection pressure (predation, disease, climate, food supply) means some variants survive and reproduce more successfully than others.
- These individuals pass their advantageous alleles to the next generation.
- Over many generations the favourable alleles become more common in the gene pool.
Selection acts on the phenotype of individuals, but evolution is measured in the population. Individuals do not evolve; populations do, as allele frequencies shift.
Other mechanisms of evolution
Natural selection is not the only driver of allele frequency change.
- Genetic drift is random change in allele frequencies, strongest in small populations. Chance events can remove or fix alleles regardless of their fitness.
- The bottleneck effect occurs when a population is drastically reduced (for example by disaster), leaving a small, less diverse gene pool.
- The founder effect occurs when a few individuals start a new population, carrying only a sample of the original gene pool.
- Gene flow is the movement of alleles between populations through migration, which tends to keep populations similar.
Selection patterns
- Directional selection favours one extreme phenotype, shifting the mean (for example, larger body size).
- Stabilising selection favours the intermediate phenotype and reduces variation (for example, human birth mass).
- Disruptive selection favours both extremes over the middle and can split a population.
Trace the change in allele frequency
A bacterial population varies in resistance to an antibiotic. When the antibiotic is applied it is the selection pressure: susceptible cells die, resistant cells survive and reproduce. The resistance allele rises in frequency, so the next generation is more resistant. Note the resistant variants existed before the drug; the drug did not create them, it selected for them.
Speciation
A species is commonly defined (the biological species concept) as a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of new species from existing ones, and it requires reproductive isolation so that two groups stop exchanging alleles.
Allopatric speciation is the most common pathway:
- A geographic barrier (mountain range, river, ocean, new desert) physically separates a population.
- Gene flow between the groups stops.
- Each group experiences different mutations, selection pressures and drift.
- The gene pools diverge until, even if the groups meet again, they can no longer interbreed to produce fertile offspring.
Sympatric speciation occurs without a geographic barrier, through reproductive isolation arising within the same area (for example, polyploidy in plants or a shift in food source or breeding time).
Isolating mechanisms
Reproductive isolation can be prezygotic (preventing mating or fertilisation) or postzygotic (acting after fertilisation):
- Prezygotic: temporal (different breeding times), behavioural (different courtship), geographic, morphological (incompatible body structures), and gametic incompatibility.
- Postzygotic: hybrid inviability, hybrid sterility (such as the mule), and hybrid breakdown in later generations.
Gene pool: all the alleles in a population. Allele frequency: how common an allele is in the gene pool. Speciation: formation of a new species. Reproductive isolation: barriers preventing interbreeding between populations.
Evidence for evolution
You should be able to cite supporting evidence: the fossil record showing transitional forms and change over time, comparative anatomy (homologous structures from common ancestry, analogous structures from convergent evolution, and vestigial structures), comparative embryology, biogeography, and molecular evidence such as similarities in DNA and protein sequences that allow phylogenetic trees to be built.
Saying organisms evolve to suit their needs or that they "want" to adapt. Selection acts on existing variation; it has no purpose or foresight.
Confusing an individual changing with a population evolving. An organism cannot evolve in its lifetime; allele frequencies change across generations.
Treating mutation as the same thing as evolution. Mutation supplies variation; evolution is the change in frequency of that variation over time.
Forgetting that speciation requires reproductive isolation, not just visible difference. Two groups can look different yet remain one species if they still interbreed.
Exam-style questions
Questions in the style of SCSA exam questions on this dot point, each with a worked answer. They are written by ExamExplained unless tagged "Past paper"; the year shows the paper a question is modelled on.
WACE 2022-style7 marksA population of beetles varies in colour. A new predator that hunts by sight moves into the area. Over many generations the population becomes predominantly the cryptic (camouflaged) colour. Explain this change using the process of natural selection.Show worked answer →
A 7 mark explain answer should walk through the steps of natural selection applied to the example.
- Variation
- The beetle population already shows heritable variation in colour (some cryptic, some conspicuous) due to different alleles.
- Selection pressure
- The sight-hunting predator is a selection pressure. Conspicuous beetles are seen and eaten more often than cryptic ones.
- Differential survival and reproduction
- Cryptic beetles are more likely to survive and reproduce, so they leave more offspring.
- Inheritance
- The alleles for cryptic colour are passed to offspring, so the cryptic phenotype becomes more common.
- Outcome over generations
- Repeated over many generations, the frequency of cryptic-colour alleles rises in the gene pool, so the population becomes predominantly cryptic. The variation pre-existed; selection changed its frequency.
Markers reward variation, the named selection pressure, differential reproduction, inheritance and the population-level frequency change over generations.
WACE 2024-style6 marksExplain how a single population can give rise to two separate species by allopatric speciation, referring to gene flow and reproductive isolation.Show worked answer →
A 6 mark explain answer needs the geographic mechanism linked to isolation.
- Geographic separation
- A geographic barrier (such as a river, mountain range or rising sea) physically divides one population into two.
- Gene flow stops
- The barrier prevents the two groups interbreeding, so alleles no longer move between them (no gene flow).
- Independent divergence
- Each group experiences different mutations, selection pressures and genetic drift, so their gene pools change in different directions over many generations.
- Reproductive isolation
- The gene pools diverge enough that, even if the barrier is removed and the groups meet, they can no longer interbreed to produce fertile offspring. They are now two species (biological species concept).
Markers reward the barrier, loss of gene flow, independent divergence of gene pools and the resulting reproductive isolation defining two species.