HSC Exam Practice

Sample response. Speciation is the formation of new and distinct species from an ancestral species [1]. The key criterion from the biological species concept is that members of the same species can interbreed and produce fertile offspring; two populations have become separate species when they can no longer produce fertile offspring together, indicating reproductive isolation [1].

Marking notes. 1 mark for defining speciation (new species from ancestral species / divergence into reproductively isolated species). 1 mark for the biological species concept criterion (interbreed and produce fertile offspring; reproductive isolation marks species boundary).

Sample response. Allopatric speciation begins when a geographic barrier (e.g. ocean, mountain range, river) divides one ancestral population into two isolated groups, sharply reducing gene flow [1]. Each population then independently accumulates different mutations and experiences different selection pressures, causing allele frequencies to diverge over time [1]. Eventually, enough divergence builds up that even if the barrier is removed, the two populations cannot successfully interbreed to produce fertile offspring — reproductive isolation is established and speciation is complete [1].

Marking notes. 1 mark each for: geographic barrier splits population and reduces gene flow; independent divergence through different mutations/selection; reproductive isolation established so fertile interbreeding no longer occurs.

Sample response. Pre-zygotic isolation prevents mating or fertilisation from occurring; examples include different mating seasons, incompatible courtship behaviours, and mechanical incompatibility of reproductive structures [1]. Post-zygotic isolation occurs after fertilisation; the zygote forms but the hybrid offspring either die young or are infertile; the mule (from horse × donkey) is a post-zygotic example because mating and fertilisation occur but the mule is sterile [1]. The key difference is whether the barrier acts before or after the zygote forms [1 for clear distinction].

Marking notes. 1 mark for pre-zygotic (prevents mating/fertilisation; named example). 1 mark for post-zygotic (fertilisation occurs; hybrid fails/sterile; mule example). 1 mark for clear distinction (before vs after zygote forms).

Sample response. Australian marsupials are useful because Australia became geographically isolated after separating from Gondwana, reducing gene flow with placental mammal populations elsewhere [1]. This long-term isolation allowed marsupial lineages to accumulate different adaptations through independent divergence, producing many distinct endemic species over millions of years — illustrating how geographic isolation can lead to population divergence and eventually to the formation of new species through allopatric speciation [1].

Marking notes. 1 mark for identifying Australia’s isolation from Gondwana reducing gene flow. 1 mark for explaining how this led to independent divergence and species formation (endemic marsupials as the outcome).

Sample response. Sympatric speciation can occur when reproductive isolation arises within the same geographic area, commonly through polyploidy — where chromosome number changes suddenly, making the new lineage reproductively incompatible with the original population [1]. Bread wheat is a classic example: it is a hexaploid formed through hybridisation involving three ancestral species, creating a new lineage that could not interbreed with its diploid ancestors, in the same geographic area [1].

Marking notes. 1 mark for mechanism (polyploidy / chromosome change creates immediate reproductive isolation without geographic separation). 1 mark for bread wheat or any valid plant example from the lesson.

Sample response. These populations are most likely separate species (or approaching speciation) [1]. Under the biological species concept, the same species requires interbreeding and fertile offspring. The table shows that when crossed in the lab, hybrid offspring are produced but grow slowly and rarely reproduce. This post-zygotic barrier means that even when fertilisation occurs, successful gene flow does not follow — satisfying the condition for separate species. Additionally, the different breeding seasons (spring vs autumn) would prevent natural mating in the wild, reinforcing reproductive isolation [1].

Marking notes. 1 mark for concluding likely separate species. 1 mark for justifying using biological species concept: hybrid offspring fail to reproduce (post-zygotic barrier) / different breeding seasons prevent natural interbreeding.

Sample response. (i) Different breeding seasons: pre-zygotic barrier [1]. The populations breed at different times of year (spring vs autumn), so they are unlikely to encounter each other during mating periods, preventing mating and fertilisation from occurring in the first place [1]. (ii) Hybrid offspring failing to reproduce: post-zygotic barrier [1]. Fertilisation can occur (offspring are produced), but the hybrids grow slowly and rarely reproduce, meaning gene flow cannot be maintained. The barrier acts after zygote formation.

Marking notes. 1 mark for pre-zygotic classification of different breeding seasons. 1 mark for explanation (different times; prevents mating/fertilisation). 1 mark for post-zygotic classification of hybrid failure (fertilisation occurs; barrier is infertility/failure of hybrids).

Sample response. The mountain range created geographic isolation, separating one ancestral lizard population into two groups on either side and sharply reducing gene flow [1]. Over 800,000 years of independent evolution, each group experienced different selection pressures and accumulated different mutations, leading to divergence in body colouration, mating call frequency and breeding season. As this divergence accumulated, reproductive isolation mechanisms developed (different breeding seasons as pre-zygotic; hybrid failure as post-zygotic), meaning the two populations can no longer successfully exchange genes even if they were brought together. This allopatric speciation process explains the differences observed in Table 2.1 [1].

Marking notes. 1 mark for explaining geographic isolation (mountain range reduced gene flow; started independent divergence). 1 mark for linking divergence to the development of reproductive isolation mechanisms and correctly using all three required terms (geographic isolation, divergence, reproductive isolation).

Sample response. The claim that speciation can only occur when populations are physically separated is incorrect. Geographic isolation is a common and important mechanism for speciation, but it is not the only one.

Speciation is the formation of new and distinct species from an ancestral species. Under the biological species concept, two populations have become separate species when they can no longer interbreed to produce fertile offspring — the decisive criterion is reproductive isolation, not geographic separation per se.

Allopatric speciation does require geographic isolation. A barrier such as an ocean, mountain range, or river divides one population, reducing gene flow. Each group then diverges through different mutations and selection pressures. Over time, reproductive isolation builds up, and even if the barrier is removed, the populations cannot successfully interbreed to produce fertile offspring. Australian marsupials illustrate this: Australia’s separation from Gondwana created a geographic barrier that allowed marsupial lineages to diverge independently over millions of years, producing many endemic species today.

However, sympatric speciation demonstrates that geographic isolation is not required. Sympatric speciation occurs in the same geographic area, most commonly through polyploidy in plants. Polyploidy is when chromosome number doubles (or multiplies), creating a new lineage that is immediately reproductively incompatible with the original diploid population because hybrid offspring between the polyploid and diploid cannot produce functional gametes. Bread wheat is a hexaploid formed through hybridisation of three ancestral species — a new, reproductively isolated lineage created without geographic separation.

The lesson’s misconceptions box explicitly states: “Allopatric speciation requires geographic isolation, but sympatric speciation can occur without it — through polyploidy in plants or resource partitioning that creates reproductive isolation. Geographic isolation is a common mechanism, not a requirement.”

The claim is therefore false: speciation can occur without geographic separation, as demonstrated by sympatric speciation through polyploidy. The essential requirement is that reproductive isolation develops — this can result from geographic isolation (allopatric) or from chromosomal/ecological mechanisms in the same area (sympatric).

Marking criteria (9 marks).

• 1 mark — Explicit evaluative judgement: the claim is incorrect.
• 1 mark — Speciation defined correctly.
• 1 mark — Biological species concept stated with fertile offspring criterion.
• 1 mark — Allopatric speciation mechanism explained (geographic barrier; gene flow; divergence; reproductive isolation).
• 1 mark — Australian marsupial example used correctly (Gondwana isolation; endemic species).
• 1 mark — Sympatric speciation defined and mechanism explained (no geographic barrier; reproductive isolation through other means).
• 1 mark — Polyploidy explained as a mechanism of sympatric speciation.
• 1 mark — Named example of sympatric speciation (bread wheat or other valid example).
• 1 mark — Synthesised conclusion: reproductive isolation (not geographic isolation) is the essential requirement; claims accurately assessed as false.