Speciation

Q1 — Sample Band 6 response (7 marks), annotated

The claim that speciation always requires geographic isolation is incorrect. [1 — explicit judgement]

Speciation is the formation of new and distinct species from an ancestral species, and requires reproductive isolation so that gene flow between diverging groups stops. The biological species concept defines a species as a group of organisms that can interbreed and produce fertile offspring. This means that speciation has occurred when two populations can no longer produce fertile offspring together — not simply when they look different or live apart. [1 — speciation definition + biological species concept with fertile offspring criterion]

Allopatric speciation begins when a geographic barrier divides one ancestral population. The two groups stop exchanging genes and experience different selection pressures, mutations and adaptations. Over generations, divergence builds until reproductive isolation develops. Australian marsupials illustrate this: when Australia separated from Gondwana, geographic isolation reduced gene flow with mammal populations elsewhere. Over millions of years, marsupial lineages diversified under distinctive Australian conditions, producing many endemic species today. [1 — allopatric speciation mechanism with Australian example]

However, sympatric speciation demonstrates that geographic isolation is not required. Sympatric speciation occurs in the same geographic area, most commonly in plants through polyploidy — where chromosome number doubles suddenly, creating a reproductively isolated lineage that can no longer interbreed with the ancestral diploid population. Bread wheat provides an example: it is a hexaploid formed through hybridisation involving three ancestral species, creating a new lineage in the same geographic area. [1 — sympatric speciation with polyploidy example]

Both allopatric and sympatric speciation produce reproductive isolation, but through different mechanisms. Geographic isolation is a common cause of speciation but it is not the only cause. The key requirement in both cases is that gene flow stops, allowing divergence to build until the populations can no longer produce fertile offspring. [1 — comparison identifying gene flow cessation as the true requirement]

The claim is therefore partly correct (allopatric speciation via geographic isolation is common and important) but flawed in its absolutism. Geographic isolation is a mechanism, not a requirement. The lesson’s misconceptions box explicitly states: “Allopatric speciation requires geographic isolation, but sympatric speciation can occur without it.” [1 — uses lesson’s misconception correction explicitly]

A corrected version: “Speciation commonly occurs through geographic isolation (allopatric speciation), but it can also occur in the same area through mechanisms such as polyploidy (sympatric speciation). The essential requirement in both cases is that reproductive isolation develops, stopping gene flow between diverging groups.” [1 — biologically accurate reformulation]

Marking criteria.

• 1 mark — Explicit judgement: claim is incorrect/flawed.
• 1 mark — Speciation definition + biological species concept including fertile offspring criterion.
• 1 mark — Allopatric speciation mechanism with Australian marsupial example.
• 1 mark — Sympatric speciation with polyploidy and named example (bread wheat).
• 1 mark — Comparison: gene flow cessation (not geographic isolation) is the true requirement.
• 1 mark — Uses lesson’s misconception correction explicitly.
• 1 mark — Biologically accurate reformulation.

Q2 — Sample Band 6 response (8 marks), annotated

Applying the biological species concept: a species is a group of organisms that can interbreed and produce fertile offspring. Horses and donkeys can mate (interbreed), but the mule offspring is sterile and cannot reproduce. Because the fertility criterion is not met, horses and donkeys are separate species under the biological species concept. [1 — applies biological species concept correctly: mule fails fertile offspring criterion; separate species]

The reproductive barrier is post-zygotic. Post-zygotic barriers act after fertilisation has occurred. In this case, mating and fertilisation can and do occur, but the resulting mule is sterile. This means the barrier acts after the zygote is formed (the mule is born), rather than preventing mating or fertilisation from occurring in the first place (which would be pre-zygotic). [1 — classifies as post-zygotic with correct justification (fertilisation occurs; offspring sterile)]

The mule’s sterility is important as evidence of species status because it shows that gene flow between horses and donkeys cannot be maintained even when they mate. If the mule were fertile, horse and donkey alleles would flow into each other’s populations through mule offspring, and the two groups might eventually merge back into one population. Sterility permanently prevents this gene flow, confirming that the two lineages are reproductively isolated. The lesson notes explicitly: “A horse and a donkey can mate and produce a mule, but the mule is sterile, so gene flow does not continue between the parent lineages. That makes the horse and donkey separate species.” [1 — explains why mule sterility is important evidence (no gene flow; lineages remain separate)]

The chromosomal explanation — horses have 64 and donkeys have 62 chromosomes, giving the mule 63 — means the mule cannot produce functional gametes during meiosis due to chromosome pairing problems. This chromosomal incompatibility is a physical consequence of 4 million years of independent evolution in the two lineages. [1 — explains chromosomal basis of mule sterility]

A plausible allopatric speciation scenario: approximately 4 million years ago, the common ancestor of horses and donkeys was separated by a geographic barrier — for example, the expansion of the Sahara desert, ocean level changes, or mountain formation could have divided one continuous population into two isolated groups, one ancestral to modern horses (Eurasia) and one ancestral to donkeys (Africa). [1 — identifies a plausible geographic barrier]

With the two groups geographically isolated, gene flow between them was sharply reduced. Each population faced different selection pressures in its respective environment: horses adapted to open grasslands with different herd behaviours; donkeys adapted to arid, rocky terrain. Different mutations accumulated in each lineage, and allele frequencies diverged over millions of years. [1 — explains divergence due to different selection pressures and mutations]

As chromosomal changes accumulated independently in each lineage (horses developing 64 chromosomes; donkeys 62), reproductive compatibility decreased. Even if the geographic barrier was removed, the chromosomal incompatibility meant that any hybrid offspring (mule) would be sterile. Reproductive isolation was now established post-zygotically, independent of whether the two groups encountered each other. [1 — links divergence to the development of post-zygotic reproductive isolation]

In conclusion, horses and donkeys are separate species under the biological species concept because their hybrid (the mule) is sterile. This is an example of post-zygotic isolation resulting from chromosomal divergence during approximately 4 million years of allopatric speciation. [1 — synthesised conclusion]

Marking criteria (8 marks).

• 1 mark — Correctly applies biological species concept: mule fails fertile offspring criterion; horses and donkeys are separate species.
• 1 mark — Classifies barrier as post-zygotic with correct justification.
• 1 mark — Explains why mule sterility is critical evidence (prevents gene flow; confirms reproductive isolation).
• 1 mark — Explains chromosomal basis (63 chromosomes; cannot form functional gametes).
• 1 mark — Identifies a plausible geographic barrier for the allopatric speciation scenario.
• 1 mark — Explains divergence: different selection pressures, mutations, allele frequency changes over time.
• 1 mark — Links divergence to post-zygotic reproductive isolation (chromosomal incompatibility).
• 1 mark — Synthesised conclusion.

Q3 — Sample Band 6 response (6 marks)

The claim is partly defensible but largely flawed. [1 — overall evaluative judgement]

What is defensible: different appearance between populations is often a signal that divergence has occurred — populations that have evolved in isolation for long periods often do look different. Physical differences can be a useful starting point for identifying possible species boundaries, and historically appearance was the primary taxonomic tool. [1 — concedes the defensible element]

What is flawed: “If two populations look different they must be different species.” The biological species concept requires reproductive isolation that prevents fertile interbreeding — not just physical difference. The lesson’s misconceptions box states: “populations can look different before they are different species. The decisive point is not appearance alone, but whether reproductive isolation blocks continued fertile gene flow.” Populations can diverge visibly while still being able to interbreed and produce fertile offspring, meaning they are not yet separate species. [1 — refutes “looks different = different species” using the biological species concept and lesson text]

“Physical appearance is the most reliable criterion.” This is also flawed. Convergent adaptation (studied earlier in this module) shows that unrelated species can look very similar due to similar selection pressures — sharks and dolphins are an example. Conversely, closely related species (cryptic species) can look nearly identical while being reproductively isolated. Appearance is often unreliable and must be supplemented by evidence of reproductive isolation (and ideally genetic analysis). [1 — refutes “most reliable” using convergent adaptation and/or cryptic species reasoning]

The decisive criterion is reproductive isolation: whether two populations can exchange genes by producing fertile offspring. Without that, even similar-looking organisms may be separate species (cryptic species), and different-looking organisms may still be the same species if they can interbreed. [1 — states the correct criterion clearly]

Defensible reformulation: “Physical appearance can be a useful initial indicator that divergence may have occurred between populations, but it is not a reliable criterion for determining species status. The biological species concept defines species by reproductive isolation — the ability to interbreed and produce fertile offspring — which is the most biologically meaningful criterion for whether speciation has occurred.” [1 — biologically defensible reformulation]

Marking criteria.

• 1 mark — States an overall evaluative judgement.
• 1 mark — Identifies the defensible element (appearance can signal divergence; historically useful).
• 1 mark — Refutes “looks different = different species” using biological species concept and lesson text.
• 1 mark — Refutes “most reliable” using convergent adaptation / cryptic species argument.
• 1 mark — States the correct criterion: reproductive isolation / fertile offspring.
• 1 mark — Reformulates the claim into a biologically defensible statement.