Speciation and isolating mechanisms: WACE Year 12 Biology
“Explain how reproductive isolation leads to allopatric and sympatric speciation”
Speciation is the formation of new species, which occurs when populations become reproductively isolated and their gene pools diverge until they can no longer interbreed. In allopatric speciation a physical barrier separates populations; in sympatric speciation isolation arises without a geographic barrier. Isolating mechanisms are prezygotic (preventing mating or fertilisation) or postzygotic (causing inviable or sterile hybrids).
What this dot point is asking
SCSA wants you to explain how a single species splits into two, distinguish allopatric from sympatric speciation, and classify isolating mechanisms. A strong answer links isolation to the divergence of gene pools and the breakdown of gene flow.
The biological species concept
A biological species is a group of organisms that can interbreed in nature to produce fertile offspring. The key to speciation is therefore the loss of the ability to interbreed: once gene flow between two populations stops, their gene pools can diverge independently until they become separate species.
Allopatric speciation
Allopatric speciation happens when a population is split by a geographic barrier such as a mountain range, river, sea or expanding desert. The steps are:
- A barrier divides one population into two, stopping gene flow.
- Each population experiences different selection pressures, mutations and drift.
- Their gene pools diverge over many generations.
- The populations become so different that, even if reunited, they can no longer interbreed.
In allopatric speciation the geographic barrier is what first stops gene flow. Australia's long isolation as a continent is a large-scale example: separation allowed unique lineages such as marsupials to diverge from those on other continents.
Sympatric speciation
Sympatric speciation occurs without a physical barrier, while populations share the same area. Reproductive isolation arises through other means, such as a shift to a different food source, a change in breeding time, or chromosomal changes that prevent interbreeding. It is less common than allopatric speciation but can occur rapidly.
Isolating mechanisms
These are the barriers to interbreeding that keep gene pools separate. They fall into two groups.
Prezygotic mechanisms act before fertilisation:
- temporal isolation (breeding at different times),
- behavioural isolation (different mating calls or courtship displays),
- geographic or habitat isolation,
- mechanical isolation (incompatible reproductive structures).
Postzygotic mechanisms act after fertilisation:
- hybrid inviability (the hybrid does not survive),
- hybrid sterility (the hybrid survives but cannot reproduce, such as a mule),
- hybrid breakdown (later generations are weak or infertile).
Follow the populations to divergence
A single population of a small lizard becomes split when a region dries into desert, leaving two isolated patches of suitable habitat. Gene flow between the patches stops. Over thousands of generations, different temperatures, predators and food in each patch drive different selection, and drift adds chance change. Eventually the two populations differ so much in mating behaviour that, even where their ranges later meet, they no longer interbreed. They are now two species.
Putting it together
Speciation always comes down to the same logic: reproductive isolation stops gene flow, and the isolated gene pools diverge through selection, drift and mutation until interbreeding is no longer possible. Whether the isolation begins with a geographic barrier (allopatric) or by other means within the same area (sympatric) determines the type of speciation, but the divergence of gene pools is common to both.
Speciation: the formation of new species. Reproductive isolation: the inability of populations to interbreed. Allopatric: speciation with a geographic barrier. Sympatric: speciation without a geographic barrier.
Why speciation matters for continuity
Speciation is how the diversity of life is generated. The accumulation of small changes within populations, when combined with reproductive isolation, eventually produces entirely new species. Over Australia's long isolation this process produced the continent's distinctive flora and fauna, showing how the same mechanisms studied in single populations scale up to explain biodiversity.
- Confusing allopatric and sympatric
- Allopatric involves a geographic barrier; sympatric does not.
- Calling all isolation postzygotic
- Most isolating mechanisms are prezygotic, preventing mating or fertilisation in the first place.
- Saying speciation needs a long time always
- It usually does, but sympatric speciation through chromosomal change can be rapid.
- Forgetting that fertile offspring define a species
- Producing sterile hybrids (a mule) does not make two species one.
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-style6 marksDistinguish between prezygotic and postzygotic isolating mechanisms, giving two examples of each, and explain how isolating mechanisms maintain separate species.Show worked answer →
A 6 mark answer needs the timing distinction, examples and the species-maintenance link.
- Prezygotic mechanisms (before fertilisation)
- They prevent mating or fertilisation. Examples (any two): temporal isolation (breeding at different times), behavioural isolation (different mating calls or courtship), mechanical isolation (incompatible reproductive structures), gametic incompatibility.
- Postzygotic mechanisms (after fertilisation)
- They act once a hybrid zygote forms. Examples (any two): hybrid inviability (the hybrid does not survive), hybrid sterility (the hybrid survives but cannot reproduce, such as a mule), hybrid breakdown (later generations are weak or infertile).
- Maintaining species
- Both types stop gene flow between populations, so their gene pools cannot mix. This keeps the populations genetically distinct, preserving them as separate species.
Markers reward the before- versus after-fertilisation distinction, two valid examples each and the link to preventing gene flow.
WACE 2024-style5 marksExplain how sympatric speciation can occur without a geographic barrier, and contrast it with allopatric speciation.Show worked answer →
A 5 mark answer needs the sympatric mechanism plus the contrast.
Sympatric speciation. Occurs while populations share the same area, with no physical barrier. Reproductive isolation arises by other means: for example a shift to a different food source or habitat, a change in breeding time, or chromosomal changes such as polyploidy in plants that immediately prevent interbreeding with the parent population. These barriers stop gene flow, so the gene pools diverge into separate species.
Contrast with allopatric speciation. In allopatric speciation a geographic barrier (mountain, river, sea, desert) first physically separates the populations and stops gene flow; in sympatric speciation there is no geographic separation, and isolation arises within the shared range. In both, divergence of isolated gene pools produces new species.
Markers reward a valid sympatric isolating mechanism, the no-barrier point and the explicit contrast with the geographic barrier of allopatric speciation.