NCEA Level 3 Biology 91605 Speciation: past papers, practice and marking
- NCEA Level 3
- Biology
- AS91605
- 4 credits
- External exam
About 91605
AS91605, Demonstrate understanding of evolutionary processes leading to speciation. Natural selection, drift, isolation and the origin of species, with the New Zealand examples the exam actually uses.
What 91605 covers
- Natural selectionMutation, named selection pressures, directional, stabilising and disruptive selection, key terms (species, endemic, niche, gene pool)
- Genetic driftGenetic drift, founder effect and bottleneck, genetic diversity and allele frequency change
- IsolationGene flow, allopatric and sympatric speciation, geological events (tectonic movement, orogeny, glaciation, sea level change)
- Isolating mechanismsPrezygotic and postzygotic mechanisms named in context, polyploidy, why groups are not yet separate species
- Patterns of evolutionDivergence, adaptive radiation, convergence, co-evolution, gradualism and punctuated equilibrium
- Evidence for speciationFossils, comparative anatomy, mtDNA and nDNA, biogeography, phylogeny and clades, ring species
91605 past papers
Open a 91605 past paper beside your own Ako page, then get it marked, part by part.
Work through a past paper All NCEA Level 3 Biology past papers
Practice questions
Question 1: Natural selection
A native stick insect lives on two kinds of plant in the same patch of coastal scrub: brown-twigged mānuka and a green-leaved shrub. Green, brown and olive forms occur. Insect-eating birds that hunt by sight take far more olive insects than green or brown ones.
- Explain how bird predation would change the frequency of the colour alleles in this population over many generations.(up to Merit)
- Predict, with a reason, what would happen to the frequency of olive insects if the insect-eating birds were removed from the scrub.(up to Merit)
Check your answer
Part a: what gets the marks
- Predation by insect-eating birds is the selection pressure, because the birds find stick insects that do not match the plant they rest on.
- Green insects on green leaves and brown insects on brown twigs are camouflaged, so they survive and reproduce more than olive insects, which are seen on either plant.
- Green and brown insects pass on their colour alleles, so over many generations the alleles for green and brown are passed on more often and the olive form becomes rarer, splitting the population towards two colour forms.
A full answer: The selection pressure is predation by birds that hunt by sight. Green insects on green leaves and brown insects on mānuka twigs are camouflaged, so they survive and reproduce more than olive insects, which stand out on both plants. Green and brown insects pass on their colour alleles, so over many generations those alleles are passed on more often and the olive form becomes rarer: the population is pushed towards two colour forms.
Part b: what gets the marks
- Without birds hunting by sight, olive insects would no longer be eaten more than green or brown insects.
- Olive insects would survive and reproduce as well as the others, so their colour alleles would no longer be removed from the gene pool and the olive form would become more common or stop declining.
- Green and brown insects would lose their survival advantage, so the population would tend towards a mixture of all three forms.
A full answer: The selection pressure (predation by birds hunting by sight) would be gone, so olive insects would no longer be eaten more than the others. They would survive and reproduce as well as green and brown insects, so their alleles would stop being removed and the olive form would become more common or at least stop declining. Green and brown insects would lose their camouflage advantage, so the population would move towards a mix of all three forms.
What gets each grade
Question 2: Natural selection
In a patch of coastal scrub, a native stick insect has green and brown forms; olive intermediates are eaten more by birds. Green insects live mainly on a green-leaved shrub and brown insects on mānuka. Stick insects walk only a few metres in their lives, and they usually mate on the plant where they live.
- Discuss how disruptive selection could lead to two species forming in this one patch of scrub. In your answer, include what would have to happen to gene flow, and why speciation might not happen.(up to Excellence)
- Predict, with reasons, what would happen to the brown form of the stick insect if all the mānuka were cleared from the patch.(up to Merit)
Check your answer
Part a: what gets the marks
- Sympatric speciation is the formation of new species in the same area, with no geographical barrier between the populations.
- Because the insects move only a few metres and mate on the plant they live on, green insects mostly mate with green and brown with brown, so gene flow between the two forms is reduced (habitat isolation).
- Disruptive selection removes olive offspring, so insects that mate with their own colour leave more surviving young; mating within each form is favoured, new mutations arising in one form stay in it, and the two gene pools build up different alleles until a reproductive isolating mechanism, such as a different mating time, stops them producing fertile offspring.
- If insects often move between the two plants and mate with the other form, gene flow keeps mixing the alleles and olive young keep forming, so the population stays one species with two colour forms; speciation needs gene flow to fall low enough for differences to build up.
A full answer: Sympatric speciation is new species forming in one area with no geographical barrier. Here the insects move only a few metres and mate on their own plant, so green mostly mates with green and brown with brown: habitat isolation reduces gene flow. Disruptive selection removes olive offspring, so insects that mate with their own form leave more surviving young, which strengthens mating within each form. New mutations in one form then stay in it, and the two gene pools build up different alleles until a reproductive isolating mechanism (for example mating at different times) means they can no longer produce fertile offspring. But if insects regularly cross between the plants and mate with the other form, gene flow keeps mixing the alleles and the population stays one species with two colour forms.
Part b: what gets the marks
- Brown insects would only be on green-leaved shrubs, where they stand out, so birds hunting by sight would eat more of them.
- Fewer brown insects would survive to breed, so the brown colour alleles would become rarer and could be lost from the gene pool.
- With only one background, the population would be mostly green, so there would be no selection splitting it into two forms and no pathway to two species.
A full answer: Brown insects would be left only on green-leaved shrubs, where they stand out, so birds hunting by sight would eat more of them. Fewer would survive and reproduce, so the brown colour alleles would become rarer and might be lost. With one background the population would end up mostly green, so disruptive selection would stop and there would be no route to two species forming.