Speciation and reproductive isolation (SACE Stage 2 Biology)
“Explain how reproductive isolation leads to speciation, including allopatric speciation”
Speciation is the formation of new species. It happens when populations become reproductively isolated so that gene flow between them stops, allowing them to diverge through selection, drift and mutation until they can no longer interbreed to produce fertile offspring. Allopatric speciation is the common pathway, where a geographic barrier first separates the populations.
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What this dot point is asking
You need to define a species and speciation, explain the role of reproductive isolation and isolating mechanisms, and describe allopatric speciation step by step.
What a species is
A species is usually defined as a group of organisms that can interbreed to produce fertile, viable offspring. Members of different species normally cannot do this. Speciation is the evolutionary process by which one species splits into two or more.
Reproductive isolation
For one population to become two species, gene flow between them must stop - they must become reproductively isolated. Once they no longer interbreed, the two populations evolve independently and accumulate genetic differences.
Isolating mechanisms prevent interbreeding and fall into two groups:
- Prezygotic mechanisms prevent mating or fertilisation, for example differences in breeding season (temporal), behaviour (different courtship), or habitat.
- Postzygotic mechanisms occur after mating, for example hybrid offspring that are inviable or sterile (the mule is a sterile hybrid).
Allopatric speciation
The most commonly described pathway is allopatric speciation ("allo" = different, "patric" = place), driven by geographic isolation:
- Geographic barrier. A physical barrier (a mountain range, river, ocean, or new desert) divides one population into two.
- No gene flow. The barrier stops the two groups from interbreeding, so no alleles are exchanged.
- Independent change. Each population experiences different selection pressures and different mutations and drift, so they accumulate different genetic changes over many generations.
- Reproductive isolation. Eventually the populations become so genetically different that they can no longer interbreed to produce fertile offspring, even if the barrier is removed. They are now separate species.
Sympatric speciation
In sympatric speciation new species form without a geographic barrier, while the populations share the same area. The clearest mechanism in plants is polyploidy: an error in cell division produces offspring with extra sets of chromosomes that can no longer interbreed with the diploid parent population but can reproduce among themselves, creating an instant reproductive barrier in a single generation. Other routes include divergence onto different host plants or food sources within the same habitat. The key point for SACE is that sympatric speciation still requires reproductive isolation; it simply achieves it without physical separation.
Isolating mechanisms in detail
Reproductive isolation is the heart of speciation, so be able to classify and give examples of isolating mechanisms. Prezygotic mechanisms act before a zygote forms and prevent mating or fertilisation: temporal isolation (breeding at different times), behavioural isolation (different courtship signals), ecological or habitat isolation (living in different parts of an area), and mechanical isolation (incompatible reproductive structures). Postzygotic mechanisms act after fertilisation: hybrid inviability (the hybrid embryo does not survive) or hybrid sterility (the hybrid survives but cannot reproduce, like the mule). Both types stop gene flow, which is what keeps diverging gene pools separate and allows two species to remain distinct.
Speciation needs two things: an end to gene flow (isolation) and enough independent genetic change (divergence) that the populations can no longer interbreed. Geographic isolation is a common trigger for the first step.
A geographic barrier in action
A single population of squirrels lives across an area that a deep canyon later divides into a north rim and a south rim.
- The canyon is a geographic barrier; squirrels cannot cross it.
- Gene flow between the two groups stops.
- The two sides have slightly different climates and food, so different selection pressures, plus independent mutation and drift, cause the populations to diverge.
- Over many generations they become so different that, if reunited, they could no longer interbreed successfully. Two species have formed by allopatric speciation.
The real-world example often cited is the Kaibab and Abert squirrels on either side of the Grand Canyon.
- Saying a geographic barrier alone creates a new species
- The barrier stops gene flow; speciation still requires enough genetic divergence that the populations can no longer interbreed.
- Forgetting the fertile-offspring part of the species definition
- Members of one species produce fertile offspring; hybrids like mules are sterile, showing the parents are different species.
- Confusing prezygotic and postzygotic isolation
- Prezygotic prevents mating or fertilisation; postzygotic acts after, through inviable or sterile hybrids.
- Assuming speciation is fast
- It usually takes many generations of divergence.
Exam-style questions
Questions in the style of SACE Board 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.
SACE 2018-style1 markPolyploid plants can form spontaneously within populations of diploid plants, giving rise to new species in the same habitat. State the term used for the process by which a new species that shares a habitat with its ancestral species is formed.Show worked answer →
Sympatric speciation. This is the formation of a new species without geographic separation, so the new species arises in the same habitat as its ancestral population (here, by polyploidy). The mark is for naming sympatric speciation.
SACE 2018-style4 marksDescribe the process that leads to the formation of a new plant species as a result of physical separation of a subpopulation from the original plant population.Show worked answer →
This is allopatric speciation. Four steps earn the marks.
A geographic barrier physically separates a subpopulation from the original population, preventing gene flow between them.
The two populations experience different environmental conditions and selection pressures, and different mutations arise in each.
Over many generations, natural selection and genetic drift cause the gene pools of the two populations to diverge, so their allele frequencies and features become increasingly different.
Eventually the populations are so genetically different that they can no longer interbreed to produce fertile offspring even if brought together. They are then reproductively isolated and are separate species.
SACE 2018-style2 marksThere is evidence that the northern hairy-nosed wombat and the southern hairy-nosed wombat are separate species. Identify one pre-zygotic isolating mechanism, and describe how it maintains distinct species.Show worked answer →
Name one pre-zygotic mechanism (one that prevents mating or fertilisation) and explain it.
Example, temporal isolation: the two species breed at different times of the year, so they do not have the opportunity to mate with each other.
Because mating between the species does not occur, no hybrid offspring are produced and no gene flow takes place between them, so their gene pools stay separate and the two species remain distinct. (Other valid pre-zygotic mechanisms include behavioural, ecological/habitat, or mechanical isolation.)
Practise this
Sources & how we know this
- Stage 2 Biology subject outline (2025 onwards) — SACE Board
- Stage 2 Biology: external assessment — SACE Board